Effects of daily medium-chain triglyceride ingestion on energy metabolism and endurance performance capacity in well-trained runners

Abstract

The purpose of this study was to assess the effect of medium-chain triglyceride (MCTG) supplementation on energy metabolism and endurance performance capacity in well-trained runners. Seven men (age 19.4±1.7 years, maximal O2 uptake 67.5±4.8 ml · min−1 · kg−1) were tested before and after MCTG as well as placebo consumption daily for 7 days. The energy and nutrient intake of the subjects did not differ in MCTG and placebo trial. The running time to exhaustion after dietary intervention was respectively 3916±1225 s in placebo trial and 3498±559 s in MCTG trial. The concentration of β-hydroxybutyric acid in plasma was the highest (1.029±0.67 mmol · l−1) in post-test samples after MCTG supplementation, significantly differing from the pre-test concentration in the same trial as well as from that observed in post-test blood after placebo treatment. These results suggest that daily MCTG supplementation increases the availability of ketone bodies for oxidation in working muscle during high intensity endurance exercise, but does not improve endurance performance capacity.

Full text

Effects of daily medium-chain triglyceride ingestion on
energy metabolism and endurance performance capacity
in well-trained runners
Vahur O
¨o¨pik
a,
*, Saima Timpmann
a
, Luule Medijainen
a
, Harry Lemberg
b
a
Institute of Exercise Biology, University of Tartu, 18 ˆ
Ulikooli St., 50090 Tartu, Estonia
b
Sports Centre, University of Tartu, 18 ˆ
Ulikooli St., 50090 Tartu, Estonia
Received 9 September 2000; received in revised form 25 March 2001; accepted 31 March 2001
Abstract
The purpose of this study was to assess the effect of medium-chain triglyceride (MCTG) supple-
mentation on energy metabolism and endurance performance capacity in well-trained runners. Seven
men (age 19.4⫾1.7 years, maximal O
2
uptake 67.5⫾4.8 ml 䡠min
-1
䡠kg
-1
) were tested before and after
MCTG as well as placebo consumption daily for 7 days. The energy and nutrient intake of the subjects
did not differ in MCTG and placebo trial. The running time to exhaustion after dietary intervention
was respectively 3916⫾1225 s in placebo trial and 3498⫾559 s in MCTG trial. The concentration of
␤
-hydroxybutyric acid in plasma was the highest (1.029⫾0.67 mmol 䡠l
-1
) in post-test samples after
MCTG supplementation, significantly differing from the pre-test concentration in the same trial as
well as from that observed in post-test blood after placebo treatment. These results suggest that daily
MCTG supplementation increases the availability of ketone bodies for oxidation in working muscle
during high intensity endurance exercise, but does not improve endurance performance capacity.
© 2001 Elsevier Science Inc. All rights reserved.
Keywords: Medium-chain triglycerides; Ketone bodies; Endurance performance
1. Introduction
Medium-chain triglycerides (MCTG) contain fatty acids (FA) with a chain length of six,
eight or ten carbon atoms. In contrast to long-chain triglycerides that delay gastric emptying,
* Corresponding author. Tel.: ⫹372-7-375-366; fax: ⫹372-7-375-366.
E-mail address: vahuro@ut.ee (V. O
¨o¨pik).
www.elsevier.com/locate/nutres
Nutrition Research 21 (2001) 1125–1135
0271-5317/01/$ – see front matter © 2001 Elsevier Science Inc. All rights reserved.
PII: S0271-5317(01)00319-0

MCTGs are rapidly emptied from the stomach and absorbed in the intestine [1]. In fact
MCTGs are absorbed as fast as glucose [2]. Because of that MCTGs readily increase
plasma medium-chain FA concentration. Moreover, while the carnitine transport system
located in mithocondrial membrane is thought to limit the rate of long-chain FA
oxidation [3], medium-chain FAs can diffuse into mitochondria independently of car-
nitine [4] and are oxidized rapidly [5]. These facts suggest, that MCTGs in comparison
with long-chain triglycerides may be more effective in modulating energy metabolism
and enhancing physical performance capacity, even if administered shortly before or
during exercise.
Indeed, the studies conducted on humans by Massicotte et al. [6] and Jeukendrup et al.
[7,8] have shown that 54–85% of MCTGs administered during endurance exercise are
oxidized. On the other hand, the available information concerning the effect of MCTG
administration on physical performance capacity is equivocal. Recently van Zyl et al. [9]
demonstrated that MCTGs ingested in combination with carbohydrate (CHO) in comparison
with the administration of MCTG or CHO alone during submaximal endurance exercise
significantly improved performance capacity of their endurance-trained male cyclists. Con-
trary to that, Jeukendrup et al. [10] in a very similar study did not observe any effect of
CHO⫹MCTG administration compared with placebo ingestion on endurance performance in
well-trained subjects.
In an animal study Fushiki et al. [11] observed enhanced swimming capacity in trained as
well as in untrained mice after chronic consumption (for 6 weeks) of MCTGs. Contrary to
that, the authors did not find any effect of acute administration of MCTGs on swimming time
in mice. The animals who chronically consumed MCTGs had a significantly higher concen-
tration of 3-oxo acid CoA transferase (an enzyme responsible for oxidation of ketone bodies)
in muscle [11]. Hence, chronic consumption of MCTGs induced metabolic adaptation which
apparently favored increase in endurance capacity.
There are commercially available dietary supplements containing MCTGs that are tar-
geted towards athletes and advertised to boost body‘s energy value and prolong endurance
by maximizing fat metabolism during intense workouts and competitions. However, it is
noteworthy that in the human studies referred to above, MCTGs were ingested only shortly
before and/or during exercise. Thus, the possible effect of prolonged dietary MCTG sup-
plementation on fat metabolism and endurance capacity in humans is actually unknown.
Moreover, the intensity of exercise used to measure endurance performance capacity of the
subjects has been around 60% of their maximal O
2
uptake (VO
2
max) in the previous
studies. At the same time it is known that the rate of oxidation on medium-chain FA in
comparison with that of long-chain FA increases especially during high intensity (80–
85% VO
2
max) exercise [12]. Consequently, the importance of medium-chain FA in
energy metabolism and their influence on endurance performance capacity should be
greater in this situation in comparison with lower intensity (60% VO
2
max or less)
exercise.
Accordingly, the aim of the present study was to test the effect of 7-day daily consumption
of MCTGs on energy metabolism and endurance performance capacity in well-trained
runners during high intensity (80% VO
2
max) treadmill run.
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2. Materials and methods
2.1. Subjects
Seven male endurance-trained runners agreed to participate in the study the protocol of
which was approved by the local Ethics Committee. Their age, body mass, height and
maximal oxygen uptake at the beginning of the study was (mean ⫾SD) 19.4⫾1.7 years,
65.6⫾2.8 kg, 179.3⫾4.1 cm and 67.5⫾4.8 ml 䡠min
-1
䡠kg
-1
, respectively. The sportsmen
followed their regular training program during the study period, running at an average 70 km
per week at an approximate intensity of 70% VO
2
max. The study was carried out during the
introductory stage of the year-round training cycle of the subjects.
2.2. Study design
The subjects participated in two series of experiments that were separated by one month
and differed from each other only by the nature of dietary manipulations used during the
7-day period before the endurance performance test. A double-blind crossover design was
used in the study. The endurance capacity of the subjects was tested on treadmill on four
occasions: before and after dietary MCTG (MCT Power, Universal, USA) supplementation
(MCTG trial) as well as before and after placebo (flavored cooking oil, Riesa, Belgium;
placebo trial) consumption during 7-day period. In the MCTG trial the subjects ingested at
an average 34.1⫾7.6 g of MCTG per day administered in two equal portions, which made
up the total quantity of 238.6⫾53.2 g over the whole 7-day period. In placebo trial flavored
commercial cooking oil (33.3⫾7.7 g per day, 233.1⫾53.9 g per week) was used instead of
MCTG, but the same procedure of administration was followed. The subjects were informed
about the essence of the dietary supplements but not about the sequence of their use. The diet
consumed by the subjects during the 7-day supplementation period was under control. They
were instructed to keep a detailed food diary, the nutritional data collected by this method
was analyzed using Micro-Nutrica 2.0 software, developed in Finland and adapted for use in
Estonia at Tallinn Technical University.
2.3. Performance test
The maximal oxygen uptake of the subjects was measured during a progressive exercise
test performed on treadmill Technogym Runrace HC 1400 (Italy). Expired gas was sampled
continuously and analyzed using analyzer True Max 2400 (Parvo Medics, USA). Individual
values of VO
2
max of the subjects were measured at the beginning of the study and were used
for adjusting the exercise intensity for the subsequent endurance performance tests.
The endurance performance capacity of the subjects was measured as their running time
till exhaustion on the treadmill at an average intensity of 80.3⫾4.6% (range 73.2–85.5%) of
their individual VO
2
max. Exhaustion was defined by subjects themselves, however, they
were verbally encouraged to continue running as long as they could.
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¨o¨pik et al. / Nutrition Research 21 (2001) 1125–1135

2.4. Biochemical analyses
Pre- as well as post-test blood samples (4.5 ml) were drawn from an arm vein keeping the
subjects in a sitting position throughout the procedure. Pre-test blood was drawn before each
test run after standardized warm-up, post-test sample being obtained 5 min after cessation of
the endurance performance test.
The ethylenediamine tetra-acetic acid treated blood sample was used for measurement of
hemoglobin concentration (cyanomethemoglobin method, Boehringer Mannheim GmbH
diagnostic kit No. 124729) and packed cell volume (by spun hematocrit). The values
obtained in this way were used to calculate changes in plasma volume [13].
An aliquot of the blood (1.5 ml) was deproteinized with the equal quantity of ice-cold
perchloric acid (0.7 mol 䡠l
-1
) and centrifuged. The concentration of
␤
-hydroxybutyric acid
(
␤
-HBA) was measured enzymatically in neutralized supernatant using diagnostic kit pur-
chased from Boehringer Mannheim GmbH No. 907979.
The remaining blood samples were cooled down immediately by placing the Vacutainer
tubes into ice-cold water. Tubes were then centrifuged, and plasma was stored at –25
o
C until
the analyses for glycerol and glucose were performed. The concentration of glycerol and
glucose was measured in samples of plasma using diagnostic kits purchased from Boehringer
Mannheim GmbH No. 148270 (glycerol) and No. 676543 (glucose).
Capillary blood samples (10
␮
l) were taken from the fingertip of the subjects before every
performance test (after warm-up), every 15 min during the exercise and 5 min after the end
of exercise. The concentration of lactate in samples was measured enzymatically (Dr Lange
Cuvette Test LKM 140) using miniphothometer LP 20 Plus (Dr Lange, Germany).
2.5. Statistical analysis
Conventional statistical analysis was used for calculating the mean and SD for each
parameter investigated. The distribution pattern of the data was tested using one-sample
Kolmogorov-Smirnov test, the differences between the means of variables were evaluated
using paired samples Student’s t-test. A p value ⱕ0.05 was considered to be significant.
3. Results
The energy and energy yielding nutrient intake of the subjects during supplementation
period did not differ in placebo and MCTG trial (Table 1). The consumption of vitamins,
minerals and water (data not shown) was also similar in the two trials.
There was no significant effect of dietary supplements on endurance capacity of the
subjects. The running time to exhaustion before and after dietary intervention was respec-
tively 3901⫾966 s and 3916⫾1225 s in placebo trial and 3892⫾1288 s and 3498⫾559sin
MCTG trial. However, it is noteworthy that the endurance performance of the subjects was
the worst after the 7-day MCTG consumption, being at an average 10.1% and 10.7% lower
than the values fixed before MCTG ingestion as well as after the placebo treatment,
respectively.
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¨o¨pik et al. / Nutrition Research 21 (2001) 1125–1135

The endurance test caused a significant loss of body mass in the placebo as well as in the
MCTG trial (Table 2). Both the concentration of hemoglobin in blood and hematocrit tended
to increase from the pre- to post-performance test samples on all occasions in both the
placebo and MCTG trial (Table 3). Plasma volume tended to decrease by 0.9%–5.0% as a
result of the performance test with no significant differences between before and after
placebo or MCTG administration.
The concentration of glycerol in blood plasma of the subjects increased significantly as a
result of each performance test in both trials (Table 4). Before and after placebo treatment
and before the MCTG ingestion the increase in the concentration of glycerol was approxi-
mately the same–at an average by 6.1, 6.9 and 6.6 times in comparison with the respective
pre-test values. However, after the MCTG consumption the increase in the glycerol concen-
tration was considerably greater–by 12.3 times compared with the pre-test concentration. On
the other hand, the absolute concentration of glycerol in post-test plasma of the subjects after
the MCTG treatment was significantly lower in comparison with the value observed after the
placebo treatment (Table 4). Moreover–if expressed in absolute numbers, the extent of the
increase of the concentration of glycerol in blood as a result of test exercise performed after
MCTG treatment was significantly lower in comparison with respective value fixed in the
placebo trial (0.078⫾0.06 mmol 䡠l
-1
and 0.135⫾0.05 mmol 䡠l
-1
, respectively; p⫽0.05).
The concentration of
␤
-hydroxybutyric acid in blood plasma of the subjects increased
significantly (by 1.6–3.4 times) as a result of each performance test (Table 4). The concen-
Table 1
Energy and nutrient intake of the subjects
Item Placebo trial MCTG trial
Energy (kcal 䡠24 h
⫺1
) 3269 ⫾453 3387 ⫾549
incl: food 2988 ⫾418 3134 ⫾524
supplement 281 ⫾69 253 ⫾60
Fat (g 䡠24 h
⫺1
) 123.7 ⫾21.0 130.1 ⫾30.1
incl: food 92.4 ⫾19.4 98.4 ⫾27.3
supplement 31.3 ⫾7.7 31.6 ⫾7.5
Protein (g 䡠24 h
⫺1
) 91.0 ⫾15.4 95.1 ⫾17.9
Carbohydrate (g 䡠24 h
⫺1
) 423.3 ⫾82.1 458.6 ⫾92.3
Alcohol (g 䡠24 h
⫺1
) 3.1 ⫾4.3 1.8 ⫾4.4
The values are mean ⫾S.D. n ⫽7, MCTG—medium-chain triglycerides.
Table 2
Changes in body mass of the subjects
Trial Body mass before supplementation (kg) Body mass after supplementation (kg)
Pre Post Change Pre Post Change
Placebo 66.2 ⫾3.2 64.4 ⫾3.5* ⫺1.6 ⫾0.2 65.9 ⫾3.1 64.0 ⫾3.4* ⫺1.9 ⫾0.3
MCTG 65.9 ⫾2.9 64.7 ⫾2.6* ⫺2.1 ⫾0.6 66.1 ⫾2.8 64.3 ⫾3.0* ⫺1.6 ⫾0.2
The values are mean ⫾S.D. n ⫽7, MCTG—medium-chain triglycerides, Pre-value measured before
performance test, Post-value measured after performance test.
* Significantly different from the corresponding pre-test value (pⱕ0.05).
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¨o¨pik et al. / Nutrition Research 21 (2001) 1125–1135

Table 3
Hematocrit, hemoglobin concentration in blood and changes in plasma volume of the subjects
Trial Before supplementation After supplementation
Hct (%) Hb (g 䡠dl
⫺1
) PV Hct (%) Hb (g 䡠dl
⫺1
)PV
Pre Post Pre Post (%) Pre Post Pre Post (%)
Placebo 44.6 ⫾2.2 44.8 ⫾1.8 14.3 ⫾1.0 14.6 ⫾0.8 ⫺3.0 ⫾3.8 43.9 ⫾1.9 44.9 ⫾1.3 14.1 ⫾0.7 14.6 ⫾0.5 ⫺5.0 ⫾3.7
MCTG 44.1 ⫾1.5 43.9 ⫾1.7 14.1 ⫾0.7 14.3 ⫾0.5 ⫺0.9 ⫾4.3 44.5 ⫾2.4 44.4 ⫾2.1 14.3 ⫾1.0 14.6 ⫾0.7 ⫺2.1 ⫾4.1
The values are mean ⫾S.D. n ⫽7, Hct—hematocrit, Hb—hemoglobin, PV—changes in plasma volume, the other abbreviations: see Table 2.
Table 4
Concentration of metabolites in blood plasma of the subjects
Metabolite Placebo trial MCTG trial
Before supplementation After supplementation Before supplementation After supplementation
Pre Post Pre Post Pre Post Pre Post
Glycerol 0.030 ⫾0.03 0.183 ⫾0.05* 0.023 ⫾0.02 0.159 ⫾0.05* 0.026 ⫾0.03 0.172 ⫾0.09* 0.007 ⫾0.01 0.086 ⫾0.07*,**
(mmol 䡠1
⫺1
)
␤
-HBA 0.207 ⫾0.08 0.531 ⫾0.30* 0.110 ⫾0.13 0.369 ⫾0.19* 0.217 ⫾0.11 0.364 ⫾0.14 0.453 ⫾0.58 1.029 ⫾0.67*,**
(mmol 䡠1
⫺1
)
Glucose 4.74 ⫾1.00 7.03 ⫾2.49* 5.38 ⫾0.42 7.67 ⫾2.58 4.79 ⫾0.89 7.69 ⫾2.75* 5.37 ⫾0.54 7.43 ⫾2.68
(mmol 䡠1
⫺1
)
Lactate 2.30 ⫾0.47 4.34 ⫾2.46 1.81 ⫾0.36 4.97 ⫾1.98* 1.86 ⫾0.52 4.24 ⫾1.39* 2.02 ⫾0.45 4.03 ⫾1.61*
(mmol 䡠1
⫺1
)
The values are mean ⫾S.D. n ⫽7,
␤
-HBA—
␤
-hydroxybutyric acid, the other abbreviations: see Table 2.
* Significantly different from the corresponding pre-test value (pⱕ0.05).
** Significantly different from the corresponding value in placebo trial (pⱕ0.05).
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tration of this metabolite was the highest in post-test blood samples after MCTG supple-
mentation, being significantly different from the pre-test value in the same trial as well as
from that observed in post-test blood after placebo treatment.
The increase in glucose concentration (Table 4) was statistically significant in endurance
tests performed before dietary manipulations and was revealed as an explicit tendency in
tests performed after 7-day placebo and MCTG treatment (p⫽0.06 and 0.08, respectively).
The concentration of lactate in blood of the subjects increased by 2.0–2.5 mmol 䡠l
-1
(Table 4) as a result of each endurance test in placebo as well as in MCTG trial. The
concentration of lactate reached 5.4–6.6 mmol 䡠l
-1
15 min after the beginning of exercise
and remained comparatively stable after that until cessation of exertion in all performance
tests in both trials (data not shown).
4. Discussion
During endurance exercise skeletal muscle can rely on both fat and carbohydrate oxidation
to cover the need for chemical energy. Under resting conditions and during low to moderate
intensity exercise, fatty acid oxidation contributes considerably to the total energy provision.
With increasing exercise intensity, however, CHO utilization becomes increasingly impor-
tant in maintaining muscle power output at the necessary level. The contribution of CHO
oxidation to the total energy provision increases particularly at the intensities above 70–80%
maximal O
2
uptake [14,15]. As the amount of CHO stored in the body is relatively small, it
poses a limitation to the ability to perform prolonged high intensity exercise. It is known that
adaptation to high CHO diet prior to exercise [16] and CHO ingestion during exercise [17]
enhances endurance performance at an intensity more than 70% VO
2
max. Thus, in order to
increase the availability of CHO to working muscles, endurance athletes are usually advised
to consume high CHO diets during training and to ingest CHO during competition.
Alternatively, attempts have been made to induce a greater oxidation of fat during
exercise, that should reduce the utilization of the limited CHO resources of the body and
thereby improve endurance capacity. The rate of plasma FA oxidation is apparently regulated
by physiological concentrations of circulating FA. For example, Romijn et al. [18] induced
an increase in plasma FA concentration by 1–2 mmol 䡠l
-1
with intravenous infusions of
long-chain triglycerides and heparin during high-intensity exercise (85% VO
2
max). As a
result of this intervention, fat oxidation during exercise increased by 27% and CHO
oxidation decreased by 11%.
Intravenous infusion of nutrients is acceptable in laboratory experiments, but its use in real
training and competition situation is practically impossible. For this reason high-fat diets
have attained considerable interest as a potential tool to enhance the capacity to oxidize FA
and to improve performance in endurance athletes. However, critical reviews [19–21] of the
research data available so far reveal that the efficacy of high fat diets in this context is
equivocal.
Chronic MCTG feeding induces metabolic adaptation and increase in endurance perfor-
mance capacity in trained as well as in untrained mice, whereas acute MCTG administration
turned out to be ineffective [11]. There are commercially available dietary supplements,
containing MCTGs and targeted towards athletes, that are recommended by the manufac-
1131V. O
¨o¨pik et al. / Nutrition Research 21 (2001) 1125–1135

turers to be consumed regularly in order to stimulate fat catabolism and enhance endurance
performance. However, to our knowledge there are no published research findings which
could support or deny the benefits of daily dietary MCTG supplementation in humans.
Our data do not confirm any positive effect of 7-day MCTG supplementation on endur-
ance capacity in well-trained runners performing high intensity exercise. In fact the endur-
ance performance of our subjects tended to be the worst after the 7-day MCTG supplemen-
tation period in comparison with any other time of measurement in the present study (see
Results). This observation is generally in accordance with the results of Jeukendrup et al.
[10], who found a significant fall in endurance performance of their subjects as a result of
MCTG administration compared with placebo ingestion during exercise. It is difficult to
compare the results of the present study with that of van Zyl et al. [9], because in the latter
case there was no placebo treatment. Their subjects ingested drinks containing CHO only,
CHO ⫹MCTG or MCTG only during moderate to high-intensity exercise. However,
endurance capacity of the subjects was significantly lower in MCTG only trial compared
both with CHO only and CHO ⫹MCTG treatment. Thus, the results of all three studies
[9,10, the present one] suggest that MCTG supplementation may deteriorate but not improve
endurance performance in humans.
Jeukendrup et al. [10] noticed that the negative effect of MCTG ingestion on performance
was associated with increased gastrointestinal complaints. They concluded that large
amounts of MCTGs (85 g) ingested during prolonged submaximal exercise may provoke
serious gastrointestinal problems leading to decreased exercise performance. The maximal
amount of MCTGs that can be ingested without causing gastrointestinal distress, has earlier
been estimated to be approximately 30 g [7,22]. In the present study an average serving size
was 17 g MCTGs, this amount was administered two times per day. Despite that, there were
five subjects out of seven who suffered under more or less severe gastrointestinal problems,
including abdominal cramping and diarrhea, during MCTG supplementation period. Con-
trary to that, there were no complaints during placebo treatment. Hence, in case of prolonged
usage even as small amount as 34 g of MCTGs per day should be taken with caution.
However, it is noteworthy that two subjects in the present study did not experience any
discomfort in MCTG trial and another two persons tolerated MCTGs comparatively well,
having mild gastrointestinal problems only during the first couple of days of the supplemen-
tation period. Moreover, the latter two subjects were the only ones who improved their
endurance performance capacity in the test performed after 7 days of MCTG supplementa-
tion in comparison with that achieved before MCTG treatment. These facts suggest that the
tolerance of people in respect of MCTG ingestion as well as the effects of MCTGs on
endurance performance capacity may be highly individual.
Changes in plasma glycerol concentration during exercise most likely reflect the intensity
of lipolysis [23]. An increase in blood glycerol concentration is a well-known effect of
endurance exercise [7–9]. In the present study the greatest relative increase (by 12.3 times
compared with the pre-test concentration) in the concentration of blood glycerol was
observed as a result of the test exercise performed after MCTG supplementation (Table 4).
This fact suggests that the stimulating effect of exercise on lipolysis was also the greatest
after MCTG supplementation in comparison with all other conditions. However, if expressed
1132 V. O
¨o¨pik et al. / Nutrition Research 21 (2001) 1125–1135

in absolute numbers, the increase in blood glycerol concentration was much lower in test
exercise performed after MCTG ingestion in comparison with the respective value observed
after placebo treatment (see Results). Thus, the data concerning changes in blood glycerol
concentration in different conditions do not confirm any stimulating effect of 7-day dietary
MCTG supplementation on lipolysis during exercise.
The concentration of
␤
-HBA was the highest in post-test blood samples after MCTG
supplementation, exceeding significantly the pre-test value in the same trial as well as that
observed in post-test blood after placebo treatment (Table 4). It is most likely that the
between-trials difference in the post-test blood
␤
-HBA concentration was caused by the
nature of the dietary supplements, because there were no other differences in the energy and
nutrient intake during the two phases of the study (Table 1).
It is known that the oxidation of medium-chain FAs results in the production of consid-
erably more ketones than does long-chain FAs [24,25]. Consequently, the higher concen-
tration of
␤
-HBA in post-test blood after dietary manipulations in MCTG trial compared with
placebo treatment indicates that the medium-chain FAs were remarkably oxidized during
exercise after MCTG supplementation. Theoretically, ketone bodies produced from the
oxidation of medium-chain FAs could be delivered as an alternative substrate to the
exercising muscle and thereby preserve muscle glycogen during an exercise bout [26].
Indeed, calculations based on measured differences between the rates of total CHO oxidation
and plasma glucose oxidation revealed reduced oxidation of muscle glycogen during endur-
ance exercise after MCTG ingestion [9]. An expected advantage of sparing glycogen would
be an increased time to exhaustion, since more muscle glycogen would be available,
especially in the latter stages of high-intensity endurance exercise. However, the results of
the present study show that, despite the availability of ketone bodies being significantly
increased during test exercise performed after MCTG supplementation, endurance capacity
of the subjects did not improve. The concentration of glycogen in skeletal muscle was not
measured in the present study, but similar changes of the concentrations of glucose and
lactate in blood (Table 4) in placebo and MCTG trial suggest that generally CHO metabolism
during exercise was not influenced by different dietary manipulations. This is in accordance
with the results of Decombaz et al. [5] and Horowitz et al. [27] who demonstrated that, with
normal carbohydrate stores in the body, the ingestion of MCTGs does not change the
contribution of CHO (including muscle glycogen) to energy metabolism during endurance
exercise. It is important to note that in the two latter studies the utilization of muscle
glycogen during exercise was measured using biopsy samples and not calculated indirectly
as done by van Zyl and associates [9]. Also our data (Table 4) do not confirm any
hypoglycemic effect of MCTG ingestion per se, which has been described by some authors
in animals [28] as well as in humans [29]. Similar changes in body mass (Table 2) and
plasma volume (Table 3) of the subjects in all performance tests indicate that the water
balance during exercise did not differ in two trials.
These results suggest that 7-day MCTG supplementation increases the availability of
ketone bodies for oxidation in working muscle during high intensity endurance exercise.
However, this metabolic adaptation does not improve endurance performance capacity in
well-trained runners.
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Acknowledgements
This work was supported by the Estonian Science Foundation, grant No. 2982. The
authors would like to thank Dr. Kalle Karelson, Dr. Tamara Janson and Ms. Mare Vene for
their excellent technical assistance.
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