A PROPOSITO DI....
Per quanto riguarda quella discussione sul post workout PROTEINE SI PROTEINE NO ecco un purtroppo lungo articolo di uno con le palle quadrate, ciò non cambierà le abitudini e le convinzioni di nessuno ma è solo per arricchire la nostra cultura in materia:Nutrient consumption before, during, and following exercise
R.J. Bloomer
Table of Contents
Exercise Performance
Substrate Usage
Exercise Recovery
Recommendations
Much debate centers around meal timing and selection of foods related
to exercise and subsequent athletic performance. With the number of
recommendations that exist concerning the correct nutrients to consume
prior to, during, and immediately following exercise, one would be led
to believe that volumes of data are available on the topic. However,
that which is marketed and spoken about as factual may in reality be
simply speculative. Therefore, the purpose of this article is to
provide some concrete scientific evidence, in addition to practical
applications, for feeding as it centers around an acute bout of
exercise.
Specific to this topic, three distinct areas exist that individuals
should be interested in. These include exercise performance and
related factors (strength, endurance, mood state, etc.), substrate
usage (proportion of fat, carbohydrate, and protein used to fuel the
exercise), and exercise recovery (glycogen resynthesis, protein
turnover, immunocompetence, etc.). While some studies have been done
to look at how feeding effects the above variables, it is important
that readers be aware of a few issues. First of all, from a
scientific standpoint, 'one or two' studies supporting/refuting a
certain claim really does not mean very much. In this case, while
data may look promising, more information would be necessary for firm
recommendations to be made. Secondly, subject number in many of these
scientific experiments is relatively small (< 20) which makes
generalizability to larger sample populations difficult. Thirdly,
data is typically collected in a laboratory setting under the careful
supervision of research staff (i.e., poorly paid graduate students)
and may not apply directly to unsupervised people existing in a
free-living environment. Lastly, there exist other variables such as
route of administration of a particular nutrient (intravenous, oral,
etc.), subject population being tested (obese, diseased, diabetic,
sedentary), etc. that should be thoroughly considered relatively to
study results. One must question whether or not the same results
would be obtained if these things were manipulated to more closely
represent his or her situation. Clearly, certain areas contain
sufficient data to support recommendations, while others are not
nearly as well researched. A well-informed individual should always
take these things into consideration prior to accepting the latest
claim/advice from the local guru. With this understanding, the
following is a brief overview of the few areas that have in fact been
sufficiently studied relative to feeding as it relates to exercise.
Back to Top
Exercise Performance
Although it is widely accepted that protein acts sparingly as an
exercise fuel, claims of strength increases resulting from high
protein intakes are common. While protein supplementation has been
proposed to increase the availability of essential amino acids during
exercise, enhance anabolic processes promoting tissue growth, and
accelerate the rate of recovery following training, no studies to date
have demonstrated conclusively that an (acute or chronic) increase in
protein intake will lead to increased strength performance-when
factoring out other confounding variables (e.g., the increase in total
energy consumed, trained vs. untrained state, etc.).
The results are slightly more conclusive related to endurance
exercise. Amino acid supplementation has been proposed to improve
physiological and psychological responses, perhaps by altering the
ratio of free tryptophan to branch-chain amino acids. It has been
noted that supplementation with branch-chain amino acids (leucine,
isoleucine, and valine) may increase aerobic work capacity and
decrease muscle fatigue (Davis & Bailey, 1997; Mero, 1999). The role
of these amino acids may be of greater importance during periods of
prolonged exercise, in particular when glycogen levels are depleted,
at which time supplemental amino acids may function to provide energy
to fuel aerobic work. However, further research is needed for a
definitive recommendation.
Dietary fat intake has been proposed over the past few years to
improve aerobic exercise performance, possibly by decreasing
carbohydrate oxidation (sparing glycogen usage) during exercise and by
prolonging the onset of fatigue. Although some work has been done to
support this hypothesis (Muoio et al., 1994), the bulk of scientific
evidence at the present time refutes this claim (Sherman & Leenders,
1995). More long-term studies are needed in order to fully understand
the role of dietary fat intake and it's effect on exercise
performance. (Note: most studies involve chronic increases in fat
consumption rather than simply consuming a high fat meal prior to an
acute bout of exercise. In addition, no real claims have been made
for this nutrient pertaining to strength performance).
The one nutrient that has been shown repeatedly to favorably improve
exercise performance is carbohydrate. Specifically, evidence clearly
demonstrates that ingestion of adequate dietary energy and
carbohydrate prior to and during exercise will improve performance by
delaying fatigue, improving mood, and making available additional
glucose to fuel the exercise bout (Balsom et al., 1999; DeMarco et
al., 1999). Data suggest that either liquid or solid carbohydrate may
be consumed with similar results, however liquid feedings may prove
easier to digest and assimilate. In addition, the type, specifically
the glycemic index (GI) of carbohydrate, may be of concern, as lower
GI choices have been found to be more beneficial than their higher GI
counterparts (DeMarco et al., 1999). Pertaining to postexercise
intake, consumption of carbohydrate rich foods and beverages may lead
to enhanced performance of subsequent athletic events, perhaps due to
the effects of loading glycogen into the muscle to be used during
these future (second daily session, next day session, etc.) training
bouts.
Despite the potential benefits of carbohydrate ingestion for
performance of strength exercise (e.g., weight lifting), at present,
little evidence is available to support this hypothesis. Although,
noting that carbohydrate consumption is associated with positive mood
changes (Keith et al., 1991), it is reasonable to expect an increase
strength output under certain mood elevating conditions. However,
systematic studies are necessary before recommendations are made
regarding this practice.
Back to Top
Substrate Usage
Pertaining to substrate utilization (proportion of carbohydrate,
protein, and fat oxidized) during the exercise session, it appears
that one only be concerned with the preexercise feeding in addition to
that which is consumed during the actual session. The postexercise
meal does not seem to influence substrate utilization of subsequent
training bouts unless under conditions of severe nutrient restriction
(e.g., carbohydrate depletion) when fatty acids (and possibly protein)
would largely contribute to fuel the work.
In this regard, feeding in close proximity to the onset of exercise
tends to elevate plasma glucose and insulin, allowing for carbohydrate
sources to be oxidized, with minimal fatty acid usage. While this may
prove beneficial for performance, one should not expect a great deal
of energy to be derived from fatty acid oxidation (little fat will be
used as fuel). In addition, it is known that short-term restriction
of dietary energy (i.e., fasting prior to exercise) is associated with
a greater increase in growth hormone (GH) output. GH is a very
powerful lipolytic (fat burning) hormone. As such, if elevated on a
systematic basis, it would be expected to increase fat
utilization/loss over time. This topic has proven controversial in
the fitness industry, as individuals interested in losing excess body
fat through exercise have been instructed to abstain from feeding for
the period before and during the exercise session. However, while it
is true that GH and fatty acid usage may be elevated following a brief
fast, data addressing the issue of whether or not someone will
actually be more successful at losing body fat long-term if adhering
to this plan is inadequate. As such, recommendations to minimize
nutrient intake for purposes of increasing fatty acid usage, at the
possible expense of decreasing exercise performance, should be
carefully considered.
Back to Top
Exercise Recovery
Perhaps the area of greatest research interest over the past several
years is that of facilitating recovery after an acute bout of
exercise. While disagreement may still exist over the ideal energy
intake and macronutrient composition of the feeding, postexercise
nutrient consumption is a well-accepted practice recommended to all
athletes and fitness enthusiasts.
Both dietary protein and carbohydrate intake have properties that
enhance the training effect on muscle. Specifically, both of these
nutrients support glycogen resynthesis (Roy & Tarnopolsky, 1998;
Zawadzki et al., 1994) and help to facilitate recovery. Studies
support carbohydrate intake in the range of 7-10 g(kg-1 body weight
within 24 hours after exhaustive exercise to normalize muscle glycogen
levels (Applegate, 1999). It has been suggested that glucose/glucose
polymers (preferably high GI choices) and fructose are most effective
at replacing muscle and liver glycogen, respectively (Ivy, 1998).
These may be consumed in either liquid or solid form with similar
results (Reed et al., 1989). However, liquid supplements are
recommended because of their greater ease in digestion. Also,
carbohydrate intake following exercise serves to enhance cellular
hydration when consumed with adequate fluid (Gonzalez-Alonzo et al.,
1992). Specifically, a 6-8% carbohydrate solution should function to
provide rapid rehydration by optimizing gastric emptying and
intestinal absorption. Actually, this type of mixture is best
consumed during the exercise bout to aid in performance, with the
percentage of carbohydrate increasing following exercise in order to
provide adequate carbohydrate to replenish glycogen stores.
The timing of carbohydrate ingestion in relation to an exercise bout
also appears important, particularly for competitive athletes, as
researchers reported an increased rate of glycogen storage in subjects
consuming carbohydrate immediately, rather than two hours postexercise
(Ivy et al., 1988). This may only be of significance if subsequent
training bouts are merely hours (3-4) away. However, for the average
trainee exercising 3-4 days per week, the rate of glycogen resynthesis
following exercise should be of little concern, assuming adequate
carbohydrate is consumed regularly in the normal diet. Further, the
typical 45-60 minute exercise bout is not of sufficient duration (nor
intensity) to markedly exhaust glycogen stores.
Other than carbohydrate, dietary protein, when added to this nutrient,
enhances glycogen storage, as a result of the heightened insulin
response caused by the amino acid ingestion (Roy & Tarnopolsky, 1998;
Zawadzki et al., 1994). In this way, insulin functions to stimulate
both muscle glucose transport and activation of glycogen synthase, the
rate-limiting enzyme in the glycogen synthetic pathway, and leads to
greater glycogen deposition.
The insulin elevating effect of consuming these nutrients
simultaneously has further reaching implications other than replacing
depleted glycogen. For example, such an acute state of
hyperinsulinemia may promote both anabolic and anticatabolic activity
in muscle by stimulating protein synthesis (Fryburg et al., 1995),
decreasing proteolysis (Biolo et al., 1995) and enhancing nitrogen
retention (Okamura et al., 1997). These positive effects on protein
turnover may, over time, favorably influence lean tissue accrual.
Also important is the role of these nutrients in supporting immune
function. High quality whey proteins have been shown to enhance
adaptive immunocompetence when provided as a dietary supplement (Wong
and Watson, 1995). The same is true for carbohydrate supplementation,
particularly during prolonged or intensive exercise (Nieman, 1999).
Mechanistically, carbohydrate supplementation can attenuate the
typical exercise-induced rise in cortisol, and other immune responses
that are related to physiologic stress and inflammation.
In sum, the synergistic effect of carbohydrate and protein consumed
postexercise may have implications for both exercise recovery and
tissue growth, largely as a result of the hyperinsulinemic effect of
consuming these nutrients in combination. As such, nutritional means
of increasing insulin to promote glycogen resynthesis and favorably
effect protein turnover, as well as to support optimal immune
function, should be considered.
Back to Top
Recommendations
Based on the available science, the following conclusions can be
drawn. Individuals attempting to maximize the results of their
exercise training through nutritional means may consider these
suggestions:
1. Consume a moderate sized, balanced meal (200-600 kcal) 2-3
hours prior to exercise. The meal should be composed of foods that
are familiar to the individual and that leave him or her neither
hungry nor with undigested food in the stomach (this may involve
minimizing dietary fat in order to speed digestion time). Extreme
endurance athletes may find that a greater amount of energy and
carbohydrate is beneficial. General fitness enthusiasts exercising
for purposes of fat loss, may consider abstaining from food for the
three to four hour period prior to exercise (or simply train first
thing in the morning). Individuals should experiment with different
regimens to determine which strategy works best for them.
2. Consume fluid liberally throughout exercise. Consider using a
carbohydrate solution (6-8%) containing added electrolytes, rather
than simply pure water. This may be especially beneficial if
exercising in the heat, when fluid loss from sweat is a concern. For
exercise of extreme duration (>2 hours) consider supplementing
carbohydrate (30-60 grams per hour).
3. Consume adequate carbohydrate (1.5 g(kg-1 body weight) and
protein (0.5 g(kg-1 body weight) immediately following exercise in
order to replenish depleted glycogen stores and to favorably impact
protein turnover and immune function.
Back to Top
References
Applegate, E. (1999). Effective nutritional ergogenic aids.
International Journal of Sport Nutrition, 9: 229-39.
Balsom, P.D., Wood, K., Olsson, P., & Ekblom, B. (1999). Carbohydrate
intake and multiple sprint sports: with special reference to football
(soccer). International Journal of Sports Medicine, 20, 48-52.
Biolo, G., Declan Fleming, R.Y., & Wolfe, R.R. (1995). Physiologic
hyperinsulinemia stimulates protein synthesis and enhances transport
of selected amino acids in human skeletal muscle. Journal of Clinical
Investigation, 95:811-819.
Davis, J.M., & Bailey, S.P. (1997). Possible mechanisms of central
nervous system fatigue during exerise. Medicine and Science in Sports
and Exercise, 29: 45-57.
DeMarco, H.M., Sucher, K.P., Cisar, C.J., & Butterfield, G.E. (1999).
Pre-exercise carbohydrate meals: application of glycemic index.
Medicine and Science in Sports and Exercise, 31, 164-70.
Fryburg, D.A., Jahn, L.A., Hill, S.A., Oliveras, D.M., & Barrett, E.J.
(1995). Insulin and insulin-like growth factor-I enhance human
skeletal muscle protein anabolism during hyperaminoacidemia by
different mechanisms. Journal of Clinical Investigation, 96:
1722-1729.
Gonzalez-Alonzo, J., Heaps, C.L., & Coyle, E.F. (1992). Rehydration
after exercise with common beverages and water. International Journal
of Sports Medicine, 13: 399-406.
Ivy, J.L. (1998). Glycogen resynthesis after exercise: effect of
carbohydrate intake. International Journal Sports Medicine, 19:
S142-45.
Ivy, J.L., Katz, A.L., Cutler, C.L., Sherman, W.M., & Coyle, E.F.
(1988). Muscle glycogen synthesis after exercise: effect on time of
carbohydrate ingestion. Journal of Applied Physiology, 64, 1480-85.
Mero, A. (1999). Leucine supplementation and intensive training.
Sports Medicine, 27(6): 347-58.
Muoio, D.M., Leddy, J.J., Horvath, P.J., Awad, A.B., & Pendergast,
D.R. (1994). Effect of dietary fat on metabolic adjustments to maximal
VO2 and endurance in runners. Medicine and Science in Sports and
Exercise. 26:81-88.
Nieman, D.C. (1999). Nutrition, exercise, and immune system function.
Clinics in sports Medicine, 18(3): 537-48.
Okamura, K., Doi, T., Hamada, K., Sakurai, M., Matsumoto, K.,
Imaizumi, K., Yoshioka, Y., Shimizu, & Suzuki, M. (1997). Effect of
amino acid and glucose administration during postexercise recovery on
protein kinetics in dogs. American Journal of Physiology, 272(6A):
E1023-E1030.
Reed, M.J., Brozinixk, J.T., Lee, M.C., & Ivy, J.L. (1988). Muscle
glycogen storage postexercise: effect of mode of carbohydrate
administration. Journal of Applied Physiology, 66: 720-26.
Roy, B.D., & Tarnopolsky, M.A. (1998). Influence of differing
macronutrient intakes on muscle glycogen resynthesis after resistance
exercise. Journal of Applied Physiology, 84: 890-896.
Sherman, W.M., & Leenders, N. (1995). Fat loading: The next magic
bullet? International Journal of Sport Nutrition, 5: S1-S12.
Wong, C.W., & Watson, D.L. (1995). Immunomodulatory effects of dietary
whey proteins in mice. Journal of Dairy Res., 2: 359-68.
Zawadzki, K.M., Yaspelkis, B.B. & Ivy, J.L. (1992).
Carbohydrate-protein complex increases the rate of muscle glycogen
storage after exercise. Journal of Applied Physiology, 72:1854-1859.
R.J. Bloomer
Table of Contents
Exercise Performance
Substrate Usage
Exercise Recovery
Recommendations
Much debate centers around meal timing and selection of foods related
to exercise and subsequent athletic performance. With the number of
recommendations that exist concerning the correct nutrients to consume
prior to, during, and immediately following exercise, one would be led
to believe that volumes of data are available on the topic. However,
that which is marketed and spoken about as factual may in reality be
simply speculative. Therefore, the purpose of this article is to
provide some concrete scientific evidence, in addition to practical
applications, for feeding as it centers around an acute bout of
exercise.
Specific to this topic, three distinct areas exist that individuals
should be interested in. These include exercise performance and
related factors (strength, endurance, mood state, etc.), substrate
usage (proportion of fat, carbohydrate, and protein used to fuel the
exercise), and exercise recovery (glycogen resynthesis, protein
turnover, immunocompetence, etc.). While some studies have been done
to look at how feeding effects the above variables, it is important
that readers be aware of a few issues. First of all, from a
scientific standpoint, 'one or two' studies supporting/refuting a
certain claim really does not mean very much. In this case, while
data may look promising, more information would be necessary for firm
recommendations to be made. Secondly, subject number in many of these
scientific experiments is relatively small (< 20) which makes
generalizability to larger sample populations difficult. Thirdly,
data is typically collected in a laboratory setting under the careful
supervision of research staff (i.e., poorly paid graduate students)
and may not apply directly to unsupervised people existing in a
free-living environment. Lastly, there exist other variables such as
route of administration of a particular nutrient (intravenous, oral,
etc.), subject population being tested (obese, diseased, diabetic,
sedentary), etc. that should be thoroughly considered relatively to
study results. One must question whether or not the same results
would be obtained if these things were manipulated to more closely
represent his or her situation. Clearly, certain areas contain
sufficient data to support recommendations, while others are not
nearly as well researched. A well-informed individual should always
take these things into consideration prior to accepting the latest
claim/advice from the local guru. With this understanding, the
following is a brief overview of the few areas that have in fact been
sufficiently studied relative to feeding as it relates to exercise.
Back to Top
Exercise Performance
Although it is widely accepted that protein acts sparingly as an
exercise fuel, claims of strength increases resulting from high
protein intakes are common. While protein supplementation has been
proposed to increase the availability of essential amino acids during
exercise, enhance anabolic processes promoting tissue growth, and
accelerate the rate of recovery following training, no studies to date
have demonstrated conclusively that an (acute or chronic) increase in
protein intake will lead to increased strength performance-when
factoring out other confounding variables (e.g., the increase in total
energy consumed, trained vs. untrained state, etc.).
The results are slightly more conclusive related to endurance
exercise. Amino acid supplementation has been proposed to improve
physiological and psychological responses, perhaps by altering the
ratio of free tryptophan to branch-chain amino acids. It has been
noted that supplementation with branch-chain amino acids (leucine,
isoleucine, and valine) may increase aerobic work capacity and
decrease muscle fatigue (Davis & Bailey, 1997; Mero, 1999). The role
of these amino acids may be of greater importance during periods of
prolonged exercise, in particular when glycogen levels are depleted,
at which time supplemental amino acids may function to provide energy
to fuel aerobic work. However, further research is needed for a
definitive recommendation.
Dietary fat intake has been proposed over the past few years to
improve aerobic exercise performance, possibly by decreasing
carbohydrate oxidation (sparing glycogen usage) during exercise and by
prolonging the onset of fatigue. Although some work has been done to
support this hypothesis (Muoio et al., 1994), the bulk of scientific
evidence at the present time refutes this claim (Sherman & Leenders,
1995). More long-term studies are needed in order to fully understand
the role of dietary fat intake and it's effect on exercise
performance. (Note: most studies involve chronic increases in fat
consumption rather than simply consuming a high fat meal prior to an
acute bout of exercise. In addition, no real claims have been made
for this nutrient pertaining to strength performance).
The one nutrient that has been shown repeatedly to favorably improve
exercise performance is carbohydrate. Specifically, evidence clearly
demonstrates that ingestion of adequate dietary energy and
carbohydrate prior to and during exercise will improve performance by
delaying fatigue, improving mood, and making available additional
glucose to fuel the exercise bout (Balsom et al., 1999; DeMarco et
al., 1999). Data suggest that either liquid or solid carbohydrate may
be consumed with similar results, however liquid feedings may prove
easier to digest and assimilate. In addition, the type, specifically
the glycemic index (GI) of carbohydrate, may be of concern, as lower
GI choices have been found to be more beneficial than their higher GI
counterparts (DeMarco et al., 1999). Pertaining to postexercise
intake, consumption of carbohydrate rich foods and beverages may lead
to enhanced performance of subsequent athletic events, perhaps due to
the effects of loading glycogen into the muscle to be used during
these future (second daily session, next day session, etc.) training
bouts.
Despite the potential benefits of carbohydrate ingestion for
performance of strength exercise (e.g., weight lifting), at present,
little evidence is available to support this hypothesis. Although,
noting that carbohydrate consumption is associated with positive mood
changes (Keith et al., 1991), it is reasonable to expect an increase
strength output under certain mood elevating conditions. However,
systematic studies are necessary before recommendations are made
regarding this practice.
Back to Top
Substrate Usage
Pertaining to substrate utilization (proportion of carbohydrate,
protein, and fat oxidized) during the exercise session, it appears
that one only be concerned with the preexercise feeding in addition to
that which is consumed during the actual session. The postexercise
meal does not seem to influence substrate utilization of subsequent
training bouts unless under conditions of severe nutrient restriction
(e.g., carbohydrate depletion) when fatty acids (and possibly protein)
would largely contribute to fuel the work.
In this regard, feeding in close proximity to the onset of exercise
tends to elevate plasma glucose and insulin, allowing for carbohydrate
sources to be oxidized, with minimal fatty acid usage. While this may
prove beneficial for performance, one should not expect a great deal
of energy to be derived from fatty acid oxidation (little fat will be
used as fuel). In addition, it is known that short-term restriction
of dietary energy (i.e., fasting prior to exercise) is associated with
a greater increase in growth hormone (GH) output. GH is a very
powerful lipolytic (fat burning) hormone. As such, if elevated on a
systematic basis, it would be expected to increase fat
utilization/loss over time. This topic has proven controversial in
the fitness industry, as individuals interested in losing excess body
fat through exercise have been instructed to abstain from feeding for
the period before and during the exercise session. However, while it
is true that GH and fatty acid usage may be elevated following a brief
fast, data addressing the issue of whether or not someone will
actually be more successful at losing body fat long-term if adhering
to this plan is inadequate. As such, recommendations to minimize
nutrient intake for purposes of increasing fatty acid usage, at the
possible expense of decreasing exercise performance, should be
carefully considered.
Back to Top
Exercise Recovery
Perhaps the area of greatest research interest over the past several
years is that of facilitating recovery after an acute bout of
exercise. While disagreement may still exist over the ideal energy
intake and macronutrient composition of the feeding, postexercise
nutrient consumption is a well-accepted practice recommended to all
athletes and fitness enthusiasts.
Both dietary protein and carbohydrate intake have properties that
enhance the training effect on muscle. Specifically, both of these
nutrients support glycogen resynthesis (Roy & Tarnopolsky, 1998;
Zawadzki et al., 1994) and help to facilitate recovery. Studies
support carbohydrate intake in the range of 7-10 g(kg-1 body weight
within 24 hours after exhaustive exercise to normalize muscle glycogen
levels (Applegate, 1999). It has been suggested that glucose/glucose
polymers (preferably high GI choices) and fructose are most effective
at replacing muscle and liver glycogen, respectively (Ivy, 1998).
These may be consumed in either liquid or solid form with similar
results (Reed et al., 1989). However, liquid supplements are
recommended because of their greater ease in digestion. Also,
carbohydrate intake following exercise serves to enhance cellular
hydration when consumed with adequate fluid (Gonzalez-Alonzo et al.,
1992). Specifically, a 6-8% carbohydrate solution should function to
provide rapid rehydration by optimizing gastric emptying and
intestinal absorption. Actually, this type of mixture is best
consumed during the exercise bout to aid in performance, with the
percentage of carbohydrate increasing following exercise in order to
provide adequate carbohydrate to replenish glycogen stores.
The timing of carbohydrate ingestion in relation to an exercise bout
also appears important, particularly for competitive athletes, as
researchers reported an increased rate of glycogen storage in subjects
consuming carbohydrate immediately, rather than two hours postexercise
(Ivy et al., 1988). This may only be of significance if subsequent
training bouts are merely hours (3-4) away. However, for the average
trainee exercising 3-4 days per week, the rate of glycogen resynthesis
following exercise should be of little concern, assuming adequate
carbohydrate is consumed regularly in the normal diet. Further, the
typical 45-60 minute exercise bout is not of sufficient duration (nor
intensity) to markedly exhaust glycogen stores.
Other than carbohydrate, dietary protein, when added to this nutrient,
enhances glycogen storage, as a result of the heightened insulin
response caused by the amino acid ingestion (Roy & Tarnopolsky, 1998;
Zawadzki et al., 1994). In this way, insulin functions to stimulate
both muscle glucose transport and activation of glycogen synthase, the
rate-limiting enzyme in the glycogen synthetic pathway, and leads to
greater glycogen deposition.
The insulin elevating effect of consuming these nutrients
simultaneously has further reaching implications other than replacing
depleted glycogen. For example, such an acute state of
hyperinsulinemia may promote both anabolic and anticatabolic activity
in muscle by stimulating protein synthesis (Fryburg et al., 1995),
decreasing proteolysis (Biolo et al., 1995) and enhancing nitrogen
retention (Okamura et al., 1997). These positive effects on protein
turnover may, over time, favorably influence lean tissue accrual.
Also important is the role of these nutrients in supporting immune
function. High quality whey proteins have been shown to enhance
adaptive immunocompetence when provided as a dietary supplement (Wong
and Watson, 1995). The same is true for carbohydrate supplementation,
particularly during prolonged or intensive exercise (Nieman, 1999).
Mechanistically, carbohydrate supplementation can attenuate the
typical exercise-induced rise in cortisol, and other immune responses
that are related to physiologic stress and inflammation.
In sum, the synergistic effect of carbohydrate and protein consumed
postexercise may have implications for both exercise recovery and
tissue growth, largely as a result of the hyperinsulinemic effect of
consuming these nutrients in combination. As such, nutritional means
of increasing insulin to promote glycogen resynthesis and favorably
effect protein turnover, as well as to support optimal immune
function, should be considered.
Back to Top
Recommendations
Based on the available science, the following conclusions can be
drawn. Individuals attempting to maximize the results of their
exercise training through nutritional means may consider these
suggestions:
1. Consume a moderate sized, balanced meal (200-600 kcal) 2-3
hours prior to exercise. The meal should be composed of foods that
are familiar to the individual and that leave him or her neither
hungry nor with undigested food in the stomach (this may involve
minimizing dietary fat in order to speed digestion time). Extreme
endurance athletes may find that a greater amount of energy and
carbohydrate is beneficial. General fitness enthusiasts exercising
for purposes of fat loss, may consider abstaining from food for the
three to four hour period prior to exercise (or simply train first
thing in the morning). Individuals should experiment with different
regimens to determine which strategy works best for them.
2. Consume fluid liberally throughout exercise. Consider using a
carbohydrate solution (6-8%) containing added electrolytes, rather
than simply pure water. This may be especially beneficial if
exercising in the heat, when fluid loss from sweat is a concern. For
exercise of extreme duration (>2 hours) consider supplementing
carbohydrate (30-60 grams per hour).
3. Consume adequate carbohydrate (1.5 g(kg-1 body weight) and
protein (0.5 g(kg-1 body weight) immediately following exercise in
order to replenish depleted glycogen stores and to favorably impact
protein turnover and immune function.
Back to Top
References
Applegate, E. (1999). Effective nutritional ergogenic aids.
International Journal of Sport Nutrition, 9: 229-39.
Balsom, P.D., Wood, K., Olsson, P., & Ekblom, B. (1999). Carbohydrate
intake and multiple sprint sports: with special reference to football
(soccer). International Journal of Sports Medicine, 20, 48-52.
Biolo, G., Declan Fleming, R.Y., & Wolfe, R.R. (1995). Physiologic
hyperinsulinemia stimulates protein synthesis and enhances transport
of selected amino acids in human skeletal muscle. Journal of Clinical
Investigation, 95:811-819.
Davis, J.M., & Bailey, S.P. (1997). Possible mechanisms of central
nervous system fatigue during exerise. Medicine and Science in Sports
and Exercise, 29: 45-57.
DeMarco, H.M., Sucher, K.P., Cisar, C.J., & Butterfield, G.E. (1999).
Pre-exercise carbohydrate meals: application of glycemic index.
Medicine and Science in Sports and Exercise, 31, 164-70.
Fryburg, D.A., Jahn, L.A., Hill, S.A., Oliveras, D.M., & Barrett, E.J.
(1995). Insulin and insulin-like growth factor-I enhance human
skeletal muscle protein anabolism during hyperaminoacidemia by
different mechanisms. Journal of Clinical Investigation, 96:
1722-1729.
Gonzalez-Alonzo, J., Heaps, C.L., & Coyle, E.F. (1992). Rehydration
after exercise with common beverages and water. International Journal
of Sports Medicine, 13: 399-406.
Ivy, J.L. (1998). Glycogen resynthesis after exercise: effect of
carbohydrate intake. International Journal Sports Medicine, 19:
S142-45.
Ivy, J.L., Katz, A.L., Cutler, C.L., Sherman, W.M., & Coyle, E.F.
(1988). Muscle glycogen synthesis after exercise: effect on time of
carbohydrate ingestion. Journal of Applied Physiology, 64, 1480-85.
Mero, A. (1999). Leucine supplementation and intensive training.
Sports Medicine, 27(6): 347-58.
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