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Nitric oxide (NO) has led a revolution in physiology and pharmacology research during the last two decades. This labile molecule plays an important role in many functions in the body regulating vasodilatation, blood flow, mitochondrial respiration and platelet function. Currently, it is known that NO synthesis occurs via at least two physiological pathways: NO synthase (NOS) dependent and NOS independent. In the former, L-arginine is the main precursor. It is widely recognized that this amino acid is oxidized to NO by the action of the NOS enzymes. Additionally, L-citrulline has been indicated to be a secondary NO donor in the NOS-dependent pathway, since it can be converted to L-arginine. Nitrate and nitrite are the main substrates to produce NO via the NOS-independent pathway. These anions can be reduced in vivo to NO and other bioactive nitrogen oxides. Other molecules, such as the dietary supplement glycine propionyl-L-carnitine (GPLC), have also been suggested to increase levels of NO, although the physiological mechanisms remain to be elucidated. The interest in all these molecules has increased in many fields of research. In relation with exercise physiology, it has been suggested that an increase in NO production may enhance oxygen and nutrient delivery to active muscles, thus improving tolerance to physical exercise and recovery mechanisms. Several studies using NO donors have assessed this hypothesis in a healthy, trained population. However, the conclusions from these studies showed several discrepancies. While some reported that dietary supplementation with NO donors induced benefits in exercise performance, others did not find any positive effect. In this regard, training status of the subjects seems to be an important factor linked to the ergogenic effect of NO supplementation. Studies involving untrained or moderately trained healthy subjects showed that NO donors could improve tolerance to aerobic and anaerobic exercise. However, when highly trained subjects were supplemented, no positive effect on performance was indicated. In addition, all this evidence is mainly based on a young male population. Further research in elderly and female subjects is needed to determine whether NO supplements can induce benefit in exercise capacity when the NO metabolism is impaired by age and/or estrogen status.
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AUTHOR PROOF
The Effect of Nitric-Oxide-Related
Supplements on Human Performance
Rau
´l Besco
´s,
1
Antoni Sureda,
2
Josep A. Tur
2
and Antoni Pons
2
1 National Institute of Physical Education INEFC-Barcelona, Research Group on Sport Sciences, University
of Barcelona, Barcelona, Spain
2 Laboratory of Physical Activity Science, Research Group on Community Nutrition and Oxidative Stress,
University of Balearic Islands, Palma Mallorca, Spain
Contents
Abstract................................................................................... 1
1. Introduction ............................................................................ 2
2. Synthesis of Nitric Oxide (NO) from the NO Synthase-Dependent Pathway . . . . . . . . . . . . . . . . . . . . . . . 3
2.1 L-Arginine: Sources and Metabolism. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.1.1 Ergogenic Effect of L-Arginine Supplements Alone . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.1.2 Ergogenic Effect of L-Arginine Supplements in Combination with Other Components . . . . 5
2.2 L-Citrulline: Sources and Metabolism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
2.2.1 Ergogenic Effect of L-Citrulline Supplements Alone. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
2.2.2 Ergogenic Effect of L-Citrulline Supplements with Malate. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
3. Synthesis of NO from the NOS-Independent Pathway . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
3.1 Nitrate and Nitrite: Sources and Metabolism. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
3.1.1 Ergogenic Effect of Sodium Nitrate Supplementation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
3.1.2 Ergogenic Effect of Nitrate Supplementation in the Form of Beetroot Juice . . . . . . . . . . . . 11
4. Other Components Related to NO Synthesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
4.1 Glycine Propionyl-L-Carnitine (GPLC). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
4.1.1 Ergogenic Effect of GPLC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
4.2 2-(Nitrooxy) Ethyl 2-Amino-3-Methylbutanoate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
5. Side Effects of NO Supplements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
6. Conclusion and Future Perspectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Abstract Nitric oxide (NO) has led a revolution in physiology and pharmacology
research during the last two decades. This labile molecule plays an important
role in many functions in the body regulating vasodilatation, blood flow,
mitochondrial respiration and platelet function. Currently, it is known that
NO synthesis occurs via at least two physiological pathways: NO synthase
(NOS) dependent and NOS independent. In the former, L-arginine is the
main precursor. It is widely recognized that this amino acid is oxidized to NO
by the action of the NOS enzymes. Additionally, L-citrulline has been in-
dicated to be a secondary NO donor in the NOS-dependent pathway, since it
can be converted to L-arginine. Nitrate and nitrite are the main substrates to
produce NO via the NOS-independent pathway. These anions can be reduced
in vivo to NO and other bioactive nitrogen oxides. Other molecules, such as
Approval for publication Signed Date Number of amended pages returned
REVIEW ARTICLE Sports Med 2012; 42 (3): 1-19
0112-1642/12/0003-0001/$49.95/0
ª2012 Adis Data Information BV. All rights reserved.
AUTHOR PROOF
the dietary supplement glycine propionyl-L-carnitine (GPLC), have also been
suggested to increase levels of NO, although the physiological mechanisms
remain to be elucidated.
The interest in all these molecules has increased in many fields of research.
In relation with exercise physiology, it has been suggested that an increase in
NO production may enhance oxygen and nutrient delivery to active muscles,
thus improving tolerance to physical exercise and recovery mechanisms.
Several studies using NO donors have assessed this hypothesis in a healthy,
trained population. However, the conclusions from these studies showed
several discrepancies. While some reported that dietary supplementation with
NO donors induced benefits in exercise performance, others did not find any
positive effect. In this regard, training status of the subjects seems to be an
important factor linked to the ergogenic effect of NO supplementation.
Studies involving untrained or moderately trained healthy subjects showed
that NO donors could improve tolerance to aerobic and anaerobic exercise.
However, when highly trained subjects were supplemented, no positive effect
on performance was indicated. In addition, all this evidence is mainly based
on a young male population. Further research in elderly and female subjects
is needed to determine whether NO supplements can induce benefit in ex-
ercise capacity when the NO metabolism is impaired by age and/or estrogen
status.
1. Introduction
Nitric oxide (NO) is a labile lipid soluble gas
synthesized at several locations in the body. The
endogenous formation and biological significance
of NO were revealed in a series of studies in the
1980s and for these seminal discoveries, three
American researchers were subsequently awarded
the Nobel Prize in Physiology orMedicine in 1998.
Soon after the identification of NO as a signalling
molecule in mammals, it was reported that specific
nitric oxide synthase (NOS) enzymes catalyze
a complex enzymatic reaction leading to NO
formation from the substrates L-arginine and
molecular oxygen.
[1]
Later, an alternative NOS-
independent pathway of NO synthesis was dis-
covered, based on the simple reduction of nitrate
and nitrite,
[2,3]
the main oxidation products of
NO. During this period, interest in the biological
role of NO has led a revolution in pharmacologi-
cal and physiological research. Currently, NO is
known to regulate important functions as a me-
diator in noradrenergic and non-cholinergic neu-
rotransmission, in learning and memory, synaptic
plasticity, and neuroprotection.
[4]
In exercise physiology, NO has also received
much interest, and supplements of NO are
thought to be an ergogenic aid.
[5]
This fact is
based on the evidence that NO is an important
modulator of blood flow and mitochondrial res-
piration during physical exercise.
[6]
In addition, it
is suggested that the increase of blood flow de-
rived from NO synthesis may improve recovery
processes of the activated tissues.
[7]
These sup-
posed benefits are claimed in most sport supple-
ments, which are currently sold in the market and
linked with stimulation of NO production.
However, a careful examination of the composi-
tion of NO-stimulating supplements shows that,
in many cases, they are ‘cocktails’ of a great
variety of ingredients such as creatine, carbohy-
drates, amino acids, vitamins, minerals, etc. It is
known that some of these components (creatine,
carbohydrates and amino acids) may have an
ergogenic effect in themselves.
[8-10]
In addition,
the scientific evidence behind these ‘cocktails’ of
supplements related with NO stimulation is very
scarce. Only one study has evaluated the effect of
some of these products, indicating that their ef-
fectiveness at increasing NO and/or improving
2Besco
´s et al.
ª2012 Adis Data Information BV. All rights reserved. Sports Med 2012; 42 (3)
AUTHOR PROOF
performance is very limited.
[11]
In comparison
with data reported in scientific studies, it has been
suggested that the amounts of NO ingredients
(mainly L-arginine and L-citrulline) that contain
commercial NO-stimulating supplements are
extremely low and ineffective to induce changes
in NO.
[11]
For this reason, most studies involving NO
donors have used pharmaceutical products to as-
sess the effect on human performance.
[12-18]
Fur-
thermore, there are some recent studies that have
also assessed the effect of natural foods rich in NO
donors, such as beetroot juice.
[19-23]
Results from
these studies show great controversy. Some of
them showed that dietary NO supplements may
enhance human performance in healthy sub-
jects,
[19,24,25]
but others did not find any positive
effect.
[16,18,26]
One reason to explain this fact
could be the large methodological differences
between studies: duration of treatment, exercise
protocol and training status differ significantly
between studies, making a comparison between
them difficult. Additionally, many studies have
used NO donors in combination with other com-
ponents such as malate, glutamate, aspartate, etc.,
in an attempt to increase the bioavailability of NO
donors. This fact adds more difficulty because
some of these additional products may participate
in the independent NO-synthesis pathways in the
body.
Accordingly, this review focuses on pathways
and donors of NO synthesis and elucidates the
effect of NO supplements on human performance.
Scientific articles were retrieved based on an ex-
tensive search in MEDLINE (19802011) and
Google Scholar (19902011) databases. Computer
search engines used the following combined key-
words: ‘L-arginine’, ‘L-citrulline’, ‘nitrate’, ‘gly-
cine-propionyl-L-carnitine’, ‘supplementation’,
‘nitric oxide’, ‘exercise’, ‘performance’. After using
these initial keywords, the search engines were
limited to human studies excluding research with
animals, as well as in humans in pathological
states. As a result, 42 articles related to the effects
of dietary ingredients linked with NO and perfor-
mance in response to exercise, were considered.
References cited in the retrieved articles were also
considered in this review.
2. Synthesis of Nitric Oxide (NO) from the
NO Synthase-Dependent Pathway
L-arginine amino acid participates in the
NOS-dependent pathway in a reaction catalyzed
by specific NOS enzymes
[4]
(figure 1). Addi-
tionally, it has been suggested that L-citrulline
could be an alternative donor of NO, due to the
fact that it can increase the levels of L-arginine.
2.1 L-Arginine: Sources and Metabolism
L-arginine is considered a conditional essen-
tial proteinogenic amino acid that is a natural
constituent of dietary proteins. L-arginine is re-
latively high in seafood, watermelon juice, nuts,
seeds, algae, meat, rice protein concentrate and soy
protein isolate.
[27]
The typical dietary intake of
L-arginine is approximately 45g per day. Fur-
thermore, L-arginine could be endogenously syn-
thesized, mainly in the kidney, where L-arginine is
formed from L-citrulline.
[28]
The liver is also able to
synthesize considerable amounts of L-arginine,
although this is completely reutilized in the urea
cycle.
[28]
Normal plasma L-arginine concentra-
tions depend upon the age of the individual and
homeostasis is primarily achieved via its catabo-
lism.
[29]
The usual mean standard deviation
range of plasma L-arginine in humans has been
determined between 70 and 115 mmolL
-1[30]
Extracellular L-arginine can be quickly taken up
by endothelial cells; in the presence of molecular
oxygen and nicotinamide adenosine dinucleotide
phosphate, L-arginine is subsequently oxidized to
NO.
[1,4]
This is a complex reaction, which is cata-
lyzed by NOS enzymes that contain a binding site
for L-arginine. There are three isoforms of NOS
that have been recognized:typeI(neuronalNOS;
nNOS), type II (inducible NOS; iNOS) and type
III (endothelial NOS; eNOS). eNOS and iNOS are
constitutive enzymes that are controlled by intra-
cellular Ca
2+
/calmodulin. nNOS is inducible at the
level of gene transcription, Ca
2+
independent and
expressed by muscle activity
[31]
the aging pro-
cess,
[32]
as well as by macrophages and other tissues
in response to inflammatory mediators.
[33]
L-arginine participates in other metabolic
pathways independent of NO synthesis. For in-
Nitric Oxide Supplements and Performance 3
ª2012 Adis Data Information BV. All rights reserved. Sports Med 2012; 42 (3)
AUTHOR PROOF
stance, L-arginine is essential for the normal
function of the urea cycle, in which ammonia is
detoxified through its metabolism into urea.
[34]
L-arginine is also a potent hormone secreta-
gogue. L-arginine infusion at rest increases plas-
ma insulin, glucagon, growth hormone (GH),
prolactin and catecholamines concentrations.
[35]
Such hormonal changes affect the metabolism.
For instance, insulin and GH are important
anabolic hormones with a remarkable degree of
synergy in regulating glucose and fat metabolism.
While insulin facilitates glucose entry into cells
and an increase in glycogen stores, GH stimulates
lipolysis and reduces glucose oxidation to main-
tain blood glucose levels.
[36]
Thus, it has been
suggested that GH and insulin release may en-
hance exercise performance by increasing fatty
acid oxidation and sparing glycogen stores.
[36]
In
addition, GH also causes the release of insulin-
like growth factor (IGF)-1 that increases amino
acid uptake and protein synthesis.
[37]
These ef-
fects could also improve performance through
increased muscle mass and strength.
[37]
2.1.1 Ergogenic Effect of L-Arginine
Supplements Alone
Seven studies have analyzed the effect of L-
arginine supplementation alone.
[12-18]
Two of these
studies were carried out in healthy, but not well
trained, males
[12,14]
and one in healthy, postmen-
opausal women.
[13]
In this study, females were
supplemented with high doses of L-arginine
(14.2 gday
-1
) for 6 months. After this period, a
significant increase in the maximal power in rela-
tion with body mass (powerkg
-1
) measured as
peak jump power (counter-movement jump) was
found.
[13]
In male studies, it has been indicated that
L-arginine supplementation could enhance the res-
piratory response. Koppo et al.
[12]
showed a sig-
nificant increase in speed in phase II of pulmonary
oxygen consumption ( .
VO
2
)attheonsetofmoder-
ate intensity endurance cycle exercise after 14 days’
NADPH + H+
Endogenous
synthesis
(kidneys and liver)
NADPH
NOS-dependent pathway
NOS-independent pathway
Diet
L-arginine + O2
Air
Lung
NOS
NO
L-citrulline
Nitrate/nitrite
Blood
Nitrate/nitrite
Kidney/urine
Arginino
succinate
O2
Fig. 1. Metabolic pathways of NO synthesis in humans. NADPH =nicotinamide adenine dinucleotide phosphate-oxidase; NO =nitric oxide;
NOS =NO synthases.
4Besco
´s et al.
ª2012 Adis Data Information BV. All rights reserved. Sports Med 2012; 42 (3)
AUTHOR PROOF
L-arginine supplementation (6 gday
-1
). Faster
.
VO
2
kinetics of phase II reduces the O
2
deficit
that follows the onset of exercise and can reduce
intracellular perturbation (e.g. increased lactic
acid, decreased phosphocreatine).
[38]
This fact
could be interesting in order to enhance tolerance
to endurance exercise, mainly in subjects with slow
.
VO
2
kinetics (time it takes to reach 63%of steady
state [tau] >30 seconds). However, all these find-
ings were not linked with NO synthesis since the
above studies did not report data related to NO
markers, such as plasma ratio of L-arginine :
L-citrulline and/or plasma levels of nitrate and
nitrite. On the other hand, Olek et al.
[14]
assessed
the effect of an acute dose of L-arginine in a low
dose (2 g) 60 minutes before exercise. They showed
that this amount of L-arginine did not induce any
increase in the total work performed or mean
power output during Wingate cycle tests (30 sec-
onds), or .
VO
2
either.
[14]
Additionally, plasma levels
of nitrate/nitrite were unchanged after L-arginine
supplementation compared with placebo.
The remaining studies were performed in well
trained athletes using different types of athletic
populations such as judo athletes,
[17,18]
tennis
players
[16]
and cyclists.
[15]
Despite analyzing sup-
plements during different durations (between 1
and 28 days) and doses (between 6 and 12 g), no
benefit was indicated in parameters linked with
performance, such as power in a cycle ergometer
test
[18]
or .
VO
2
during a treadmill test.
[15,16]
Ad-
ditionally, the levels of some exercise metabolites
(lactate and ammonia) were unchanged after
L-arginine supplementation compared with pla-
cebo.
[17,18]
Moreover, three of these studies ana-
lyzed the level of plasma nitrate/nitrite as NO
markers showing that they did not increase after
dietary L-arginine ingestion.
[16-18]
The other study
did not include data regarding NO metabolites.
[15]
Apart from dietary supplementation, other
studies have analyzed the effect of intravenous
infusion of L-arginine in an attempt to increase its
bioavailability,
[39,40]
since dietary L-arginine bio-
availability is only about 60%.Thisfactisdueto
the high activity of arginases in the liver.
[41]
Argi-
nases are enzymes that participate in the fifth and
final step of the urea cycle, competing with NOS
for L-arginine.
[42]
In athletes, there is evidence that
exhaustive exercise increases arginase activity in
lymphocytes nearly 6-fold, limiting L-arginine
availability for lymphocyte iNOS activity.
[43]
However, despite the fact that the bioavailability
of intravenous infusion of L-arginine could be
high compared with dietary consumption, no po-
sitive effect in parameters of performance, such as
maximal workload during an incremental cycle
ergometer test or the amount of work completed
in a 15-minute test after intravenous L-arginine
infusion, has been reported.
[39,40]
2.1.2 Ergogenic Effect of L-Arginine Supplements in
Combination with Other Components
There are several studies that have shown an
increase in exercise performance after L-arginine
supplementation in combination with other com-
ponents in untrained or moderately trained sub-
jects. For instance, a recent study of Bailey et al.
[24]
showed that L-arginine (6 g ·3 days) in combina-
tion with other amino acids and vitamins induced a
decrease in .
VO
2
(L-arginine: 1.48 0.12 Lmin
-1
;
placebo: 1.59 0.14 Lmin
-1
;p<0.05) in a low to
moderate bout of exercise (6 minutes at 82 14 W);
and an increase in time to exhaustion (L-arginine:
707 232 seconds; placebo: 562 145 seconds;
p<0.05) during an incremental cycling test.
[21]
In
another recent study, Camic et al.
[25]
foundanin-
crease in power output (5.4%)duringanincre-
mental test to exhaustion (cycle ergometer) when
L-arginine (3 g) in combination with grape seed
extract was administered for 28 days. Similarly, in
elderly males, Chen et al.
[44]
found that supple-
mentation of L-arginine (5.2 gday
-1
·21 days)
with L-citrulline and antioxidants increased power
output (~21%) during an incremental cycle erg-
ometer test until exhaustion. These surprising
findings have been related to an increase in gas
exchange threshold after L-arginine supplementa-
tion.
[44,45]
It has been suggested that the attenua-
tion of metabolic products such as potassium,
ammonia and lactate, may be the result of in-
creased clearance from the circulation related to
NO synthesis and increased blood flow.
[45]
How-
ever, it is only speculation, since there is evidence
indicating that higher doses of dietary L-arginine
(>10 g) are ineffective to increase blood flow in
healthy humans.
[46,47]
Nitric Oxide Supplements and Performance 5
ª2012 Adis Data Information BV. All rights reserved. Sports Med 2012; 42 (3)
AUTHOR PROOF
Other studies have also reported benefits of a
mixture of L-arginine supplements in strength and
power performance in moderately trained subjects.
Campbell et al.
[48]
indicated a significant increase
of one-maximum repetition (1-RM) of bench
press, as well as peak power during a 30-second
Wingate test after L-arginine supplementation
(6 gday
-1
·56 days) in combination with a-keto-
glutarate. Futhermore, Buford et al and Koch
[49]
and Stevens et al.
[50]
showed that an acute dose
of L-arginine (6 g of L-arginine) in the form of
a-ketoisocaproic increased the mean power per-
formed during Wingate tests (10 seconds) and work
sustained during continuous isokinetic concentric/
eccentric knee extension repetitions, respectively.
In well trained athletes, two studies have as-
sessed dietary L-arginine supplementation in com-
bination with aspartate. In the first, Colombani
et al.
[51]
supplemented (15 gday
-1
·14 days) en-
durance-trained runners. They showed that the
plasma level of somatotropic hormone (STH),
glucagon, urea and arginine were significantly in-
creased, and the level of plasma amino acids was
significantly reduced after a marathon run follow-
ing L-arginine supplementation. The conclusion of
this study was that there was no metabolic or per-
formance benefit derived from L-arginine. More
recently, similar findings were reported by Abel
et al.
[52]
They supplemented endurance-trained
cyclists with L-arginine and aspartate at high (5.7 g
of L-arginine; 8.7 g of aspartate) and low (2.8 g of
L-arginine; 2.2 g of aspartate) doses for 28 days.
After an incremental endurance exercise test (cycle
ergometer) in laboratory conditions, no modifica-
tion was found in endurance performance ( .
VO
2
peak [ .
VO
2peak
], time to exhaustion), or in endocrine
(concentration of growth hormone, glucagon, cor-
tisol and testosterone) or in metabolic (concentra-
tion of lactate, ferritine and urea) parameters.
[52]
Therefore, including all studies with L-arginine
supplementation alone and with other compo-
nents (tables I and II), no study in well trained
athletes reported benefits in human perfor-
mance.
[15-18,51,52]
One important factor that may
explain the reduced effect of L-arginine in well
trained athletes, could be explained by the phy-
siological and metabolic adaptation derived from
chronic physical training. The effect of exercise
training on the enhancement of endothelial func-
tion has been well established.
[69]
Repetitive ex-
ercise over weeks results in an upregulation of
endothelial NO activity. This is not a localized
but rather a systemic response in endothelial
function when large muscle mass is regularly acti-
vated, as in aerobic exercise.
[70]
Perhaps benefits in
pulmonary, cardiovascular and neuromuscular
systems induced by long-term training may over-
come any potential effects of dietary L-arginine
supplementation in well trained athletes. How-
ever, there are other factors that may also reduce
the effect of dietary L-arginine, such as the
L-arginine : lysine ratio. The amino acid lysine
competes with L-arginine for entry into cells and
also inhibits arginase activity.
[71]
Under normal
feeding conditions, the total amount of L-arginine
in the diet should not be more than 150%greater
than that of lysine (namely, L-arginine : lysine
<2.5).
[72]
In addition, in most of the above mentioned
studies, there is a lack of data concerning NO
metabolites. Only Bailey et al.
[24]
analyzed the
plasma levels of nitrite, reporting a significant
increase after L-arginine supplementation. How-
ever, only one study
[24]
states that there is too
little scientific evidence to corroborate that die-
tary L-arginine supplementation increases NO
synthesis in healthy humans. Some of the benefits
shown in the previous studies could be related to
other metabolic pathways independent of NO
synthesis, as well as to the other ingredients in-
cluded in L-arginine supplements. For example,
there is evidence that L-arginine supplementation
in combination with glutamate and aspartate is
effective at reducing blood levels of ammo-
nia,
[55,56]
as well as blood lactate
[40,57]
during
exercise. This response could explain the results
reported by the aforementioned studies of Camic
et al.
[45]
and Stevens et al.
[50]
Moreover, L-arginine
is known to actively participate in the synthesis of
creatine.
[73]
Diets supplemented with L-arginine
increase intramuscular creatine phosphate con-
centrations between 1%and 2%in laboratory
animals; thus, this may enhance the response to
anaerobic exercise.
[74]
This finding may be a sui-
table response to the study of Buford et al. and
Koch
[49]
who indicated that a supplement of
6Besco
´s et al.
ª2012 Adis Data Information BV. All rights reserved. Sports Med 2012; 42 (3)
AUTHOR PROOF
glycine-arginine-a-ketoisocaproic acid (GAKIC)
enhances performance during repeated bouts of
anaerobic cycling performance.
In summary, current evidence of L-arginine
supplementation in sports performance suggests
that (i) L-arginine, mainly in combination with
other components, could induce some benefit in
untrained or moderately trained subjects, im-
proving tolerance to aerobic and anaerobic phy-
sical exercise. However, as the studies do not
show a well defined relationship between dietary
L-arginine supplementation and NO synthesis,
the benefit in exercise performance shown in
some studies could be derived from other in-
gredients of supplements, as well as other meta-
bolic pathways independent of NO synthesis; and
(ii) in well trained athletes, there is a lack of data
indicating that L-arginine supplementation in-
duces benefits in performance. A recent review
analyzing the potential ergonegic effects of acute
and chronic L-arginine supplementation did not
reach a clear conclusion as to the benefits in ex-
ercise performance either.
[75]
2.2 L-Citrulline: Sources and Metabolism
The organic compound L-citrulline, is a non-
essential alpha amino acid. Its name is derived
from Citrullus, the Latin word for watermelon
from which it was first isolated in 1930, and which
Table I. Studies with nitric oxide supplements that reported an increase in performance
Substance Dose per
day
Duration
(days)
Other components Design Sample
size
Training
status
Effects References
L-arg 14.2 g »180 DB, R 23 U Maximal power 13
L-arg 6.0 g 3 Vitamins and
amino acids
DB, CO 9 M Efficiency and time to
exhaustion
24
L-arg 1.5 g 28 Grape seed extract DB, R 50 U Work capacity 25
L-arg 1.5 g 28 Grape seed extract DB, R 41 U Increase of gas
exchange threshold and
power output
45
L-arg 5.2 g 21 L-citrulline and
antioxidants
DB, R 16 M Power output 44
L-arg 6.0 g 56 a-ketoglutarate DB, R 35 M Increase of 1-RM 48
L-arg 6.0 g 1 Glycine
a-ketoisocaproic
DB, R 19 M Power performance 49
L-arg 6.0 g 1 Glycine
a-ketoisocaproic
DB, R, CO 13 U Work sustained during
anaerobic exercise
50
L-citr 8.0 g 1 Malat e DB, R, CO 41 M Work capacity 53
Nitrate 5.5 mmol 6 Beetroot juice DB, R, CO 8 M Efficiency and time to
exhaustion
19
Nitrate 5.1 mmol 6 Beetroot juice DB, R, CO 7 M Increase time-to-task
failure
20
Nitrate 6.2 mmol 6 Beetroot juice DB, R, CO 9 M Efficiency and time to
exhaustion
22
Nitrate 6.2 mmol 1 Beetroot juice DB, R, CO 9 M Power output 23
Nitrate 5.2 mmol 15 Beetroot juice DB, R, CO 8 U Increase efficiency
and peak power
21
GPLC 4.5 g 1 DB, R, CO 24 M Peak power and
reduced power
decrement
54
1-RM =one-repetition maximum; CO =crossover; DB =double blind; GPLC =glycine propionyl-L-carnitine; L-arg =L-arginine; L-citr =
L-citrulline; M=moderately-trained subjects; R=randomized; U=untrained subjects; indicates improvement in performance.
Nitric Oxide Supplements and Performance 7
ª2012 Adis Data Information BV. All rights reserved. Sports Med 2012; 42 (3)
AUTHOR PROOF
Table II. Studies with NO supplements that reported no or negative effects on performance
Substance Dose per day Duration (days) Other components Design Sample size Training status Performance Other findings References
L-arg 6.0 g 14 DB, CO 7 M NM Increase of phase II pulmonary .
VO
2
12
L-arg 2.0 g 1 DB, R, CO 6 M None 14
L-arg 12.0 g 28 DB, R 18 H None 15
L-arg 20.5 g 3 DB, R 9 H NM 16
L-arg »7.6 g 1 R 15 H NM Increase of glucose and insulin 17
L-arg 6.0 g 3 DB, R 10 H None 18
L-arg 30.0 g
a
1 DB, R 9 H None Increase of glucose 39
L-arg 3.0 g
a
1 DB, R 8 M None Lower blood lactate and ammonia 40
L-arg 5.7 g 28 L-asp R 30 H None 52
L-arg 15.0 g 14 L-asp DB, CO 20 H NM Increase of somatotropic hormone, glucagon and urea 51
L-arg 20.0 g 1 L-glut DB, CO 3 U NM Lower ammonia 55
L-arg 5.0 g 10 L-asp DB, R 15 U NM Lower ammonia 56
L-arg 3.0 g 21 L-asp DB 16 M NM Lower blood lactate and .
VO
2
57
L-citr 6.0 g 1 Malate DB, R 17 H NM Increased levels of NO metabolites 58
L-citr 9.0 g 1 DB 17 M Decrease of time to exhaustion 59
L-citr 6.0 g 15 Malate ? 18 U NM Increase ATP production 60
L-citr 6.0 g 1 Malate DB, R 17 H NM Increase of plasma nitrite 61
Nitrate 10.0mgkg
-1
1 SN DB, R, CO 11 H None Reduce .
VO
2peak
26
Nitrate 0.1mmolkg
-1
3 SN DB, R, CO 9 M None Increase efficiency 62
Nitrate 0.1mmolkg
-1
2 SN DB, R, CO 9 M None Reduce .
VO
2peak
63
Nitrate 0.1mmolkg
-1
3 SN DB, R, CO 14 M NM Increase mitochondrial efficiency 64
GPLC 4.5g 1 DB, R, CO 19 M None Decrease of malondialdehyde 11
GPLC 3.0g 28 DB, R, CO 15 M NM Increased levels of NO metabolites 65
GPLC 1.54.5 g
b
56 DB, R 30 U NM Increased levels of NO metabolites 66
GPLC 1.54.5 g
b
56 DB, R, CO 32 U None 67
2-ethyl 1 R, CO 10 M NM No changes in plasma nitrate/nitrite 68
a Intravenous supplementation.
b Data is presented in ranges where stated.
2-ethyl =2-(nitrooxy) ethyl 2-amino-3-methylbutanote; ATP =adenosine triphosphate; CO =crossover; DB =double blind; GPLC =glycine propionyl-L-carnitine; H=highly trained
subjects; L-arg =L-arginine; L-asp =L-aspartate; L-citr =L-citrulline; L-glut =L-glutamate; M=moderately trained subjects; NM =none measured; R=randomized; SN =sodium
nitrate; U=untrained subjects; .
VO
2
=oxygen consumption; .
VO
2peak
=peak .
VO
2
;indicates decrease in performance; indicates no supplement composition shown therefore
amount not shown.
8Besco
´s et al.
ª2012 Adis Data Information BV. All rights reserved. Sports Med 2012; 42 (3)
AUTHOR PROOF
is the main dietary source of this amino acid.
[76]
L-citrulline is also produced endogenously via the
following two main pathways: (i) it is synthesized
from glutamine in enterocytes, by condensation of
ornithine and carbamyl phosphate in a reaction
catalyzed by ornithine carbamyl-transferase;
[77,78]
and (ii) L-citrulline is produced via the conversion
of L-arginine to NO in a reaction catalyzed by
NOS enzymes (figure 1). The normal value of
L-citrulline reported in healthy populations is ap-
proximately 25 mmolL
-1
,
[79]
although lower val-
ues have recently been found (1015 mmolL
-1
)in
professional cyclists.
[58]
The dietary interest for this amino acid has
substantially increased in the last decade as a result
of the importance of L-citrulline as a precursor of
L-arginine.
[80,81]
It is interesting, because, unlike
L-arginine, it bypasses the hepatic metabolism and
is not a substrate of arginase enzymes. For this
reason, it has been indicated that systemic admin-
istration of L-citrulline could be a more efficient
way to elevate extracellular levels of L-arginine by
itself.
[82]
Dietary L-citrulline is taken up and re-
leased by enterocytes in the portal circulation, by-
passes metabolism by periportal hepatocytes and is
transported to the kidneys where around 80%is
catabolized to L-arginine by cells of the proximal
tubules.
[83]
Apart from the function as a precursor
of L-arginine, it is known that L-citrulline is an es-
sential component participating in the urea cycle in
the liver.
[77]
2.2.1 Ergogenic Effect of L-Citrulline
Supplements Alone
Only one study has been carried out involving
L-citrulline supplementation without the addition
of other products. In this study, Hickner et al.
[59]
assessed the effect of one dose of L-citrulline ad-
ministered 3 hours (3 g) or 24 hours (9 g) before an
incremental treadmill test until exhaustion in
young healthy subjects. Contrary to the hypothesis
of the authors, the results showed that L-citrulline
supplementation impaired exercise performance
measured as time to exhaustion compared with
placebo. To explain this surprising response, it was
indicated that L-citrulline ingestion might reduce
nitric-oxide-mediated pancreatic insulin secretion
or increase insulin clearance. This hypothesis was
based on the lower plasma insulin levels found
after L-citrulline ingestion.
[59]
Additionally, lower
levels of plasma NO markers (nitrates/nitrites)
were also indicated following L-citrulline supple-
mentation compared with placebo.
2.2.2 Ergogenic Effect of L-Citrulline Supplements
with Malate
The other studies that have analyzed the effect
of L-citrulline combined this amino acid with
malate, which is an intermediate component of
the tricarboxylic acid cycle (TCA). The first of
these studies examined the rate of adenosine
triphosphate (ATP) production during an ex-
ercise of finger flexions using
31
P-magnetic res-
onance spectroscopy (
31
P-MRS).
[60]
This study
concluded that 6 gday
-1
of L-citrulline with
malate for 16 days resulted in a significant increase
(34%) in the rate of oxidative ATP production
during exercise, and a 20%increase in the rate of
phosphocreatine recovery after exercise.
[60]
How-
ever, there is some criticism around this research,
because it used a very simple design without a
placebo group or a blind condition. More recently,
two studies conducted by the same research group
showed an increase in plasma NO metabolites in
well trained endurance athletes after a cycling
competition; these athletes were supplemented
with only one dose of L-citrulline with malate (6 g)
2 hours before exercise.
[58,61]
In addition, an in-
crease in plasma arginine availability was linked
with substrate for NO synthesis, as well as poly-
morphonuclear neutrophils (PMNs).
[58]
PMNs
play an important role in the defense against in-
fections, the inflammatory response, and muscle
repair and regeneration.
[84,85]
Unfortunately, these
findings were unable to be associated with vari-
ables of exercise performance because of the
characteristics of the study design. Many factors,
such as strategy, environmental conditions, nutri-
tion, drafting and breakdown of material, can af-
fect the results during field sport events, limiting the
use of these data to assess the association between
dietary supplement and performance. Another re-
cent study by Pe
´rez-Guisado and Jakeman
[53]
showed that a single dose of L-citrulline with ma-
late (8 g) increased work capacity by an average
of 19%, measured as the number of repetitions
Nitric Oxide Supplements and Performance 9
ª2012 Adis Data Information BV. All rights reserved. Sports Med 2012; 42 (3)
AUTHOR PROOF
performed until exhaustion during a flat barbell
bench-press test at 80%of 1-RM. However, this
finding cannot be related to NO delivery because
plasma NO markers were not determined in this
study.
[53]
Taking all this overview together, it is evident
that there is a lack of data linking an increase in
exercise performance to an increase in NO pro-
duction derived from L-citrulline supplementation
(tables I and II). Performance enhancement re-
ported by L-citrulline in combination with malate
could be explained by the interaction of these mo-
lecules in other metabolic pathways independent
of NO production. For example, L-citrulline in-
creases levels of plasma L-arginine indirectly; it
could also enhance the synthesis of creatine, since it
has been reported that L-arginine supplementation
stimulates an increase in intramuscular creatine
concentration.
[74]
Therefore, this mechanism may
improve the response to anaerobic exercise. In ad-
dition, malate may be involved in the beneficial
effects on energy production because it is an in-
termediate of TCA.
[59,86]
It has been suggested that
hyperactivation of aerobic ATP production cou-
pled to a reduction in anaerobic energy supply,
may contribute to the reduction in fatigue sensa-
tion reported by the subjects.
[87]
In short, the conclusions that we can extract
from the studies using L-citrulline as a dietary
supplementation in sport are as follows:
Dietary supplementation with L-citrulline
alone does not improve exercise performance.
Addition of malate to dietary L-citrulline supple-
ments may increase levels of NO metabolites.
However, this response has not been related to
an improvement in athletic performance.
3. Synthesis of NO from the NOS-
Independent Pathway
The NOS-independent pathway is a novel
pathway that was discovered by two independent
research groups during the 1990s.
[2,3]
Nitrate and
nitrite are the main precursors for NO synthesis
in this alternative system. Interestingly, the NOS-
dependent pathway is O
2
dependent; whereas the
nitrate/nitrite-NO pathway is gradually activated
as O
2
tension falls
[88]
(figure 1).
3.1 Nitrate and Nitrite: Sources and
Metabolism
The main providers of nitrate in the diet of
humans are vegetables such as lettuce, spinach or
beetroot.
[89]
Drinking water can also contain
considerable amounts of nitrate. It has been esti-
mated that nitrate consumption derived from food
and beverages is on average 100150 mgday
-1
in adults.
[90]
However, the amount of nitrate in
food has been regulated for a long time and there
is currently an acceptable daily intake (ADI)
for humans of 5 mg sodium nitrate or 3.7 mg ni-
tratekg
-1
of body weight, which equals 222 mg
for a 60 kg adult. This is due to the fact that nitrate
has been considered a carcinogenic substance and a
toxic residue in our food and water. The supposed
carcinogenic mechanism is the nitritedependent
formation of nitrosating agents, which can react
with dietary amines, forming nitrosamines, sub-
stances with known carcinogenic properties.
[91]
However, despite extensive research, no causal link
between dietary nitrate intake and gastric cancer in
humans has been found.
[92]
Apart from the diet, nitrate and nitrite is gen-
erated endogenously in our bodies. The NO
generated by L-arginine and NOS enzymes is
oxidized in the blood and tissues to form nitrate
and nitrite.
[4]
Thus, the NOS-dependent pathway
significantly contributes to the overall nitrate and
nitrite production, which indicates an active re-
cycling pathway for generating NO in the human
body. The normal plasma level of nitrate is within
the 2040 mM range, while the nitrite level is sub-
stantially lower (501000 nM), although many
factors such as training and diet can modify these
levels.
[93]
Nitrate circulating in plasma distributes to
the tissues and has a half life of approximately
5 hours. By not yet fully defined mechanisms,
circulating nitrate is actively taken up by the sal-
ivary glands and concentrated in the saliva (10- to
20-fold higher than in the blood).
[94]
In the oral
cavity, facultative anaerobic bacteria on the sur-
face of the tongue reduces nitrate to nitrite by the
action of nitrate reductase enzymes.
[95]
In the
absence of O
2
, these bacteria use nitrate as an
alternative electron acceptor to gain ATP. When
10 Besco
´s et al.
ª2012 Adis Data Information BV. All rights reserved. Sports Med 2012; 42 (3)
AUTHOR PROOF
swallowed, one part of nitrite in the saliva is me-
tabolized to NO locally in the acidic environment
of the stomach, but the other part of swallowed
nitrite is absorbed intact to increase circulating
plasma nitrite.
[93]
Such nitrite can be converted to
NO and other bioactive nitrogen oxides in the
blood and tissues under appropriate physiologi-
cal conditions.
[3]
These findings demonstrate that
a complete reverse pathway (nitratenitriteNO)
exists in mammals.
3.1.1 Ergogenic Effect of Sodium Nitrate
Supplementation
This alternative pathway of NO generation
has not gone unnoticed in exercise physiology.
Currently, four studies have assessed the effect of
dietary nitrate supplementation in the form of
sodium nitrate.
[26,62-64]
The first was carried out
by Larsen et al.,
[62]
which showed that the inges-
tion of sodium nitrate (0.1 mmolkg
-1
·3 days)
reduced .
VO
2
(~160 mLmin
-1
) during work
rates at mean intensities of 4080%of .
VO
2peak
performed on a cycle ergometer. Gross efficiency,
defined as the ratio of mechanical work output to
the metabolic energy input, was also significantly
improved (~0.4%). This highly surprising effect
occurred without changes in other cardiorespi-
ratory parameters (ventilation, carbon dioxide pro-
duction, heart rate and respiratory exchange ratio)
or lactate concentration, which suggests that energy
production became more efficient after dietary ni-
trate consumption. Interestingly, in the second
study by Larsen et al.,
[63]
it was reported that .
VO
2
at maximal intensity of exercise ( .
VO
2peak
) was also
significantly reduced (~100 mLmin
-1
) after ni-
trate supplementation (0.1 mmolkg
-1
·2days).
Despite this decrease in .
VO
2peak
, exercise perfor-
mance measured until time to exhaustion during an
incremental exercise test did not decrease compared
with placebo (nitrate: 564 30 seconds; placebo:
524 31 seconds; p >0.05). To explain this physio-
logical response during endurance exercise, it was
suggested that nitrate and nitrite modulated mi-
tochondrial respiration via NO synthesis, since
both studies showed a significant increase in plasma
NO metabolites (nitrate and nitrite) after nitrate
treatment.
[62,63]
This hypothesis was investigated by
the same research group in an interesting recent
study.
[64]
They reported that human mitochondrial
efficiency, measured in vitro astheamountofO
2
consumed per ATP produced, termed P/Oratio,
was significantly improved after sodium nitrate
ingestion, compared with placebo using a similar
amount of supplementation as in previous studies
(0.1 mmolkg
-1
·3days).
[64]
Nevertheless, despite
these interesting findings, these studies did not
report an enhancement in specific parameters of
sports performance such as power output, time to
exhaustion or total work performed.
While all the above studies assessed moder-
ately trained subjects, one recent study by an-
other independent research group assessed the
effect of nitrate supplementation in well trained
endurance athletes.
[26]
Following acute supple-
mentation of sodium nitrate (10 mgkg
-1
of body
mass) 3 hours before exercise, 11 trained cyclists and
triathletes completed a cycle ergometer test per-
forming four intermittent workloads at submaximal
intensities (between 2 and 3.5 Wkg
-1
of body mass)
and one continuous incremental test until volitional
exhaustion.
[26]
Interestingly, this study showed that
plasma nitrate levels increased at the same level after
only one dose of nitrate ingestion (10 mgkg
-1
of
body mass) compared with 2-days’ supplementation
(8.5 mgkg
-1
of body massday
-1
).
[63]
Further-
more, results of exercise tests showed that, contrary
to previous studies,
[62,63]
.
VO
2
at low-to-moderate
intensities and increase in gross efficiency were not
improved. However, in agreement with Larsen
et al.,
[62,63]
at maximal intensity of exercise, the
.
VO
2peak
was significantly reduced (~180 mLmin
-1
)
without a decrease in time to exhaustion after nitrate
supplementation.
3.1.2 Ergogenic Effect of Nitrate Supplementation
in the Form of Beetroot Juice
Five studies by the same research group have
used dietary nitrate supplementation in the form of
beetroot juice to assess the effect on human perfor-
mance.
[19-23]
Interestingly, to isolate the effects of
dietary nitrate from the other potentially active in-
gredients found in beetroot juice (betaine, quercetin
and resveratrol), a process was developed to selec-
tively remove the nitrate from beetroot juice using a
commercially available resin.
[22]
In the first of these
studies, Bailey et al.
[19]
showed a significant en-
Nitric Oxide Supplements and Performance 11
ª2012 Adis Data Information BV. All rights reserved. Sports Med 2012; 42 (3)
AUTHOR PROOF
hancement of .
VO
2
kinetics after supplementation
with 500 mLday
-1
of nitrate-rich beetroot juice
(468 mg of sodium nitrateday
-1
)for6days.
During low- to moderate-intensity exercise (cycle
ergometer exercise), there was a 19%reduction in
the amplitude of the pulmonary response. In addi-
tion, it was shown that the .
VO
2
-slow component
was reduced (~23%) and the time-to-exhaustion
during an incremental cycle ergometer test was ex-
tended (~14%) after beetroot juice supplementation
compared with placebo.
[19]
In an attempt to extend
these findings to other forms of exercise (walking
and running on a treadmill), the same research
group performed another study using the same
protocol of beetroot juice ingestion (500 mLday
-1
equivalent to 527 mg of sodium nitrate ·6 days).
[22]
This study concluded that beetroot supplementa-
tion induced similar changes in respiratory response
in a treadmill exercise compared with the previous
data in a cycle ergometer test.
[19]
However, the effects of beetroot juice derived
from nitrate seem to be fast, and acute ingestion
of food rich in nitrate can affect the cardio-
vascular response in a few hours.
[96]
This fact was
analysed in the study of Vanhatalo et al.
[21]
In
this research, subjects ingested only one dose of
beetroot juice (500 mL equivalent to 434 mg
of sodium nitrate) 2.5 hours before a cycle erg-
ometer test that included two moderate work-
loads at 90%of the gas exchange threshold
followed by a ramp test. Moreover, subjects per-
formed the same test after 5 and 15 days of beet-
root juice ingestion and placebo. The steady-state
.
VO
2
during moderate-intensity exercise was sig-
nificantly reduced 2.5 hours after supplementa-
tion and remained low on day 5 and 15 compared
with placebo. However, contrary to the previous
studies, the gas exchange threshold, maximal
.
VO
2
(.
VO
2max
) and peak power output were not
affected 2.5 hours post-ingestion or after 5 days
of supplementation. Surprisingly, these parame-
ters showed a significant increase (peak power:
~3%;.
VO
2max
:~4%) after 15 days of beetroot
juice ingestion. However, several factors such as
training or resting conditions, as well as diet
(subjects did not follow a nitrate-restricted diet at
any time during the study period) could be the
reason for these changes after 15 days of beetroot
juice ingestion. Furthermore, in another very re-
cent study by Lansley et al.
[23]
following the same
protocol of supplementation (500 mL of beetroot
juice equivalent to 527 mg of sodium nitrate
2.5 hours before exercise), a significant improve-
ment of average power output (5%) and mean
completion time (2.8%) was indicated during
4 and 16.1 km cycle ergometer time trials com-
pared with placebo.
To explain all these findings derived from
beetroot juice ingestion, Bailey et al.
[20]
suggested
that the nitrate content of beetroot juice could
play an important role in the reduction of ATP
turnover in contracting myocytes. With the utili-
zation of
31
P-MRS, these authors reported that
the decrease of O
2
cost at moderate and high in-
tensities after beetroot ingestion (500 mLday
-1
equivalent to 468 mg of sodium nitrate ·6 days)
was accompanied by a reduction in muscle
phosphocreatine of a similar magnitude.
[20]
From
this viewpoint, one of the most costly energy
processes during skeletal muscle contraction is
sarcoplasmic reticulum calcium pumping, which
may account for up to 50%of the total ATP
turnover.
[97]
There is evidence that small eleva-
tions of NO improves muscle metabolism, pre-
venting excess calcium release and subsequently
modulates the ATP cost of force production.
[98]
Interestingly, in beetroot juice studies, plasma
nitrite levels measured as NO markers showed a
significant increase after beetroot juice ingestion.
Therefore, this is another alternative metabolic
pathway to mitochondrial respiration indicated
by Larsen et al.,
[64]
which may explain the re-
duction of O
2
demands during exercise derived
from ingestion of food rich in nitrate.
Nevertheless, in all studies involving beetroot
juice supplementation, moderately trained but not
well trained subjects participated. In only one stu-
dy that evaluated nitrate supplementation (sodium
nitrate) in well trained endurance athletes, no re-
duction of O
2
consumption was found, or gross
efficiency at low-to-moderate intensities of ex-
ercise either.
[26]
This study concluded that at low-
to-moderate intensities of exercise, dietary nitrate
supplementation could have a low effect in well
trained endurance athletes compared with moder-
ately trained subjects. Further research is needed in
12 Besco
´s et al.
ª2012 Adis Data Information BV. All rights reserved. Sports Med 2012; 42 (3)
AUTHOR PROOF
highly trained athletes to assess the effect of so-
dium nitrate or beetroot juice supplementation on
performance.
In conclusion, in the field of exercise physiol-
ogy, studies indicate that nitrate supplementation
(i) could be effective at enhancing exercise effi-
ciency and tolerance to exercise in untrained or
moderately-trained subjects; and (ii) have shown
that there is a lack of data assessing the effect of
nitrate supplementation in the form of sodium
nitrate as well as beetroot juice in well trained
athletes (tables I and II). Results derived from the
only study that assessed well trained endurance
athletes, concluded that sodium nitrate does not
enhance efficiency at low to moderate intensities
of exercise.
4. Other Components Related to
NO Synthesis
4.1 Glycine Propionyl-L-Carnitine (GPLC)
Glycine propionyl-L-carnitine (GPLC) is a
new United States Pharmacopeial Convention-
grade dietary supplement that consists of a
molecular bonded form of propionyl-L-carnitine
and one of the carnitine precursor amino acids,
glycine. This molecule has also been proposed to
improve NO metabolism
[65]
via two mechanisms:
first, it has been reported in animal studies that the
protective action of GPLC is derived from its an-
tioxidant action, which may prevent vessels from
peroxidative damage.
[99]
In accordance with this
fact, other authors have suggested that the lower
release of reactive oxygen species could be linked
with a decrease in NO breakdown.
[100]
Second,
eNOS gene expression has been demonstrated to
increase within cultured human endothelial cells
following carnitine incubation.
[101]
Thus, it has also
been hypothesized that GPLC could stimulate NO
synthesis via eNOS expression.
[102]
Separating the components of GPLC, glycine
is considered a glucogenic amino acid, in that it
helps to regulate blood glucose levels, and is also
important in the formation of creatine. Interest-
ingly, glycine has been shown to have its own
independent vasodilatory effects in rats.
[103]
On
the other, hand L-carnitine in combination with
propionyl (propionyl-L-carnitine) is a pharma-
ceutical agent that has been examined primarily
as a treatment in clinical populations with ap-
parent muscle carnitine deficiencies.
[104,105]
4.1.1 Ergogenic Effect of GPLC
Recent studies assessed the effect of GPLC as
a NO donor in sport exercise with different con-
clusions. First, Bloomer et al.
[65]
showed an in-
crease in plasma NO metabolites (nitrate/nitrite)
in active males after GPLC supplementation
(4.5 gday
-1
·4 weeks). These findings were
confirmed in the second study by the same re-
search group.
[66]
However, contrary results were
found in the third study published by Bloomer et
al.
[11]
They showed that an acute dose (4.5 g) of
GPLC did not increase NO markers. This con-
troversy was attributed to the fact that in the latter
study,
[11]
a single dose of GPLC was provided prior
to exercise, whereas in the first two studies, GPLC
was administered for 4 and 8 weeks, respective-
ly.
[65,66]
Nevertheless, an important limitation of
the studies performedby Bloomer et al.
[11,65,66]
was
the lack of evidence indicating some benefit of
GPLC in exercise performance. Two recent studies
assessed this issue showing different results. Smith
et al.
[67]
showed that ingestion of 3 gday
-1
of
GPLC for 8 weeks did not enhance peak power,
mean power or total work during a 30-second
Wingate test. In contrast, Jacobs et al.
[54]
indicated
that only one dose of GPLC (4.5 g) 90 minutes be-
fore performing a test consisting of five 10-second
Wingate cycle sprints separated by 1-minute of ac-
tive recovery periods, significantly improved peak
power (~5.2%) and reduced power decrement
(~5.2%) through sprints, compared with placebo.
In addition, lactate measures taken 14-minutes
post-exercisewere16.2%(p <0.05) lower with
GPLC.
[54]
However, these findings were not linked
to NO delivery.
In summary, current scientific evidence of
GPLC supplementation indicates that (i) in
healthy and moderately trained subjects, GPLC
could induce a mild increase in plasma NO meta-
bolites, although the mechanism behind this re-
sponse has not been defined; and (ii) evidence
seems to indicate that the ergogenic effect of GPLC
could be very limited; only one study indicated
Nitric Oxide Supplements and Performance 13
ª2012 Adis Data Information BV. All rights reserved. Sports Med 2012; 42 (3)
AUTHOR PROOF
benefits in exercise performance after GPLC sup-
plementation.
[54]
However, this finding could not
be related to NO production as a result of the ab-
sence of analysis of NO markers
4.2 2-(Nitrooxy) Ethyl 2-Amino-3-
Methylbutanoate
Recently, a new molecule 2-(nitrooxy) ethyl
2-amino-3-methylbutanoate has been claimed to
increase NO delivery in the body, with this being
more efficient and effective than the traditional
NO donors.
[68]
However, to the best of our
knowledge, there is only one study that has as-
sessed the acute effect of 2-(nitrooxy) ethyl
2-amino-3-methylbutanoate in plasma NO mar-
kers, measured as nitrate/nitrite in moderately
trained resistance males.
[68]
This study concluded
there was no effect on circulating nitrate and ni-
trite within 1-hour post-ingestion of two tablets
(no data was given on dose).
5. Side Effects of NO Supplements
Dietary supplementation with L-arginine and
L-citrulline is not lacking in side effects, with
gastrointestinal disturbances such as nausea, vom-
iting or diarrhoea as the most common adverse
effects.
[106]
However, there is great inter-individual
variation in the tolerance of these amino acids;
high doses (>9g
day
-1
) can increase the risk of
gastrointestinal distress. It has been suggested that
smaller, divided doses might lead to fewer side ef-
fects.
[34]
In addition, the pre-existing proabsorp-
tive or prosecretory state of the intestine may be
important. The combination of secretory state
and the extra stimulus provided by an acute dose
(>9g
day
-1
)ofL-arginineand/or L-citrulline
may overwhelm the reserve-absorption capacity
of the colon.
[106]
As has been indicated previously, the amount of
inorganic nitrate in food and water has been
strictly regulated because of their proposed role in
the development of malignancies such as meta-
haemoglobinemia and cancer;
[107]
however, this
view is currently changing. It is now thought that
the nitrate concentrations commonly encountered
in food and water are unlikely to cause metahae-
moglobinemia.
[108,109]
Moreover, an effect of exog-
enous nitrite on cancer seems less likely, because
large amounts of nitrite are formed endogenously.
Fasting saliva contains »2mg
L
-1
, and after
consumption of an amount of nitrate equivalent to
200 g of spinach, nitrite concentration in saliva
mayrisetoasmuchas72mg
L
-1
.Thisismuch
higher than the ADI of 4.2mg of nitriteday
-1
.
Interestingly, all the above studies assessing nitrate
supplementation in sports performance used
amounts that were possible to achieve with natural
foods, such as beetroot juice or green leafy vege-
tables (lettuce, spinach).
Studies have not reported adverse effects related
with GPLC and 2-(nitrooxy) ethyl 2-amino-3-
methylbutanoate supplementation. Nevertheless,
this lack of data does not mean that this supple-
ment is completely safe. For instance, it is known
that amounts larger than 2 gday
-1
of L-carnitine
may induce slight gastrointestinal distress.
[110,111]
6. Conclusion and Future Perspectives
The current available data indicate that L-
arginine supplementation, mainly in combination
with other components, may be effective in mod-
erately trained or untrained subjects for enhancing
cardiorespiratory adaptation and tolerance to en-
durance exercise,
[24,25,45]
although a relationship
between these findings and NO synthesis has not
been established. On the other hand, L-citrulline in
combination with malate could be a more efficient
way to elevate extracellular levels of L-arginine by
itself and with plasma NO markers. However, de-
spite these effects, there is a lack of data indicating
an improvement in exercise performance after
L-citrulline supplementation. Therefore, it seems
that some of the benefits shown after L-arginine and
L-citrulline supplementation could be derived from
the other ingredients included in supplements, as
well as other metabolic pathways, independently of
NO synthesis, which these amino acids participate
in. Alternatively, a NOS-independent pathway has
been reported by nitrate and nitrite oxidation. There
is evidence that nitrate supplementation reduces the
O
2
cost of endurance exercise, increasing the effi-
ciency of energy production. An explanation for this
intriguing physiological response has been linked
14 Besco
´s et al.
ª2012 Adis Data Information BV. All rights reserved. Sports Med 2012; 42 (3)
AUTHOR PROOF
with an increase in mitochondrial efficiency,
[64]
as
well as ATP turnover functions.
[20]
While some of the benefits linked with NO
donors have been shown in moderately trained
subjects, in well trained athletes, scientific data
show that the effect of these supplements is low.
Thus, it seems that the training status is an im-
portant factor linked with the effectiveness of
dietary NO donors. One reason to explain this
fact may be attributable, in part, to the positive
effect of exercise in the regulation of NO metab-
olism. While short-term training rapidly in-
creases NO bioactivity, if training is maintained,
the short-term functional adaptation is succeeded
by NO-dependent structural changes, leading to
arterial remodelling and structural normalization
of shear.
[112]
This structural remodelling and
consequent normalization of shear obviates the
need for ongoing functional dilatation, including
enhanced NO dilator system function. The con-
clusion that we can extract is that training
performed by competitive athletes has a greater
effect on improving the NO system compared
with NO supplementation. This fact raises the
intriguing possibility of a threshold effect vol-
ume and intensity for training and mechanisms
associated with NO production. Further in-
vestigation is needed to elucidate where this limit
of physical exercise lies.
Moreover, almost all studies analysing NO do-
nors and exercise performance were carried out,
mainly, in young male subjects. It is known that
vascular function and NO availability is impaired
with age; thus, other studies should be planned in
order to assess the effect of NO supplements in
healthy adults (>40 years). Additionally, almost all
research has been focused on endurance perfor-
mance and few data exist concerning the effect of
NO supplements in the regulation of hypertrophy
and stimulation of satellite cells. This point needs
more attention, not only for sports performance,
but also for muscle mass losses associated with age
and convalescence periods after injuries. Finally,
gender differences have not been analysed, al-
though it is recognized that there are structural/
morphological differences between adult males
and females for many, if not all, organ systems,
which may have a significant impact on physiolog-
ical function.
[113]
The female reproductive system is
highly sensitive to physiological stress, and re-
productive abnormalities including delayed men-
arche, primary and secondary amenorrhoea and
oligomenorrhoea occur in 679%of women en-
gaged in athletic activity.
[114]
Theprevalenceof
observed irregularities varies with athletic dis-
cipline and level of competition.
[115]
It has also
been indicated that amenorrhoea is associated with
altered endothelial function.
[116]
We consider this
point needs further research to analyse the poten-
tial effects of NO supplementation and physical
exercise, specifically in females.
Acknowledgements
This paper has been prepared with funding DPS2008-
07033-C03-03 from the Spanish Government and FEDER.
No present or past conflicts of interest exist for any of the
authors or their institutions.
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AUTHOR PROOF
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Correspondence: Dr Rau
´l Besco
´s, National Institute of
Physical Education (INEFC), Physiology laboratory, Av., de
l’Estadi s/n 08038 Barcelona, Spain.
E-mail: raul.bescos@inefc.net; raulbescos@gmail.com
Nitric Oxide Supplements and Performance 19
ª2012 Adis Data Information BV. All rights reserved. Sports Med 2012; 42 (3)
... These include health-enhancing effects such as the regulation of blood flow and blood pressure, the maintenance of gastric integrity, and protection against ischemic tissue damage [17][18][19]. They have also been found to be related to improved performance in different types of disciplines and through different supplements, improving vasodilation and angiogenesis, causing an increase in mitochondrial respiration and biogenesis, positively impacting glucose uptake, optimizing oxygen regulation, and improving muscle contraction [20][21][22][23][24][25][26]. ...
... For this reason, Arg and Cit supplementation has been used in athletes from different disciplines and conditions to improve athletic performance [20,[33][34][35][36][37]. A recent systematic review and meta-analysis [24] showed a positive effect of Arg supplementation in different sports disciplines, depending on the energy metabolism used. ...
... Therefore, the training, nutrition, and supplementation methods used are of particular importance for athletes in these disciplines. They are therefore of great interest considering the potential beneficial effects that the intake of supplements that exogenously and endogenously stimulate NO synthesis have in relation to the demands required by long-duration exercise [20]. The increased synthesis of this bioactive compound could ultimately result in multiple beneficial effects [17,20,[38][39][40]. ...
Article
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Supplementation with Citrulline (Cit) has been shown to have a positive impact on aerobic exercise performance and related outcomes such as lactate, oxygen uptake (VO2) kinetics, and the rate of perceived exertion (RPE), probably due to its relationship to endogenous nitric oxide production. However, current research has shown this to be controversial. The main objective of this systematic review and meta-analysis was to analyze and assess the effects of Cit supplementation on aerobic exercise performance and related outcomes, as well as to show the most suitable doses and timing of ingestion. A structured literature search was carried out by the PRISMA® (Preferred Reporting Items for Systematic Reviews and Meta-Analyses) and PICOS guidelines in the following databases: Pubmed/Medline, Scopus, and Web of Science (WOS). A total of 10 studies were included in the analysis, all of which exclusively compared the effects of Cit supplementation with those of a placebo group on aerobic performance, lactate, VO2, and the RPE. Those articles that used other supplements and measured other outcomes were excluded. The meta-analysis was carried out using Hedges’ g random effects model and pooled standardized mean differences (SMD). The results showed no positive effects of Cit supplementation on aerobic performance (pooled SMD = 0.15; 95% CI (−0.02 to 0.32); I2, 0%; p = 0.08), the RPE (pooled SMD = −0.03; 95% CI (−0.43 to 0.38); I2, 49%; p = 0.9), VO2 kinetics (pooled SMD = 0.01; 95% CI (−0.16 to 0.17); I2, 0%; p = 0.94), and lactate (pooled SMD = 0.25; 95% CI (−0.10 to 0.59); I2, 0%; p = 0.16). In conclusion, Cit supplementation did not prove to have any benefits for aerobic exercise performance and related outcomes. Where chronic protocols seemed to show a positive tendency, more studies in the field are needed to better understand the effects.
... Previous studies have shown that L-arginine supplementation improves respiratory function and exercise tolerance in patients with pulmonary diseases [22] and in those with congestive heart failure [23] as well as in heart transplant recipients [24]. In addition, supplementation with L-arginine may increase aerobic and anaerobic performance in healthy adults, especially in untrained individuals [25,26]. However, other studies found no effects of L-arginine supplementation on human performance [27,28]. ...
... Based on these findings, the authors concluded that L-arginine supplementation with 1.5−2 g daily from four to seven weeks and 10−12 g daily for eight weeks could be recommended to improve aerobic and anaerobic performance, respectively [25]. Interestingly, untrained or moderately trained individuals seem to obtain greater gains in exercise performance after L-arginine supplementation than those who are highly trained [26]. While no conclusive evidence exists on the beneficial effects of L-arginine supplementation on human performance, our findings indicate that a short course of L-arginine plus vitamin C supplementation may positively impact the exercise capacity of adults with long COVID. ...
Article
Full-text available
Long COVID, a condition characterized by symptom and/or sign persistence following an acute COVID-19 episode, is associated with reduced physical performance and endothelial dysfunction. Supplementation of l-arginine may improve endothelial and muscle function by stimulating nitric oxide synthesis. A single-blind randomized, placebo-controlled trial was conducted in adults aged between 20 and 60 years with persistent fatigue attending a post-acute COVID-19 outpatient clinic. Participants were randomized 1:1 to receive twice-daily orally either a combination of 1.66 g l-arginine plus 500 mg liposomal vitamin C or a placebo for 28 days. The primary outcome was the distance walked on the 6 min walk test. Secondary outcomes were handgrip strength, flow-mediated dilation, and fatigue persistence. Fifty participants were randomized to receive either l-arginine plus vitamin C or a placebo. Forty-six participants (median (interquartile range) age 51 (14), 30 [65%] women), 23 per group, received the intervention to which they were allocated and completed the study. At 28 days, l-arginine plus vitamin C increased the 6 min walk distance (+30 (40.5) m; placebo: +0 (75) m, p = 0.001) and induced a greater improvement in handgrip strength (+3.4 (7.5) kg) compared with the placebo (+1 (6.6) kg, p = 0.03). The flow-mediated dilation was greater in the active group than in the placebo (14.3% (7.3) vs. 9.4% (5.8), p = 0.03). At 28 days, fatigue was reported by two participants in the active group (8.7%) and 21 in the placebo group (80.1%; p <0.0001). l-arginine plus vitamin C supplementation improved walking performance, muscle strength, endothelial function, and fatigue in adults with long COVID. This supplement may, therefore, be considered to restore physical performance and relieve persistent symptoms in this patient population.
... 17 respectively).The significant difference was observed in mean anaerobic power between 29 supplemented and placebo group for right and left knee flexors (p=0.002 and p=0.005, 30 respectively) as well as for right and left knee extensors (p=0.001 and p=0.002; 31 respectively).There was also observed that the time to peak torque was significantly greater 32 in supplemented group for right and left knee flexors (p=0.002 for both legs). The significant 33 difference was also observed in mean power between supplemented and placebo group 34 during Wingate test (placebo: 8.49 ± 0.57 W/kg, and supplemented group: 8.66 ± 0. 55 The physiological effect of a training session is dependent upon the quality of the work 45 undertaken, hence athletes constantly search for methods to enhance the training outcome. 46 Consequently, pre-workout formulations are becoming increasingly popular class of dietary 47 supplements among athletes. ...
... It must 354 be noted that in light of the current evidence a single dose of L-citrulline and L-arginine is 355 insufficient to enhance sport performance and supplementation should last at least one week (53, 356 55). Moreover, a review by Bescós et al. (55) indicates a paucity of data linking an increase in 357 exercise performance and intake of NO --related supplements. 358 ...
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Background: The purpose of this study was to investigate the acute effects of commercially available pre-workout supplement on anaerobic performance in resistance trained men. Methods: Twenty-three men underwent three testing sessions administrated in a randomized and double-blind fashion separated by a seven-day break. The participants performed three exercise tests: isokinetic strength test, maximal strength test and Wingate test. Statistical analysis was conducted in R environment. Linear mixed models were estimated via R package lme4. Results: The mean knee peak torque was significantly greater in supplemented group for right and left knee flexors (placebo: 103.17 ± 37.61 Nm, and supplemented group: 131.84 ± 29.31 Nm where p=0.001, and placebo: 103.72 ± 39.35, and supplemented group: 129.38 ± 28.44, where p=0.001; respectively) as well as for right and left knee extensors (placebo: 202.65 ± 58.64 Nm, and supplemented group: 237.22 ± 54.75 Nm where p=0.001, and placebo: 203.27 ± 63.2 Nm versus supplemented group: 229.84 ± 50.8 Nm where p=0.002; respectively).The significant difference was observed in mean anaerobic power between supplemented and placebo group for right and left knee flexors (p=0.002 and p=0.005, respectively) as well as for right and left knee extensors (p=0.001 and p=0.002; respectively).There was also observed that the time to peak torque was significantly greater in supplemented group for right and left knee flexors (p=0.002 for both legs). The significant difference was also observed in mean power between supplemented and placebo group during Wingate test (placebo: 8.49 ± 0.57 W/kg, and supplemented group: 8.66 ± 0.55 W/kg where p=0.038). Moreover the mean 3-RM strength test was significantly greater in supplemented group with p=0.001. Conclusions: The results of the study indicate that the supplement significantly improves upper and lower body strength and power output in resistance trained men.
... Additionally, it should also be considered that ergogenic response of citrulline or arginine supplementation may have depended on basic fitness level of the subjects. Studies in untrained or moderately trained healthy subjects have shown that NO donors can improve tolerance of aerobic and anaerobic exercise [12]. However, when highly skilled subjects were examined, no positive impact on performance was observed [13]. ...
Article
Full-text available
Purpose: The purpose of this study was to evaluate the effect of a single intake of citrulline at 3 g and 6 g doses in adult elite soccer players performing sport-specific exercise. Materials and Methods: This randomized double-blind placebo-controlled study analyzed 18 soccer players from the top divisions of three European countries. Participants were randomized into three groups of six each and performed a field-based soccer-specific test for 18 min. Comparative analysis of heart rate, fatigue and post-exercise recovery was conducted. Results: There were no statistically significant differences in most of the analyzed parameters, nor at any of the time points for lactate concentration. Players' RPE exercise test score did not reveal any differences. Conclusions: Neither a single intake of 3 g nor of 6 g of citrulline malate affected physical performance, subjective feelings of fatigue or post-exercise recovery in adult elite soccer players who performed a soccer-specific test.
... Activation of TRPV1 may also stimulate nitric oxide synthesis, a potent vasodilator [8]. Vasodilatation may enhance exercise performance by increasing blood flow, nutrient and oxygen delivery to the working muscle, and the clearance of metabolic by-products [9]. ...
Article
Full-text available
Several studies have explored the effects of capsaicin and capsiate on endurance performance, with conflicting findings. This systematic review aimed to perform a meta-analysis examining the effects of capsaicin and capsiate vs. placebo on endurance performance in humans. Seven databases were searched to find eligible studies. The effects of capsaicin and capsiate on aerobic endurance (e.g., time-trials or time-to-exhaustion tests), muscular endurance (e.g., repetitions performed to muscular failure), and rating of perceived exertion (RPE) were examined in a random-effects meta-analysis. Fourteen studies (n = 183) were included in the review. Most studies provided capsaicin or capsiate in the dose of 12 mg, 45 minutes before exercise. In the meta-analysis for aerobic endurance, there was no significant difference between the placebo and capsaicin/capsiate conditions (Cohen’s d: 0.04; 95% confidence interval: –0.16, 0.25; p = 0.69). In subgroup meta-analyses, there were no significant differences between the placebo and capsaicin/capsiate conditions when analyzing only studies that used time-trials (p = 0.20) or time-to-exhaustion tests (p = 0.80). In the meta-analysis for muscular endurance, a significant ergogenic effect of capsaicin/capsiate was found (Cohen’s d: 0.27; 95% confidence interval: 0.10, 0.43; p = 0.002). When analyzing set-specific effects, an ergogenic effect of capsaicin/capsiate was found in set 1, set 2, and set 3 (Cohen’s d: 0.21-29). Capsaicin/capsiate ingestion reduced RPE following muscular endurance (p = 0.03) but not aerobic endurance tests (p = 0.58). In summary, capsaicin/capsiate supplementation acutely enhances muscular endurance, while its effects on aerobic endurance are less clear.
... NO has a relevant role as an intracellular second messenger and its production is also related to an increase in blood flow, which improves nutrient and hormone delivery. Furthermore, NO has a positive impact on resistance and endurance training adaptions [108,109]. Recent systematic reviews and meta-analyses about NO synthaseindependent pathway supplementation have shown that sodium nitrate and potassium nitrate are less effective than beetroot juice consumption in endurance exercise. The use of 6-12 mmol of NO 3 − contained in beetroot juice supplements produced significant im-provements in time to exhaustion in a 5-30 min cycling race, but slightly non-significant improvements in time trial or graded-exercise performance [80]. ...
Article
Full-text available
Nutritional ergogenic aids (NEAs) are substances included within the group of sports supplements. Although they are widely consumed by athletes, evidence-based analysis is required to support training outcomes or competitive performance in specific disciplines. Combat sports have a predominant use of anaerobic metabolism as a source of energy, reaching peak exertion or sustained effort for very short periods of time. In this context, the use of certain NEAs could help athletes to improve their performance in those specific combat skills (i.e., the number of attacks, throws and hits; jump height; and grip strength, among others) as well as in general physical aspects (time to exhaustion [TTE], power, fatigue perception, heart rate, use of anaerobic metabolism, etc.). Medline/PubMed, Scopus and EBSCO were searched from their inception to May 2022 for randomised controlled trials (RCTs). Out of 677 articles found, 55 met the predefined inclusion criteria. Among all the studied NEAs, caffeine (5-10 mg/kg) showed strong evidence for its use in combat sports to enhance the use of glycolytic pathways for energy production during high-intensity actions due to a greater production of and tolerance to blood lactate levels. In this regard, abilities including the number of attacks, reaction time, handgrip strength, power and TTE, among others, were improved. Buffering supplements such as sodium bicarbonate, sodium citrate and beta-alanine may have a promising role in high and intermittent exertion during combat, but more studies are needed in grappling combat sports to confirm their efficacy during sustained isometric exertion. Other NEAs, including creatine, beetroot juice or glycerol, need further investigation to strengthen the evidence for performance enhancement in combat sports. Caffeine is the only NEA that has shown strong evidence for performance enhancement in combat sports.
... These supplements play a key role in muscle function, increasing the bioavailability of nutrients and hormones as a result of their vasodilatory action [24,25]. However, the main part of studies with NO inducers have been performed in the field of sport performance [26][27][28][29], but only few studies have addressed the problem of sarcopenia [25,30]. ...
Article
Full-text available
Calcium and magnesium, together with vitamin D and the hormones testosterone and cortisol, are key elements in muscle function, to maintain physical fitness. This study aims to analyze if supplementation with NO precursors (L-arginine, L-citrulline and beetroot extract) modulates the circulating levels of calcium, magnesium, vitamin D and steroid hormones in elders. Sixty-one volunteers (65.1 years old, 164.6 cm of height and 71.2 kg of weight) susceptible to develop sarcopenia participated in a physical activity program for 6 weeks. Participants were divided into four groups: one placebo and three taking one of the indicated supplements. Physical capacity was assessed through the following tests: (a) distance covered in 6 min by walking (endurance indicator); (b) hand grip (upper-body strength indicator); (c) time to cover 4 m by walking (speed indicator); and (d) time to perform five full squats (lower-body strength indicator). We concluded that there is a disparity in the association of steroid hormones, vitamin D levels and physical fitness. However, a significant inverse correlation between speed and endurance indicators was observed. Higher circulating vitamin D levels were observed in the L-arginine- and beetroot-supplemented groups. In conclusion, vasodilators increase vitamin D circulating levels that, in the long term, could maintain mineral homeostasis, improving muscular function.
... NO supplementation or nitric oxide contained in watermelon is known to improve muscle function, resistance to fatigue during exercise, and the recovery process after exercise (Besco et al., 2012). In addition, NO supplementation is considered an ergogenic aid that is an essential modulator of blood flow and mitochondrial respiration during physical exercise, which can enhance the tissue recovery process (Petróczi & Naughton, 2010). ...
Article
Full-text available
Karate is a popular and often played martial sport. Due to the limited recovery period experienced by karate athletes during matches, they must employ an appropriate recovery plan to regain their initial condition. Watermelon research has become popular in recent years for application in sports. This clinical trial used one crossover design. The participants in this study were young karate athletes supported by the West Java student education and training center. Subjects underwent two periods: the first in which the subject would not consume watermelon juice, and the second in which the subject would consume 500ml of watermelon juice every day for seven days. The data collection procedure includes measuring perceived recovery status to evaluate the degree of perceived recovery and anaerobic ability through a running-anaerobic sprint test. After the sample had completed the protocol in the form of a match simulation, measurements were taken. Wilcoxon and Mann-Whitney analysis analyzed data. The results indicated that drinking watermelon juice significantly affected perceived recovery status 48 hours after and 72 hours after the treatment. However, after consuming watermelon juice, there was no significant difference in the athlete's anaerobic abilities. Consumption of watermelon juice can aid athletes in their recovery process, which affects the athlete's reported recovery state. It does not, however, contribute to the recovery of anaerobic capability. Therefore, watermelon juice supplementation can be used to accelerate the athlete's recovery.
... Altogether, dietary supplementation with inducers of NO production, such as Larginine or nitrates, shows positive results on muscle regeneration and repair, improving physical performance [16,[34][35][36]. However, there are few studies devoted to this type of intervention in elders who are at clear risk of sarcopenia. ...
Article
Full-text available
Aging is associated with a significant decline in neuromuscular function, leading to a reduction in muscle mass and strength. The aim of the present report was to evaluate the effect of supplementation with nitric oxide precursors (l-arginine and beetroot extract) in muscular function during a training period of 6 weeks in elderly men and women. The study (double-blind, placebo-controlled) involved 66 subjects randomly divided into three groups: placebo, arginine-supplemented and beetroot extract-supplemented. At the end of this period, no changes in anthropometric parameters were observed. Regarding other circulating parameters, urea levels were significantly (p < 0.05) lower in women of the beetroot-supplemented group (31.6 ± 5.9 mg/dL) compared to placebo (41.3 ± 8.5 mg/dL) after 6 weeks of training. In addition, the circulating creatine kinase activity, as an index of muscle functionality, was significantly (p < 0.05) higher in women of the arginine- (214.1 ± 162.2 mIU/L) compared to the beetroot-supplemented group (84.4 ± 36.8 mIU/L) at the end of intervention. No significant effects were noticed with l-arginine or beetroot extract supplementation regarding strength, endurance and SPPB index. Only beetroot extract supplementation improved physical fitness significantly (p < 0.05) in the sprint exercise in men after 6 weeks (2.33 ± 0.59 s) compared to the baseline (2.72 ± 0.41 s). In conclusion, beetroot seems to be more efficient during short-term training while supplementing, preserving muscle functionality in women (decreased levels of circulating creatine kinase) and with modest effects in men.
Article
Behavioral aspects of organized sports activity for pediatric athletes are considered in a world consumed with winning at all costs. In the first part of this treatise, we deal with a number of themes faced by our children in their sports play. These concepts include the lure of sports, sports attrition, the mental health of pediatric athletes (i.e., effects of stress, anxiety, depression, suicide in athletes, ADHD and stimulants, coping with injuries, drug use, and eating disorders), violence in sports (i.e., concepts of the abused athlete including sexual abuse), dealing with supervisors (i.e., coaches, parents), peers, the talented athlete, early sports specialization and sports clubs. In the second part of this discussion, we cover ergolytic agents consumed by young athletes in attempts to win at all costs. Sports doping agents covered include anabolic steroids (anabolic-androgenic steroids or AAS), androstenedione, dehydroepiandrostenedione (DHEA), human growth hormone (hGH; also its human recombinant homologue: rhGH), clenbuterol, creatine, gamma hydroxybutyrate (GHB), amphetamines, caffeine and ephedrine. Also considered are blood doping that includes erythropoietin (EPO) and concepts of gene doping. In the last section of this discussion, we look at disabled pediatric athletes that include such concepts as athletes with spinal cord injuries (SCIs), myelomeningocele, cerebral palsy, wheelchair athletes, and amputee athletes; also covered are pediatric athletes with visual impairment, deafness, and those with intellectual disability including Down syndrome. In addition, concepts of autonomic dysreflexia, boosting and atlantoaxial instability are emphasized. We conclude that clinicians and society should protect our precious pediatric athletes who face many challenges in their involvement with organized sports in a world obsessed with winning. There is much we can do to help our young athletes find benefit from sports play while avoiding or blunting negative consequences of organized sport activities.
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L-arginine-L-aspartate is widely used by athletes for its potentially ergogenic properties. However, only little information on its real efficacy is available from controlled studies. Therefore, we evaluated the effects of prolonged supplementation with L-arginine-L-aspartate on metabolic and cardiorespiratory responses to submaximal exercise in healthy athletes by a double blind placebo-controlled trial. Sixteen healthy male volunteers (22 ± 3 years) performed incremental cycle spiroergometry up to 150 watts before and after intake of L-arginine-L-aspartate (3 grams per day) or placebo for a period of 3 weeks. After intake of L-arginine-L-aspartate, blood lactate at 150 watts dropped from 2.8 ± 0.8 to 2.0 ± 0.9 mmol·l -l (p < 0.001) and total oxygen consumption during the 3-min period at 150 watts from 6.32 ± 0.51 to 5.95 ± 0.40 l (p = 0.04) compared to placebo (2.7 ± 1.1 to 2.7 ± 1.4 mmol•l-1; p = 0.9 and 6.07 ± 0.51 to 5.91 ± 0.50 l; p = 0.3). Additionally, L-arginine-L-aspartate supplementation effected an increased fat utilisation at 50 watts. L-arginine and L-aspartate seem to have induced synergistic metabolic effects. L-arginine might have reduced lactic acid production by the inhibition of glycolysis and L-aspartate may have favoured fatty acid oxidation. Besides, the results indicate improved work efficiency after L-arginine-L-aspartate intake. The resulting increases of submaximal work capacity and exercise tolerance may have important implications for athletes as well as patients.
Article
Endothelial metabolism of L-arginine to L-citrulline and the potent vasodilator, nitric oxide (NO), is important in the regulation of vascular tone and resting BP. L-arginine improves abnormal endothelium-dependent vasodilation in the setting of hypercholesterolemia and has a vasodilatory effect in normal vessels, effects presumed to be mediated through increased endogenous NO production, although this has not been established by direct measurement of NO. In a randomized, placebo-controlled, crossover trial, 10 healthy male subjects received a 30-min infusion of 0.5 g/kg L-arginine hydrochloride. Subjects underwent continuous monitoring of BP and heart rate (HR) as well as intermittent determination of mixed expired NO concentration and plasma L-arginine and L-citrulline levels. Infusion of L-arginine produced a significant fall in mean BP with a peak effect of −9.3±0.9% (p< 0.005). The hemodynamic effects of L-arginine were associated with an increase in mixed expired NO concentration (FeNO) of 55±15% (p<0.005) from 15±2 to 21±3 parts per billion (ppb) and an increase in the rate of pulmonary NO excretion of 118±45% (p<0.005), as well as a rise in plasma L-citrulline from 25±4 to 46±5 µmol/L (p<0.005). There was a significant correlation between the hypotensive response to L-arginine and the increase in expired NO (r=-0.68, p<0.05). The hypotensive effect of L-arginine in humans appears to be mediated, at least in part, by NO synthase metabolism of L-arginine and increased endogenous NO production as indicated both by increased plasma L-citrulline and by increased expired NO.
Article
Nitric oxide (NO) has been the topic of many studies regarding its therapeutic benefit in patients with cardiac disease. Recent studies are now revealing potential advantages for healthy individuals and endurance athletes. This article discusses current research focused on NO augmentation in relation to muscular strength and endurance. Arginine, an NO precursor, has been more extensively studied as a supplement for performance enhancement. Its role in cardiovascular endurance and strength training is assessed in individuals with various athletic backgrounds. The therapeutic role of NO in the treatment of tendinopathies is also reviewed.
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Nitrate and nitrite have been considered stable inactive end products of nitric oxide (NO). While several recent studies now imply that nitrite can be reduced to bioactive NO again, the more stable anion nitrate is still considered to be biologically inert. Nitrate is concentrated in saliva, where a part of it is reduced to nitrite by bacterial nitrate reductases. We tested if ingestion of inorganic nitrate would affect the salivary and systemic levels of nitrite and S-nitrosothiols, both considered to be circulating storage pools for NO. Levels of nitrate, nitrite, and S-nitrosothiols were measured in plasma, saliva, and urine before and after ingestion of sodium nitrate (10 mg/kg). Nitrate levels increased greatly in saliva, plasma, and urine after the nitrate load. Salivary S-nitrosothiols also increased, but plasma levels remained unchanged. A 4-fold increase in plasma nitrite was observed after nitrate ingestion. If, however, the test persons avoided swallowing after the nitrate load, the increase in plasma nitrite was prevented, thereby illustrating its salivary origin. We show that nitrate is a substrate for systemic generation of nitrite. There are several pathways to further reduce this nitrite to NO. These results challenge the dogma that nitrate is biologically inert and instead suggest that a complete reverse pathway for generation of NO from nitrate exists.
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Citrulline is a non protein amino acid involved in three important metabolic pathways, the intrahepatic transformation of ammonia to urea, the de novo synthesis of arginine from glutamine in gut and kidney, the nitric oxide synthesis. The two first pathways use the same enzyme activities but are regulated in different way. This review describe these pathways and their regulation in different tissues. In the light of our knowledge we tried to explain the physiological and pathological (inherited or acquired) variations in man.
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NITRIC OXIDE-STIMULATING DIETARY SUPPLEMENTS ARE ARGUABLY THE MOST WIDELY ADVERTIZED AND PROMOTED AGENTS IN THE SPORT/BODYBUILDING NUTRITION ARENA TODAY. UNFORTUNATELY, THESE PRODUCTS HAVE LITTLE SCIENTIFIC EVIDENCE FOR EFFECT, DESPITE THE MASSIVE HYPE THAT SURROUNDS THE AGGRESSIVE ADVERTISING CAMPAIGNS. WHILE SOME ANECDOTAL REPORTS SUGGEST A POTENTIAL BENEFIT FROM USING THESE PRODUCTS, ONE CANNOT RULE OUT THE POSSIBILITY OF A “PLACEBO EFFECT.” THE PURPOSE OF THIS REVIEW IS TO PRESENT INFORMATION RELATED TO THE ROLE OF NITRIC OXIDE IN SPORT PERFORMANCE AND TO PROVIDE AN OVERVIEW OF THE SCIENTIFIC RATIONALE FOR THE USE OF NUTRITIONAL SUPPLEMENTS AIMED AT INCREASING NITRIC OXIDE.
Article
Administration of L-arginine by intravenous infusion or via oral absorption has been shown to induce peripheral vasodilation in humans, and to improve endothelium-dependent vasodilation. We investigated the pharmacokinetics and pharmacokinetic-pharmacodynamic relationship of L-arginine after a single intravenous infusion of 30 g or 6 g, or after a single oral application of 6 g, as compared with the respective placebo, in eight healthy male human subjects. L-arginine levels were determined by h.p.l.c. The vasodilator effects of L-arginine were assessed non-invasively by blood pressure monitoring and impedance cardiography. Urinary nitrate and cyclic GMP excretion rates were measured as non-invasive indicators of endogenous NO production. Plasma L-arginine levels increased to (mean +/- s.e.mean) 6223+/-407 (range, 5100-7680) and 822+/-59 (527-955) micromol l(-1) after intravenous infusion of 30 g and 6 g L-arginine, respectively, and to 310+/-152 (118-1219) micromol l(-1) after oral ingestion of 6 g L-arginine. Oral bioavailability of L-arginine was 68+/-9 (51-87)%. Clearance was 544+/-24 (440-620), 894+/-164 (470-1190), and 1018+/-230 (710-2130) ml min(-1), and elimination half-life was calculated as 41.6+/-2.3 (34-55), 59.6+/-9.1 (24-98), and 79.5+/-9.3 (50-121) min, respectively, for 30 g i.v., 6 g i.v., and 6 g p.o. of L-arginine. Blood pressure and total peripheral resistance were significantly decreased after intravenous infusion of 30 g L-arginine by 4.4+/-1.4% and 10.4+/-3.6%, respectively, but were not significantly changed after oral or intravenous administration of 6 g L-arginine. L-arginine (30 g) also significantly increased urinary nitrate and cyclic GMP excretion rates by 97+/-28 and 66+/-20%, respectively. After infusion of 6 g L-arginine, urinary nitrate excretion also significantly increased, (nitrate by 47+/-12% [P<0.05], cyclic GMP by 67+/-47% [P= ns]), although to a lesser and more variable extent than after 30 g of L-arginine. The onset and the duration of the vasodilator effect of L-arginine and its effects on endogenous NO production closely corresponded to the plasma concentration half-life of L-arginine, as indicated by an equilibration half-life of 6+/-2 (3.7-8.4) min between plasma concentration and effect in pharmacokinetic-pharmacodynamic analysis, and the lack of hysteresis in the plasma concentration-versus-effect plot. The vascular effects of L-arginine are closely correlated with its plasma concentrations. These data may provide a basis for the utilization of L-arginine in cardiovascular diseases.