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Pre-Exercise Nutrition
Introduction
There is evidence that the manipulation of eating schedules
may be important to achieve specific goals of sports nutrition
both in daily training and in the competition setting 1). The
effect of eating practices in the pre-exercise period (generally
defined as the period four hours before exercise 1) is the focus
of this paper.
A literature
review revealed that the vast majority of research on this topic
has focused on carbohydrate consumption, particularly as it
pertains to prolonged endurance exercise. This is logical from
a physiological point of view; the total reserves of endogenous
carbohydrate (muscle and liver glycogen) are limited and are
substantially less than the fuel requirements in many athletic
events of this type1. In more intense exercise bouts lasting
less than 90 minutes, limiting factors other than glycogen availability
become more significant, therefore, the value of pre-exercise
carbohydrate consumption becomes less apparent. However, increased
muscle glycogen availability is associated with enhanced intermittent
exercise performance in repeated bouts of high-intensity exercise
2). Essentially, for pre-exercise carbohydrate consumption,
or any other dietary technique, to be effective in enhancing
exercise performance, a specific limiting factor in the performance
of the exercise must be augmented.
In general, the functions of a pre-exercise snack are as follows:
1. Prevent low blood sugar 3) by optimizing muscle and liver
glycogen stores 4,1)
2. Ensure proper hydration 3,4,5)
3. Leave the athlete neither hungry nor with undigested food
in the stomach 5)
4. Provide positive psychological reinforcement that the body
is well fueled 3)
5. Avoid the exaggerated increase in plasma insulin concentrations
that sometimes results in rebound hypoglycemia in susceptible
individuals 1)
In order
to accomplish these functions, the characteristics of the foods
selected should be:
1. High in carbohydrate to maximize glycogen stores 5)
2. Low in fat 3) and fibre to facilitate gastric emptying and
minimize gastrointestinal distress 5)
3. Moderate in protein 5)
4. Familiar and well tolerated, as determined through experimentation
in previous sessions 3,5)
3-4 Hours
Before Exercise
The general rule is that 3-4 hours are needed for a large meal
to digest (obviously, the smaller the meal, the quicker it can
be digested 3). Most (but not all) studies have shown that ingestion
of a meal containing 140-330 grams of carbohydrate 3-4 hours
before exercise has enhanced performance 2). Defining the best
amount of pre-exercise food for an individual is difficult because
tolerances vary greatly from person to person 3). Also, the
amount of carbohydrate consumed in the pre-exercise meal should
be weighed against the total daily amount of Calories required
by the individual i.e. someone with a greater energy requirement
would choose an amount of pre-exercise carbohydrate at the upper
end of this range. The most likely mechanism for the improvement
in performance is maintenance of blood glucose levels during
exercise via increased muscle 1,4) and liver glycogen1.
The 60
Minutes Before Exercise
Warnings to avoid carbohydrate consumption during the hour before
exercise have become part of sports nutrition dogma. These warnings
originated from one study conducted in 1979 whose results were
unfortunately well publicized. Every study since then has shown
either unchanged or enhanced endurance exercise performance
following the ingestion of carbohydrate in the hour prior to
exercise. Indeed, there is little evidence to support such warnings
1,2,4,5).
The theory
to explain the alleged decrease in performance is a physiological
phenomenon dubbed "reactive (or rebound) hypoglycemia".
Studies show that ingesting carbohydrate in the hour prior to
exercise results in a large increase in blood sugar, and consequently,
insulin levels. With the onset of exercise, however, there is
often a rapid but transient fall in blood glucose. This decrease
in blood sugar is attributed to several factors working in unison
2):
1. Increased energy demand of muscle with increased contractile
activity
2. The general action of insulin in taking glucose from the
blood into muscle cells
3. The inhibitory effect of insulin overcoming the stimulatory
effect of exercise induced catecholamines on liver glucose output
4. Suppressed free fatty acid availability due to inhibition
by insulin, and therefore, suppressed fatty acid oxidation i.e.
even more reliance on carbohydrate as a fuel
5. Decreased intestinal absorption of the ingested glucose with
the start of exercise
However, it must be reiterated that neither the drop in blood
sugar, nor the decreased availability of fatty acids as a fuel
has been shown to any appreciable degree to be detrimental to
performance. The drop in blood sugar is transient and the increase
in carbohydrate availability compensates for the greater carbohydrate
utilization 1,4). Although there appears to be little evidence
to support the practice of avoiding carbohydrate ingestion in
the hour before exercise, individual practice should be based
on individual experience 2). Some athletes may be more susceptible
to negative effects than others 2,3). Waiting until 10 minutes
before exercise may be an effective strategy if for some reason
previous meals were skipped or hunger cravings are especially
prominent. Insulin will not have the chance to greatly increase
in this short time period 3). Another strategy for susceptible
individuals is to experiment with low glycemic index foods1
although the data are still mixed on this issue 5).
Dealing
With Problems of Practicality
In practice, if the theoretically ideal pre-exercise meal cannot
be consumed, it is O.K. to choose the best possible pre-exercise
meal1. Oftentimes the choice is dictated by the specific situation
and the unique characteristics of the individual. For example,
each person has unique food preferences and aversions, so no
one magic food or meal will ensure top performance for everyone
3). The following are some examples of situations that complicate
pre-exercise nutrition and some ideas for dealing with them:
Exercise
in The Morning
Liver glycogen is largely the source used to maintain normal
blood glucose levels 3). However, these stores are labile and
may become substantially depleted over the night 1,2,3). Beginning
a workout with low blood sugar is likely to lead to early fatigue
3). Herein lays the importance of eating before morning events
1).
As stated in the previous sections, 3-4 hours may be needed
to digest a large carbohydrate meal. If the exercise session
is early enough to make waking up 3-4 hours before impractical,
a smaller carbohydrate snack can be consumed 30-90 minutes before
exercise 1). It is suitable for the snack to be 200-400 Calories,
with the higher end of the range being more appropriate for
consumption with more time before the exercise 3). Additionally,
if the individual knows they are likely to eat little or nothing
before morning exercise, having a quality high carbohydrate
dinner or snack the night before can compensate somewhat 3).
Digestive
Trouble
The following are some factors that may predispose one to experience
digestive discomfort related to pre-exercise food:
1. Competing
in a sport that jostles the stomach i.e. running as opposed
to a relatively stable sport like cycling
2. Low fitness level
3. Inexperience or young age
4. Female gender
5. High level of emotional and/or mental stress
6. Very high exercise intensity
7. High protein, high fat or high fibre foods as pre-exercise
meal
8. Excess sugar consumption with pre-exercise meal
9. Poor hydration 3)
Eating several
small meals over a period of time reduces the load placed upon
the stomach and may aid in digestion. Another strategy is to
hydrate by drinking fluids between meals, as the extra bulk
when consumed with solid food may upset the stomach. If an individual
knows they simply cannot eat before an event (due to nervousness,
etc.) the best advice is to make sure nutrition on the day before
is optimal 3). Some of the other non-nutritional risk factors
may require the assistance of other professionals like sports
psychologists or exercise physiologists.
Nutrients
Other Than Carbohydrates
Fluids
The fact that water is vital for peak bodily function is unequivocal.
Although a separate paper could be written on the benefits of
proper hydration and the dangers associated with dehydration,
some general guidelines for pre-exercise fluid consumption will
now be discussed.
The day before an event, drink an extra 4-8 glasses of fluid
so that you overhydrate 3). Such a practice is tolerated well
by most people 4). Up to two hours before the start of an exercise
approximately 500 mL of fluid should be consumed 3,5). Drink
another 250 mL 30 minutes before, then another 250 mL 15 minutes
before the exercise begins.
Protein
At least one study found that the addition of essential amino
acids to carbohydrate consumed immediately before exercise resulted
in increased amino acid delivery to muscle and greater net muscle
protein synthesis compared with consumption of the supplement
at various times after exercise 6). As dietitians generally
recommend the consumption of mixed-nutrient meals anyway, it
is prudent to include some protein source along with the standard
pre-exercise carbohydrates. Lean proteins should be favoured
over higher fat proteins, as fat delays gastric emptying 3).
Other Nutrients
Studies have been completed on the effects of a variety of other
nutritional compounds. Some of these are briefly discussed below:
1. Fat - no benefit has been found with high fat diets prior
to exercise 2)
2. Glycerol - has been proposed as a hydration aid, but studies
have found equivocal results 4)
3. Creatine - the practice of creatine supplementation is a
topic that goes beyond the scope of this paper. Look for a review
on this topic in subsequent Communiques.
The Dietitian's
Recommendation
- 3-4 hours before exercise, a familiar meal of well tolerated
foods containing 140-330 grams of carbohydrate should be consumed.
The meal should also be low in fat and moderate in protein.
- Proper hydration is of utmost importance. Drink lots of fluids
the day before an event and a cup (250 mL) 2 hours, 1 hour,
30 minutes and 15 minutes before exercising. Water is usually
the best choice.
- Although it is not necessary to eat during the hour before
exercise, neither is it likely to be harmful to performance
as was once speculated. If you must eat during this time period
(for example, trying to squeeze in a snack before a morning
workout) choose a smaller meal of easily digested foods.
- Oftentimes, obstacles in life prohibit the implementation
of ideal plans and adjustments are needed. For more information
specific to your situation, sport and nutritional requirements,
it is best to seek the advice of a registered dietitian.
Post-Exercise
Nutrition
Introduction
Whether
you are a high level athlete in intensive training, or an office
worker exercising to improve your health, there is little doubt
that proper nutrition is a significant factor in achieving your
goals. The contribution to such goals made by nutrition in the
period immediately following exercise will be the focus of this
review.
Three important concepts need to be considered when examining
any diet plan: Timing (i.e. when) 2) Nutrient quality (i.e.
what kind) 3) Nutrient quantity (i.e. how much) 1). In order
to determine the quality and quantity of nutrients that is of
importance post-exercise, we can look at the demands that exercising
places upon the body.
Carbohydrate is the predominant source of energy for muscle
metabolism during short (<60 sec) bouts of supra-maximal
work and the preferred fuel for muscle for prolonged, moderate
intensity exercise lasting up to 4 hours 2). Several studies
have shown that resistance exercise depletes muscle glycogen
1).
There is data supporting the theory that protein/amino acids
contribute very little as an exercise fuel 3). However skeletal
muscle contains about 50% of total body protein 4) and resistance
exercise substantially elevates protein turnover 1). Also, it
has been proposed that endurance exercise can induce muscle
protein damage, especially when the exercise has a large eccentric
component 3).
In summary,
carbohydrate (in the form of glycogen) and protein (in the form
of skeletal muscle) will be depleted during exercise (be it
resistance or endurance training). Therefore, it is logical
to speculate that these two nutrients may play a key role in
post-exercise nutrition. Indeed, most of the research has focused
on these two nutrients.
Protein
There is solid evidence that individuals who exercise (particularly
athletes) require more dietary protein than sedentary individuals
4,5,6). But exactly how much is required? The 1995 review by
Lemon is an often cited reference on this topic. He concluded
that protein requirements differ based upon the kind of activity
done by the individual. A strength/power athlete has an average
daily protein requirement of around 1.7-1.8 grams per kilogram
of body weight; an individual who regularly engages in endurance
exercise should consume 1.2-1.4 grams of protein per kilogram
per day. This level is approximately double the requirement
for sedentary individuals 3).
Despite
the seemingly dramatic increase in protein requirement for athletes,
most ingest a sufficient amount of dietary protein to meet this
increased need 4,5). A balanced diet containing 12-15 percent
of the Calories as protein will provide amounts of the individual
amino acids more than adequate to meet even the need of those
who exercise extensively 6).
Although
it is not particularly difficult to meet protein requirements
through a typical North American diet, many individuals take
protein or amino acid supplements. In most cases there is no
strong evidence calling for protein supplementation 3,4,7).
Exercising individuals should be advised to heed the requirements
established by Lemon. When total protein consumption (via diet
alone, or diet plus supplementation) exceeds 2 grams per kilogram
of body weight per day, there is no extra muscle growth, nor
any other tangible benefit 3,5). Before beginning any supplementation
regime, an athlete should have their intake assessed by a registered
dietitian.
The positive
effects of attaining the daily protein requirement may be augmented
if the protein is consumed at beneficial times, specifically,
after a workout. The bulk of studies done on the topic of post-exercise
nutrition have not focused solely on protein. However, in his
review examining interactions among diet and resistance exercise,
emphasizing the role of nutrient intake on post-prandial and
diet induced endocrine response, Volek (2004) cited four studies
in which an infusion of amino acids with or without carbohydrate
stimulated protein synthesis after exercise 1).
Carbohydrates
Common recommendations have focused on consuming carbohydrates
immediately post-exercise to capitalize on higher rates of glycogen
synthesis allegedly occurring at this time, thereby maximizing
endogenous carbohydrate stores for the next training session.
However, the bulk of the research indicates that for the average
exerciser, this may not be as important as once assumed 2).
Several recent studies have noted that various carbohydrate
re-feeding schemes produced no difference in 24 hour muscle
glycogen re-synthesis, as long as the same amount of total carbohydrate
was consumed6. Previously, it was thought that 48 hours were
needed to restore muscle and liver glycogen stores to normal
levels following exercise induced depletion. Now it is commonly
accepted that this can be accomplished within 24 hours if the
amount of carbohydrate intake is optimal 2). In other words,
failure to consume carbohydrate in the immediate phase of post-exercise
recovery results in very low rates of glycogen re-synthesis
until feeding occurs 1,2).
These findings do have some importance from at least one practical
perspective. Early carbohydrate consumption is likely beneficial
when an individual performs several exercise sessions over one
day (for example, an athlete in a tournament). This strategy
is most important when there is only 4-8 hours of recovery between
exercise bouts 2). Several studies have shown that ingesting
carbohydrate during the rest interval between two prolonged
exercise bouts improves performance in the second bout. The
carbohydrate can help restore blood glucose levels but may also
be used to re-synthesize muscle glycogen 6).
As the time for recovery becomes longer (>24 hours) it is
likely that any small differences in early rates of glycogen
re-synthesis are unimportant 2).
Protein and Carbohydtares Together
The research indicates that the most benefits are derived when
post-workout feeding includes both protein and carbohydrate
sources together.
Effect
on Muscle
It has been demonstrated that protein and carbohydrate ingestion
within the immediate hours after resistance exercise potentiates
the effect of exercise alone in stimulating protein synthesis
1,4,8,9). It has also been suggested that the earlier these
nutrients are supplied, the more the muscle protein synthesis
rate is amplified 1,8). A positive protein balance is attained
by employing this post-exercise protein/carbohydrate strategy
via the following possible mechanisms: enhanced protein synthesis,
4) decreased myofibrillar protein breakdown, 4,5) promotion
of an anabolic hormonal environment, particularly an increase
in insulin, 1,4,5,8,9) hastened recovery, possibly through enhanced
glycogen re-synthesis 5).
Effect
on Glycogen
Research shows that the rate of glycogen re-synthesis is not
slowed by incorporating protein into the post-exercise feeding1.
In fact, the combination of protein and carbohydrate may induce
an even faster rate of glycogen synthesis in 4 hours compared
to either the protein or carbohydrate given alone 6).
Possible
Chronic Adaptations
The acute effects of increased net protein balance caused by
protein and carbohydrate re-feeding immediately after sessions
of resistance exercise may lead to the chronic adaptation of
an increased muscle mass if accumulated over an extended period
of time 5,7,8). This result would be desirable for athletics,
as well as overall good health. However, there is a need for
longer term studies addressing whether these repeated metabolic
alterations are of sufficient magnitude to augment the long
term adaptations to resistance training 1,7) itself.
Conclusions
Early protein consumption after exercise promotes acute positive
net protein balance. Consumption of greater than 2 grams of
protein per kilogram of body weight per day, regardless of how
much or what type of physical activity is performed throughout
the day, is unnecessary and unproductive.Early carbohydrate
consumption after exercise is of particular importance when
the recovery time before the next exercise session is short
(4-8 hours), but full glycogen re-synthesis can occur with appropriate
eating within 24 hours, regardless of the timing of carbohydrate
ingestion.
Consumption
of a snack containing both protein and carbohydrate after exercise
is the strategy all active individuals should follow. This may
lead to increased muscle protein synthesis and faster glycogen
re-synthesis.
More studies are needed on the long term effects of post-exercise
nutrition.
The Dietitian's Recommendation
Whether you do strength/power training, endurance training or
just workout to stay in shape, you should always eat as soon
as is practical after you exercise. The meal should have carbohydrate
(to ensure your glycogen stores are topped up and ready to supply
you with the energy you`ll need for the next workout) as well
as protein (to promote the building of lean muscle tissue).
Some examples of snacks that fit this criteria are:
Turkey sandwich on whole wheat bread with 2 tablespoons of low
fat mayonnaise and an apple (61 grams carbohydrate; 17 grams
protein)
1 cup of trail mix (71 grams carbohydrate; 22 grams protein)
375 mL of 1% chocolate milk and a banana (67 grams carbohydrate;
13 grams protein)
Geni-Soy bar and 1 cup of Gatorade (49 grams carbohydrate; 14
grams protein)
Reference
List
1. Volek,
J.S. (2004). Influence of nutrition on response to resistance
training. Medicine and Science in Sports and Exercise, 36(4),
689-696
2. Hawley, J.A. & Burke, L.M. (1997). Effect of meal
frequency and timing on physical performance. British Journal
of Nutrition, 77 Suppl1, S91-103
3. Lemon, P.W. (1995) Do athletes need more dietary protein
and amino acids? International Journal of Sports Nutrition,
5, S39-61
4. Houston, M.E. (1999). Gaining weight: the scientific basis
of increasing skeletal muscle mass. Canadian Journal of
Applied Physiology, 24(4), 305-316
5. Kreider, R.B. (1999). Dietary supplements and the promotion
of muscle growth with resistance exercise. Sports Medicine,
Feb:27(2), 97-108
6. Williams, M.H. (1999). Nutrition for Health, Fitness and
Sport 5th Ed. U.S.A.; WCB McGraw-Hill
7. Gibala, M.J. (2000). Nutritional supplementation and resistance
exercise: what is the evidence for enhanced skeletal muscle
hypertrophy? Canadian Journal of Appied. Physiology, 25(6),
524-535
8. Suzuki, M. (2003). Glycemic carbohydrates consumed with
amino acids or protein right after exercise enhance muscle formation.
Nutrition Reviews, 61(5, pt.2), S88-S94
9. Chandler, R.M. et al. (1994). Dietary supplements affect
the anabolic hormones after weight training exercise. Journal
of Applied Physiology, 76(2), 839-845
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