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48
Nutr Hosp 2013;28(Supl. 4):48-56
ISSN (Versión papel): 0212-1611
ISSN (Versión electrónica): 1699-5198
CODEN NUHOEQ
S.V.R. 318
Sugar and physical exercise; the importance of sugar for athletes
Ana B. Peinado, Miguel A. Rojo-Tirado and Pedro J. Benito
Exercise Physiology Laboratory. Department of Health and Human Performance. Faculty of Sciences for Physical Activity
and Sport (INEF). Polytechnic University of Madrid, Madrid, Spain.
Abstract
Muscle glycogen, the predominant form of stored glu-
cose in the body, and blood glucose are the main energy
substrates for muscle contraction during exercise.
Sucrose is an ideal substance for athletes to incorporate
because it provides both glucose and fructose. Therefore,
it is essential that athletes monitor their diet to maintain
and increase muscle glycogen deposits, since they are a
major limiting factor of prolonged exercise performance.
Carbohydrate-rich diets are also recommended for
endurance and ultra-endurance exercise, because they
are associated with increased muscle glycogen stores, as
well as delayed onset of fatigue. In addition, high carbo-
hydrate diets and carbohydrate intake before and during
exercise have shown to be beneficial due to increased con-
centrations of hepatic glycogen and maintenance of blood
glucose. The effect of carbohydrate intake on athletic per-
formance mainly depends on the characteristics of the
exercise, the type and amount of carbohydrate ingested
and the time of intake. A combination of these factors
must be taken into account when analysing individual
athletic performance.
Nutr Hosp 2013; 28 (Supl. 4):48-56
Key words: Carbohydrate intake. Endurance sports.
Strength sports. Performance. Training.
EL AZÚCAR Y EL EJERCICIO FÍSICO;
SU IMPORTANCIA EN LOS DEPORTISTAS
Resumen
El glucógeno muscular, principal almacén de glucosa
en el organismo, y la glucemia sanguínea constituyen
uno de los principales sustratos energéticos para la con-
tracción muscular durante el ejercicio. El azúcar (saca-
rosa) es un estupendo suplemento al suministrar tanto
glucosa como fructosa. Por ello, es esencial que los
deportistas cuiden su alimentación, para mantener y
aumentar los depósitos de este combustible, ya que las
reservas de glucógeno muscular constituyen un factor
limitante de la capacidad para realizar ejercicio prolon-
gado. Las dietas ricas en hidratos de carbono se han
recomendado para el ejercicio de resistencia y ultra-
resistencia debido a su relación con el aumento de las
reservas musculares de glucógeno y la aparición tardía
de la fatiga. Además de las dietas altas en carbohidratos,
la ingesta de carbohidratos antes y durante el ejercicio,
han demostrado ser beneficiosas debido al aumento de
las concentraciones hepáticas de glucógeno y el mante-
nimiento de las concentraciones de glucosa en sangre. El
efecto de la ingesta de carbohidratos sobre el rendi-
miento deportivo dependerá principalmente de las
características del esfuerzo, del tipo y cantidad de car-
bohidratos ingeridos y del momento de la ingesta. La
combinación de todos estos factores debe ser tenida en
cuenta a la hora de analizar el rendimiento en las dife-
rentes especialidades deportivas.
Nutr Hosp 2013; 28 (Supl. 4):48-56
Palabras clave: Ingesta de carbohidratos. Deportes de resis-
tencia. Deportes de fuerza. Rendimiento. Entrenamiento.
Abbreviations
FFA: Plasma free fatty acids
ATP: Adenosine triphosphate.
HR: Heart rate Max.
HR: Maximum heart rate.
O2: Oxygen.
TG: Triglycerides.
VCO2: Carbon dioxide production.
VO2: Oxygen consumption.
VO2max: Maximum oxygen consumption.
Corresponding author: Pedro J. Benito Peinado.
Department of Health and Human Performance.
Faculty of Sciences for Physical Activity and Sport - INEF.
Polytechnic University of Madrid
C/ Martín Fierro, 7.
28040 Madrid. Spain.
E-mail: pedroj.benito@upm.es
05. Azucar y ejercicio_Nutr Hosp 25/06/14 12:28 Página 48
Energy metabolism of carbohydrates and their
importance in different types of exercise
The sugar (sucrose) we consume in our diet is an
important source of glucose for the body, as it is a
disaccharide formed from one molecule of glucose and
one of fructose. However, by extension, all carbohy-
drates are included under the term sugar. Among the
different kinds of carbohydrate that are consumed
monosaccharides (glucose, fructose and galactose) and
disaccharides (maltose, sucrose and lactose) and
glucose polymers (maltodetrin and starch) stand out.
Their differences in osmolarity and structure have an
impact on palatability, digestion, absorption, hormone
release and the availability of glucose to be oxidised in
the muscles1. All the metabolic pathways of carbohy-
drates are reduced by the breakdown (catabolic
pathways) of glucose (glycolysis) or glycogen
(glucogenolysis), or the formation (anabolic path-
ways) of glucose (glycogenesis) or glycogen
(glycogenosynthesis). Glucose is the only carbohy-
drate that circulates around the body and whose
concentration can be measured in the blood (blood
glucose). So all carbohydrates that are consumed in the
diet are converted to glucose.
Muscle glycogen, the main form of stored glucose in
the body, and blood glucose are the main energy
substrates for muscle contraction during exercise, and
they become progressively more important as exercise
intensity increases. They are the most important
substrates, as a quick source of energy for the body
because their oxidation produces 6.3 moles of ATP
(Adenosine triphosphate) per mole of oxygen (O2)
compared with 5.6 moles obtained from oxidising fats.
One of the factors that may determine muscle fatigue is
the depletion of carbohydrate reserves.
During low intensity aerobic activity (~30% of
maximal oxygen consumption[VO2max]) the total
energy produced comes from 10-15% of carbohydrate
oxidation. With an increase in intensity this percentage
increases, and could reach 70-80% when VO2max is
~85%, or even 100% during maximal or supra-
maximal intensity activities2. Besides exercise inten-
sity, their use during exercise is influenced by various
factors such as its duration (Fig. 1), the level of phys-
ical fitness, diet, gender, environmental conditions, etc.
As most sports are carried out at intensities that are
above 60-70% of VO2max, carbohydrates from muscle
glycogen and blood glucose are the main source of
energy.
The role that carbohydrates play in energy metabo-
lism during exercise highlights the importance of
analysing adequate sugar intake for sports perfor-
mance. The availability of carbohydrates during exer-
cise, as well as the subsequent recovery of muscle
glycogen deposits, plays a pivotal role in the perfor-
mance of different sports. Skeletal muscle has a higher
concentration of glycogen and is the tissue which has
the largest deposits, as the liver (another glycogen
store) only stores an eighth of the amount that muscle
stores. For example, a 70 kg person with a muscle mass
percentage of 45%, has 315 g of glycogen stored in
their muscles. while there is about 80 g in the liver. The
liver contains the glucose-6-phosphate enzyme which
enables the dephosphorylation of glucose-6-phosphate
and therefore supplies the rest of the organs and tissues
with glucose. Liver function is vital during exercise to
maintain blood sugar and supply the brain with
glucose. For their part, muscles are able to use
glycogen deposits independently. Therefore it’s essen-
tial that athletes watch their diet so that they can main-
tain and increase these fuel deposits because muscle
glycogen reserves are limiting factors in the capacity to
perform prolonged exercise3.
49
Fig. 1.—The effects of exercise intensity and duration in the use
of metabolic substrates. The percentage of energy supplied by
glucose, plasma free fatty acids (Plasma FFA), glycogen and
muscular triglycerides (Muscular TG), during exercise from 90
to 120 min (A) and from 0 to 30 min (B) at intensities of 25, 65
and 85% of maximal oxygen consumption (VO2max) (Adapted
from13).
A
B
% of energy provided during exercise
of 90 to 120 min in duration
Intensity (% VO2max)
100%
90%
80%
70%
60%
50%
40%
30%
20%
10%
0%
% of energy provided during exercise
of 0 to 30 min in duration
100%
90%
80%
70%
60%
50%
40%
30%
20%
10%
0%
25 65 85
Intensity (% VO2max)
25 65 85
Muscular TG
Plasma FFA
Glycogen
Glucose
Muscular TG
Plasma FFA
Glucose
Muscular TG
Plasma FFA
Glucose
Glycogen
Muscular TG
Plasma FFA
Glycogen
Muscular TG
Plasma FFA
Glucose Glucose
05. Azucar y ejercicio_Nutr Hosp 25/06/14 12:28 Página 49
A person can store around 1,500-2,000 kcal as blood
glucose and glycogen. In the blood we only have 50
kcal of glucose for immediate use. Hepatic glycogen
can provide around 250-300 kcal. Muscle glycogen in
trained long distance runners is around 130 mmol·kg-1,
which is a higher value than those found in sedentary
subjects or people who practice other sports that are
shorter in duration. Because carbohydrates have limita-
tions during exercise, including in cases where fats
were are used as the main energy source, an athlete’s
diet should be rich in carbohydrates to cope with the
high energy consumption and to maintain full glycogen
stores1.
Then, the role of sugar in different kinds of exercise
will be briefly analysed, specifically glucose and
glycogen: submaximal, maximal or supramaximal and
intermittent.
Long term submaximal exercise
For this type of exercise, the higher the intensity the
more muscular glycogen is used and the less energy is
obtained from fat. However, the longer the durations
the more fatty acids are used as a source of energy4
(Fig. 1). Muscles are metabolically independent thanks
to glycogen stores, although these stores are not inex-
haustible. That’s why fat tissues and the liver have to
provide the muscle fibres with fuel. This inter-relation-
ship between tissues helps prevent the complete
exhaustion of glycogen stores, because its concentra-
tion is the main limitation on the ability to perform
prolonged exercise. Furthermore. when energy mainly
comes from fats, mechanical performance is reduced,
therefore, the coordination between muscles, the liver
and fatty tissue is imperative3.4.
Short term maximal or supra-maximal
exercise
The high intensity of this type of exercise means that it
can be perform for a long period of time. Moreover, the
aerobic metabolic pathway is not able to supply energy
at the speed that it’s needed, therefore, from a quantita-
tive point of view, anaerobic metabolism is more impor-
tant in this type of exercise. In the phosphagen system
glucose and glycogen are the main sources of energy.
The contribution of muscle glycogen in short term
maximal intensity exercise could be as follows: 20% in
the first 30 seconds, 55% from 60 to 90 seconds and 70%
from 120 to 180 seconds3.
Intermittent exercise
Combined periods of exercise and rest periods are
known as intermittent exercise. These exercises are
very common in training and in many sporting activi-
ties. The fuel used during this type of exercise depends
on the intensity, the duration of the exercise, the length
of the rest period and the number of times the exercise
is repeated, therefore the possibilities are endless.
Focusing on glycogen, the four characteristics above
determine the decrease in glycogen stores, whilst their
replenishment (hepatic and muscular) depends entirely
on diet.
Specific recommendations of sugar intake
for athletes
Information on this matter is extensive and there are
numerous studies that have examined the effectiveness
of consuming different amounts of sugar. Tables I and
II summarise the recommended carbohydrate intake
guidelines for athletes.
Endurance sports
During endurance exercise muscle glycogen gradu-
ally decreases and, as we have previously mentioned,
performance deteriorates. An effective way of
improving endurance is to increase the amount of
glycogen stored in the skeletal muscles and the liver
before starting exercise5. The availability of carbohy-
drates, as a substrate for the muscles and central
nervous system, becomes a factor that limits perfor-
mance during prolonged submaximal (> 90 min) and
intermittent high intensity exercise6.
Traditionally, diets rich in carbohydrates have been
recommended for endurance and ultra-endurance
training, because of the relationship between these
diets, the increase in muscular glycogen stores and
delayed onset fatigue. More recently diets high in
50
Table I
Recommended carbohydrate intakes in athletes.
Translated and amended from13
Intake
recommendations
Daily requirements. These recommendations should take the total
individual energy expenditure, specific training needs and perfor-
mance into consideration.
Light or low intensity activities 3-5 g·kg-1·day-1
Moderate intensity exercise programme
(~1 h·day-1)5-7 g·kg-1·day-1
Moderate to high intensity exercise
programme (1-3 h·day-1)6-10 g·kg-1·day-1
High intensity exercise programme
(4-5 h·day-1)8-12 g·kg-1·day-1
05. Azucar y ejercicio_Nutr Hosp 25/06/14 12:28 Página 50
carbohydrates and carbohydrate intake before and
during exercise, have been shown to be beneficial due
to an increase in hepatic glycogen concentrations and
the maintenance of blood sugar concentrations7. The
daily carbohydrate requirements for training and
recovery are summarised in table I. To address the
specific carbohydrate needs of athletes it is important
to express them with regard to body weight. Various
articles have suggested that a carbohydrate intake of 8
to 10 g·kg-1·day-1 is needed to replenish glycogen6-9, a
higher intake (10-13 g·kg-1·day-1) is needed for athletes
whose sports disciplines generate a greater depletion of
glycogen reserves6. In female athletes it appears that
glycogen synthesis may increase during the luteal
phase, therefore, the menstrual cycle is an important
consideration when it comes to carbohydrate consump-
tion in female endurance athletes6.7.
It is vital for athletes to replenish glycogen reserves
following exercise, with a view to providing enough
energy for the next training session or competition. A
high carbohydrate diet can be effective, on its own, at
quickly replenishing glycogen stores, but there are a
variety of strategies that can increase efficiency, such
as adding proteins. Glycogen stores can be increased
1.5 times more than normal, for example, by
consuming a high carbohydrate diet for 3 days prior to
competition, after having followed a low carbohydrate
diet for the 3 previous day (for a total period of 6 days
before competition). Furthermore, if we take citric
acid, which inhibits glycolysis, at the same time as
following a high carbohydrate diet, glycogen stores
increase even more because of the inhibiting effect
they have on glycolysis5. In table II the strategies
employed by athletes to increase and replenish
glycogen stores are summarised.
Strength sports
Strength training, as a basic physical quality, has a
major impact on nearly all of the homeostatic regula-
tion systems of the body and also has significant
metabolic consequences in terms of energy supply.
Muscles do not differentiate between sporting activi-
ties, what they do differentiate between is the number
of motor units that are recruited and the amount of
time they are active (% in relation to maximal volun-
tary contraction), the rest are cultural differences that
don’t have any physiological implications. For
example running a marathon is no more than a
sequence of very small frequent muscle contractions
at a moderate or low intensity and for a very long
time, whereas running the 100 metres requires a very
high proportion of muscle fibres available in a short
space of time.
Strength training has a number of peculiarities that
have a direct and important impact on the choice and
use of different fuels. For activities that are longer than
120 seconds, total energy expenditure in strength
training is less than aerobic activities because they are
performed with an embedded rhythm. In this respect,
for stand-alone exercises like the bench press or squats,
anaerobic energy expenditure may be more than 30%
of total energy10, in the case of circuit training this
requirement is no more than 10%, as can be seen in
figure 2. Carbohydrate requirements are important,
in fact, when training intensity is high hypoglycaemias
can be very common. However the total energy expen-
diture is not high. In figure 3 we can see that circuit
training with weights does not require more than 35%
of VO2max, whereas the required heart rate is higher than
90%.
51
Table II
Carbohydrate loading strategies. Translated and amended from13
Intake recommendations
(in grammes of carbohydrates)
Strategies aimed at promoting high carbohydrates availability that enables optimal performance in competition or important training sessions.
Carbohydrate loading Preparation for events 7-12 g·kg-1·day-1
< 90 min of exercise (daily requirements)
Carbohydrate loading Preparation for events 36-48 hours of
> 90 min of exercise 10-12 g·kg-1·day-1
Rapid loading < 8 hours of recovery between two 1-1.2 g·kg-1·h-1 during for the first
intense training 4 hours, followed by daily requirements
Intake before exercise One hour before exercise 1-4 g·kg-1 consumed 1-4 hours before exercise
Intake during exercise <45 min Not necessary
Intake during high intensity exercise 45-75 min Small amounts
Intake during endurance training 1-2.5 h 30-60 g·h-1
Intake during ultra endurance training 2.5-3 h 90g·h-1
05. Azucar y ejercicio_Nutr Hosp 25/06/14 12:28 Página 51
Although there are clear benefits to following a high
sugar diet for endurance sports, the situation for
strength training isn’t as clear, as the small amount of
energy that this kind of exercise usually needs is easily
supplied by our energy system11. What can often
happen, and primarily due to the inadequate progres-
sion of training variables (amount and intensity), is
that, due to the limited amount of energy, in the form of
sugars in the blood, a very rapid demand for glucose
(intense strength training) can exhaust these small
reserves and this means that the liver is not able to
supply glucose to the bloodstream as quickly, so the
likelihood of hypoglycaemia occurring is very high.
The effects of sugar consumption
on sports performance
The effect of carbohydrate intake on sports perfor-
mance depends mainly on the kind of exertion (inten-
sity, duration, etc), the type and amount of carbohy-
drates eaten and when they are consumed. The
combination of all these factors should be taken into
consideration when analysing performance in different
sports. In the following points, the effects of
consuming sugars before, during and after exercise will
be explained, as well as the main features of low carbo-
hydrate diets.
Carbohydrates should provide 55-60% of athletes
total daily calorie intake. During periods of increased
training this percentage should be increased to 65-
70%1-6. When they should be consumed will be
discussed below.
Intake before exercise
The general recommendations for carbohydrate
intake before exercise stipulate that dinner on the day
before competition should be high in carbohydrates
(250-250 g), that the meal before (3-6 hours before)
should include the intake of 200-350g and 60-30
minutes before competing 35-50 g of glucose, sucrose
or glucose polymers should be consumed. The foods
consumed should be low in fat, fibre and proteins, well
tolerated, not in large portions and with a high or
medium glycaemic index1. On the other hand, certain
studies indicate that the intake of glucose 30 or 45
minutes before exercise causes muscle fatigue more
quickly than when it’s not consumed (due to changes in
glucose and insulin concentrations). However, if fruc-
tose is consumed plasma glucose and insulin concen-
trations do not change drastically before exercise12.
The American College of Sports Medicine (ACSM)
maintains that the amount of carbohydrates that
enhances performance varies between 200 and 300g
for meals 3-4 hours before exercise, at the rate of 30-
60 g·h-1 in intervals of 15-20 min (primarily in the form
of glucose), for exercise that lasts more than an hour9.
Furthermore, consumption of 0.15-0.25 g of protein·
kg-1, 3-4 hours before exercise, with a ratio of 3-4:1
(glucose: protein), can stimulate the synthesis of
proteins during endurance exercise, but it has not been
proven to improve performance9. Genton et al. propose
the consumption of 1-4 g·kg-1 1 to 4 hours before exer-
cise to increase carbohydrate availability during
prolonged exercise sessions, and 0.5 to 1 g·kg-1 during
moderate intensity or intermittent exercise sessions
> 1 h5.
When exercise is performed for a long period of
time, such as a marathon, consuming carbohydrates
immediately before or during exercise is an effective
way of improving endurance. Under such conditions, it
is desirable that athletes consume mono- or oligosac-
charides, because they are quickly absorbed and trans-
ported to the peripheral tissues for use. On the other
hand, carbohydrate consumption inhibits the break-
down of fats and stimulates insulin secretion. This
52
Fig. 2.—The proportion of
energy provided and oxygen
consumed during three
rounds of circuits with
weights, with 8 exercises
without rest at 65% of inten-
sity and for 54 min.
35
30
25
20
15
10
5
0
Oxygen consumption (ml·kg-1·min-1)
Anaerobic metabolism (lactate) Aerobic metabolism
Circuit using machines Circuit using free weights Combined circuit
6%
94%
5%
2%
95%
98%
05. Azucar y ejercicio_Nutr Hosp 25/06/14 12:28 Página 52
leads to a deterioration in energy production through
the metabolism of fats and accelerates glycolysis as a
way of producing energy. As a result, muscle
glycogen consumption increases. Therefore it’s
necessary to consume carbohydrates that don’t inhibit
fat metabolism. It has been suggested that supple-
ments containing fructose stimulate insulin release
less and it is unlikely that they inhibit lipolysis,
instead of common carbohydrates such as glucose and
sucrose. Moreover, the simultaneous consumption of
citric acid and arginine can facilitate energy
consumption from fats through the inhibition of
glycolysis, delaying glycogen depletion. Therefore,
consuming both together with carbohydrates that
slowly stimulate insulin secretion, before or during
exercise, could be an effective way of improving
energy metabolism and providing an optimal source
of energy during prolonged exercise7.
Some studies show a reduction in muscle glycogen
use when carbohydrates are consumed before and
during exercise. Others have reported a reduction in
hepatic glucose synthesis, the maintenance of normal
blood sugar and high levels of glucose oxidation during
the final stages of exercise, but not a reduction in
glycogenolysis. However, high levels of circulating
insulin reduce lipolysis and therefore reduce the contri-
bution of muscle fat during exercise. Consequently, the
amount of carbohydrates provided should be sufficient
to cover energy demands from exercise and the energy
lost from fat oxidation7-8.
53
Fig. 3.—Oxygen consump-
tion response (VO2) and
heart rate (HR) during a
strength training circuit,
compared with the response
during stress test until
exhaustion (Adapted from16).
7,000
6,000
5,000
4,000
3,000
2,000
1,000
0
VO2max = 5,562 ml·min-1
VO2max = 1,930 ml·min-1
35%
VO2max (ml·min-1) VCO2(ml·min-1)
0
0
25
30
35
50
75
100
125
150
175
200
225
245
270
295
320
345
370
395
420
445
470
495
520
50
50
0
0
0
0
250
200
150
100
50
0
0
0
25
30
35
50
75
100
125
150
175
200
225
245
270
295
320
345
370
395
420
445
470
495
520
50
50
0
0
0
0
HR (beats·min-1)
Max HR = 193
Max HR = 181
Heart rate during a race
Heart rate during a circuit
93,8%
VO2max: Maximum oxygen consumption; Max HR: Maximum heart rate.
05. Azucar y ejercicio_Nutr Hosp 25/06/14 12:28 Página 53
A temporary reduction in blood sugar levels usually
occurs when carbohydrates are consumed before exer-
cise. This is probably due to an increase in glucose
uptake in the plasma, as a result of increased insulin
levels and the suppression of hepatic glucose synthesis.
This imbalance causes a reduction in plasma glucose
concentration that is subsequently offset by an increase
in the intestinal absorption of glucose, with the aim of
normalising plasma glucose levels. For the majority of
individuals this reduction in blood sugar concentra-
tions is temporary and of no functional significance12.
Intake during exercise
High intensity and long duration endurance tests
(> 65% VO2max) are characterised by a steady gradual
decrease of glycogen in the active muscles. Although
glycogen is not the only source of energy, it is neces-
sary for maintaining intensity and it will be compen-
sated by plasma glucose if it decreases, which is
compensated by the liver (stored glycogen and the
conversion of substrates like lactate or alanine in
glucose). The reduction in plasma glucose, which
occurs during prolonged exercise, is an indication that
the liver cannot provide enough glucose once its
glycogen stores are exhausted. Under these conditions,
additional glucose may benefit performance8. There-
fore, the aim of eating during exercise is to provide a
readily available source of oxygen fuel, since the
endogenous glycogen stores are exhausted7.
The maximum oxidation rate for exogenous carbo-
hydrates during moderate intensity exercise is 0.8 to
1.0 g·min-1. This proportion is slightly less than 1mJ of
energy, whereas some forms of exercise need four
times this amount. This suggests that there is the poten-
tial for supplementing with fat during exercise.
Various studies have used medium-chain triglycerides
as a source of additional fuel during exercise. There-
fore, the consumption of both carbohydrates and fats
together during exercise could prevent the decrease in
fat metabolism that is observed when only carbohy-
drates are consumed13. The rate that limits the oxida-
tion of ingested carbohydrates is due to its intestinal
absorption, specifically, the type of transport mecha-
nism. So, if glucose is consumed in combination with a
glucide such as fructose, which is absorbed via a
different transport mechanism, the total rate of carbo-
hydrates consumed may be higher than 1.5 g.min-1.
Following this, recommendations for glucose and
fructose intake have been raised to 80-90 g·h-1, at a
ratio of 2:1. Furthermore, it has been shown that the
time to exhaustion increases with the consumption of
fructose and is dose-dependent12. Improvements in
performance are significantly higher when the subject
receives larger amounts of fructose. The possible
mechanism, by which fructose intake might spare
muscle glycogen, involves its influence on plasma
lipids, as they enable the increased use of fats1.9. So,
sugar (sucrose) becomes an excellent supplement to
both glucose and fructose.
On the other hand, during tests of less than 60 min. in
length recommendations suggest not giving any
specific carbohydrates. However, consuming 300-
500 ml of drink with a carbohydrate concentration of
6-10% every 15 min at a temperature of 8-12ºC, could
help preserve muscle glycogen and balance fluid loss,
especially if the exercise is carried out during high
temperatures. For events of between 1 to 3 hours it is
recommended that 800-1,400 ml·h-1 of liquid are
consumed, with a carbohydrate concentration of 6-8%
and a sodium concentration of 10-20 mmol·l-1. When
the exercise duration exceeds 3 hours it is advisable to
drink around 1,000 ml·h-1 of liquid with 23-30 mmol·
l-11of sodium.
Post exercise intake
After physical exertion of more than an hour, muscle
glycogen stores may be empty, with a loss that could be
around 90%. As a result, an exogenous supply of
substrates is requires to achieve the levels of glycogen
prior to exercising. The full replenishment of glycogen
stores following exercise takes between 24 and 48
hours, as the resynthesis rate is directly proportional to
the amount of carbohydrates in the diet during the first
24 hours13. The recovery of muscle and hepatic
glycogen is a key objective of recovery between
training sessions or competition events, particularly
when the athlete engages in multiple training sessions
during a condensed period of time6.8. Previously it was
though that 48 hours of rest were needed to recover
muscle and liver stores. Now it is accepted that, in the
absence of severe muscle damage, glycogen reserves
can normalise after 24 hours of reduced training and
adequate fuel consumption7.8.
The diet after each exercise session should contain
sufficient carbohydrates to replenish glycogen reserves
and to maximise subsequent performance (an average
of 50 g of high carbohydrate foods for every 2 hours of
exercise). The aim should be to consume a total of
approximately 600 g of high carbohydrate foods with a
high and moderate glycaemic index in a 24 hour
period6. After intense exercise, muscle glycogen
synthesis needs to recover about 100 mmol·kg-1, with a
glycogen synthesis rate of 5 mmol·kg-1·h-1, requiring
about 20 hours to recover (normalise) glycogen stores.
Carbohydrate consumption during the first 2 hours
after exercise allows a slightly faster rate of synthesis
than normal (7-8 mmol·kg-1·h-1). For this reason,
athletes should consume enough carbohydrates
following exercise as soon as possible, especially
during the first hour after exercise due to the activation
of the glycogen synthase enzyme by glycogen deple-
tion, an increase in sensitivity to insulin and the perme-
54
05. Azucar y ejercicio_Nutr Hosp 25/06/14 12:28 Página 54
ability of muscle cell membranes to glucose. Glycogen
synthesis throughout the day is similar, whether the
carbohydrates are consumed in large meals or as a
series of small snacks, with no differences between
consuming them in liquid or solid form, the only
important thing is the total amount of carbohydrates
consumed. Primarily, high carbohydrates foods should
have a high glycaemic index (they increase muscle
glycogen stores as much as possible), whereas those
with a low glycaemic index should not constitute more
than a third of recovery meals1.6.9.
The recommendation, according to the type of
activity, (grams of carbohydrates) are as follows (Table
I and II)6:
• Immediate recovery after exercise (0-4h) 1.0-1.2
g·kg-1·h-1, every 2 hours.
• Daily recovery < 1 h·day-1 from low intensity exer-
cise: 5-7 g·kg-1·day-1.
• Daily recovery < 1 h·day-1 from moderate to
intense resistance training: 7-10 g·kg-1·day-1.
• Daily recovery < 4 h·day-1 from moderate to very
intense training: 10-12 g·kg-1·day-1.
During the first few hours meals with 70-80% carbo-
hydrates should be eaten to avoid consuming lots of
protein, fibre and fats, which besides suppressing
hunger and limiting carbohydrate consumption, may
cause gastrointestinal problems, in which case liquid
preparations are preferred. At the same time, sports
drinks, which fundamentally aim to create an anabolic
environment, should cause an increase in blood sugar
and consequently raise insulin, thereby enhancing the
effects of different anabolic hormones to stimulate the
synthesis of muscle and liver glycogen14.
Low carbohydrate diets
After addressing. at great length, the importance of
consuming large quantities of carbohydrates for sports,
the opposing alternative will be discussed: low sugar
diets. It suggests that low carbohydrate diets that
provide less than 50-150 g·day-1, and their influence on
sports performance has also been studied.
Low carbohydrate and high fat diets have been
considered as a potential mechanism for improving
performance in endurance exercises. However,
amongst athletes these diets are perceived negatively
when it comes to performance. The authors that
proposed these diets suggest that this dietary practice
provides large amounts of lipids as energy substrates to
synthesise ATP. Low carbohydrate diets result in meta-
bolic and hormonal adjustments that can improve the
oxidation of fats and encourage muscle glycogen
conservation during exercise. Like with endurance
training adaptations, there is a shift towards increased
fat oxidation as fuel, at rest or during exercise, which
may be due to a combination of an increase in oxidative
enzymes, an increase in mitochondrial density, the
increased storage and use of intramuscular triglyc-
erides and the increased muscle uptake of plasma free
fatty acids. This combination of mechanisms leads to a
reduction in muscle glycogenolysis and carbohydrate
oxidation and contributes to the increased use of free
fatty acids during exercise14.
The low amount of glucose stored in the human body
restricts the ability to maintain a high power output
during prolonged endurance exercise. It had been
argued that one of the consequences of a low carbohy-
drate diet can be a reduction in muscle glycogen
content before exercise, particularly in untrained indi-
viduals, which could defeat the purpose of creating a
glycogen sparing effect. Therefore, studies indicate
that an increase in carbohydrate consumption tends to
cause less disruption in sports performance compared
to low carbohydrate diets14.15.
Weaknesses
In general physical exercise has very specific energy
and sugar demands. Therefore, metabolic activity
during physical activity and training can cause prob-
lems with homeostasis in healthy people, and more so
in at-risk populations, if the performance-based
requirements are not met. In this way, the qualified
professionals’ ignorance and lack of consultation of
these requirements could involve the implementation
of a series of initiatives (eliminating foods and encour-
aging others) that could carry an unwarranted and
unreasonable risk in many cases.
Threats
The difficulty in establishing the specific require-
ments for each physical activity, according to the inten-
sity and volume of exercise, poses a major threat, as
well as the proliferation of advertising campaigns or
miracle diets that discredit the benefits of consuming
sugar for sport. At present, the methods of quantifying
physical activity help determine the energy require-
ments of each activity, although a number of
confounding factors such as age or gender, could influ-
ence the accuracy of these measurements. Even so
there is still a long way to go before specific sugar
requirements can be quantified with real accuracy, for
each person in every situation.
Strengths
This text takes a pedagogical approach to under-
standing general sugar requirements according to the
type of physical exercise: endurance or strength.
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Nowadays the requirements for many activities are
known, as well as the significance or influence that
adequate intake may have on performance. However,
more work is needed in this line of research.
Opportunities
The demand to know the exact sugar requirements
adapted to each person and situation creates work
opportunities for research groups that are dedicated to
studying specific blood sugar requirements. These
groups are working on both healthy and populations
with medical conditions. Certain lines of work will
help improve the administering of insulin in diabetics,
as well as its interaction with exercise.
Recommendations
All the carbohydrate intake recommendations have
already been outlined throughout the text, however we
should remember that it is important to assess the type
of exercise performed, because sugar intake depends
on these characteristics. In at-risk populations, the
monitoring of blood sugar levels during exercise
should be common practice.
Conclusions
The skeletal muscle and liver are the main stores of
glycogen in the body. These stores, together with blood
glucose, are the main source of energy for most
athletes. Therefore, carbohydrate availability during
exercise, as well as the subsequent recovery of muscle
glycogen reserves, plays a pivotal role in the perfor-
mance of different sports. The reduction in muscle
glycogen levels (a substrate of the muscles and the
central nervous system) becomes a factor that limits
performance. There is evidence that consumption of a
high carbohydrate diet, before and during exercise, is
beneficial due to the increase in concentrations of
hepatic glycogen and the maintenance of and blood
sugar concentrations. Its effect on sporting perfor-
mance depends primarily on the kind of the exercise,
the type and amount of carbohydrates eaten and when
they are consumed. It is also important for athletes to
replenish glycogen stores after exercise, with a view to
providing enough energy for the next training session
or competition, through a high carbohydrate diet with a
high and moderate glycaemic index, enabling glycogen
synthesis to be enhanced through the addition of
protein. In conclusion, sugar (sucrose) becomes an
excellent supplement to both glucose and fructose.
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