|
In recent years, post-exercise nutrition has evolved as an imperative
part of training regimens among athletic populations. Athletes of all
ages, abilities, and skill levels are adopting some form of post-exercise
nutrition to improve performance and enhance the body's recovery processes
following exercise. Athletes in particular are highly susceptible to the
detriments of heavy training regimens, because they are constantly depleting
their energy substrates and stressing skeletal muscle tissues simultaneously.
The macronutrients that have drawn much attention, in reference to the
recovery phase of exercise, are protein and carbohydrates. Protein and
carbohydrates have their own distinct functions, yet both work to generate
an anabolic state within the body when ingested after the completion of
an exercise bout. It is necessary for individuals who seek to gain lean
muscle mass to induce a positive protein turnover as often as possible.
It has been confirmed that protein and/or amino acid ingestion is required
to reach a positive protein/nitrogen balance (Borsheim et al., 2004a;
Koopman et al., 2006;
Tipton et al., 2004),
and carbohydrate ingestion alone provides marginal benefits on protein
synthesis rates (Roy, 1997).
Carbohydrate intake during recovery has been shown to replenish depleted
glycogen after intense or exhaustive exercise (Ivy et al., 2002;
Ivy et al., 1988b;
Reed et al., 1989).
The addition of protein can further enhance this process (Ivy, et al.,
2002),
but only in situations when an inadequate amount of carbohydrate is made
available in the diet (van Loon et al., 2000).
A lack of glycogen stores in the muscle and liver will limit the performance
capacities of the body during prolonged or higher intensity bouts of exercise
(Coyle et al., 1986).
The provided evidence clearly denotes the importance these two macronutrients
have in regards to post-exercise nutrition and anabolism. Therefore, the
purpose of this review is to discuss the impact of dietary protein and
carbohydrate intake during the recovery state on muscle protein synthesis
and glycogen synthesis.
Resistance
training and protein turnover
It is of paramount importance to delineate the role that resistance training
plays in protein turnover. The work of Biolo and colleagues (Biolo et
al., 1995b)
examined protein synthesis and degradation rates before and three hours
after resistance training in healthy untrained men. Three hours after
the exercise bout, protein turnover and amino acid transport increased
in addition to increases in protein degradation above baseline levels,
resulting in a net negative protein balance. Similar findings were reported
by (Phillips et al., 1999),
as they measured fractional synthesis rates (FSR) and breakdown rates
(FBR) in resistance trained and untrained individuals. FSR values increased
when measured immediately following the resistance training bout. However,
FBR values were also elevated in response to resistance training, concluding
that protein turnover rates remained negative (protein degradation rates
remained higher than synthesis rates). One interesting note of this study
was that FBR, following the training session, increased higher in the
untrained subjects. This demonstrates that trained individuals with adequate
resistance training experience will undergo reduced protein turnover rates
when compared to untrained counterparts. The duration of elevated protein
synthesis rates following resistance training was analyzed by Chesley
and colleagues (Chesley et al., 1992).
Their results show a 50% and 109% increase in protein synthesis rates
at four and 24 hours post- exercise respectively. This elevation seems
to diminish and return to near baseline values between 36 hours (MacDougall
et al., 1995)
and 48 hours (Phillips et al., 1997)
after the exercise bout. Several other previous investigations have also
supported these findings on the relationship between resistance exercise,
void of nutritional intervention, and protein turnover (Biolo et al.,
1995a;
Durham et al., 2004;
Hasten et al., 2000;
Phillips et al., 2002;
Pitkanen et al., 2003;
Tipton et al., 1996).
Protein
intake
A number of studies have identified the effects of dietary protein intake,
void of any exercise intervention, on markers of protein synthesis. Tipton
and colleagues (1999b)
supplemented four healthy volunteers with 13.4 g of essential amino acids
(EAA) and 35 g of sucrose which elevated arterial EAA concentrations between
100% and 400% between 10 and 30 minutes post-ingestion. This demonstrates
that the combination of a small quantity of amino acids coupled with a
carbohydrate (an approximate 3:1 ratio) can effectively stimulate protein
synthesis at rest. Furthermore, an EAA dosage similar to the Tipton investigation
(15 g) increased protein (fractional) synthesis rates in young (10.3%/hr)
and older (8.8% hr) adults in a parallel fashion measured by arterial
phenylalanine concentrations (Paddon-Jones et al., 2004).
Absorption speed of amino acids is a critical factor in regulating post-prandial
protein accretion. Whey protein was superior to that of casein in upregulating
protein synthesis (Boirie et al., 1997)
due to its ability to digest more rapidly than casein protein. Free form
amino acid ingestion acts similarly to whey by displaying a rapid and
strong increase in aminoacidemia (Dangin et al., 2001).
This is especially important in elderly populations, because a quickly
digested protein can increase protein synthesis/stores sufficiently and
provide some alleviation against protein losses accompanied with aging
when compared to a slower digested protein (Dangin et al., 2003).
In the larger scope of things, it appears that protein synthesis rapidly
increases for up to two hours after amino acid administration (Bohe et
al., 2001),
and that a dose-dependent response between amino acid availability and
protein synthesis exists up to a point of complete saturation and possible
inhibitory mechanisms of amino acid uptake by muscles (Bohe, et al., 2001;
Monneret et al., 2003).
Several other previous investigations have also supported these findings
in regards to protein ingestion void of exercise intervention (Kobayashi
et al., 2003;
Paddon-Jones et al., 2003;
Tang et al., 2009;
Volpi et al., 2000;
2003).
Protein
intake and resistance training
The intervention of dietary protein or amino acid supplementation in conjunction
with resistance training has proven to effectively increase protein synthesis
rates. An original investigation in this area of research (Biolo et al.,
1997)
evaluated the effects of intravenous infusion of amino acids (alanine,
phenylalanine, leucine, and lysine) at rest and following a lower extremity
resistance exercise bout. Their findings revealed a 291% increase in protein
synthesis following the exercise bout, while protein degradation remained
unchanged from baseline quantities, a response most largely influenced
from the 30% - 100% increase in amino acid transport to the active muscle
tissue following exercise. Similar protocols in the elderly resulted in
augmented rates of protein synthesis accompanied with unchanged rates
in muscle protein breakdown which generated a positive protein balance
(Volpi et al., 1998).
The practicality, however, for these study designs comes into question
due to difficulties associated with intravenous infusion of amino acids
after resistance training. Therefore, other researchers have assessed
the efficiency of oral administration of amino acids and protein following
resistance training. Borsheim and colleagues (2002)
found that 3g of EAA ingested one and two hours following a resistance
training bout increased protein balance in a similar fashion. Furthermore,
it has been established that post-exercise EAA supplementation stimulates
protein synthesis, in conjunction with a positive protein balance, comparable
to that of intravenous infusion of amino acids (Tipton et al., 1999a),
and non-EAA are not necessary to achieve post-exercise anabolism (Borsheim
et al., 2002;
Tipton et al., 1999a).
Esmarck and colleagues (2001)
investigated the effectiveness of an oral supplement containing 10g of
protein, 7g of carbohydrates, and 3g of fat when taken immediately after
or two hours following resistance training on muscle hypertrophy and strength
in thirteen elderly men. The cross-sectional area of the vastus lateralis
following twelve weeks of resistance training increased when subjects
ingested the post-workout supplement immediately upon completion of all
training sessions, whereas when taken two hours after completion, no changes
in muscle cross-sectional area were observed. In this study, it was not
necessary to measure protein synthesis levels, since the increase in muscle
cross-sectional area (hypertrophy) is indicative of a net positive protein
balance. Also, these conclusions give insight on the possible time course
for consuming post-exercise protein and/or amino acids, as hypertrophy
resulted only when protein was immediately ingested upon cessation of
resistance training. A more recent study (Tipton et al., 2004)
explored the acute protein balance after exercise when two different proteins
were consumed following resistance training. Twenty three non-resistance
trained subjects ingested a placebo, 20g of whey, or 20g of casein one
hour after completing ten sets of eight repetitions of leg extensions
at 80% of their respective one-repetition maximums. Casein and whey protein
ingestion yielded similar values of net positive protein balance, and
thus an overall increase in protein synthesis (see Figure
1). A later analysis revealed that soy protein increased protein synthesis
in rats similar to that of whey after a treadmill exercise protocol (Anthony
et al., 2007).
A human trial, however, concluded that milk proteins (caseins and whey)
in comparison to soy promoted greater muscle protein accretion when they
were ingested after regular resistance training (Wilkinson et al., 2007);
a response linked closely to their known impacts on splanchnic and peripheral
metabolism, respectively (Fouillet et al., 2002).
Whey hydrolysate ingested after a resistance exercise bout acutely stimulated
mixed muscle protein synthesis 31% greater than soy (Tang, et al., 2009),
and post-exercise ingestion of fat-free milk significantly increased lean
body mass to a greater extent than soy protein after 12 weeks of resistance
training (Hartman et al., 2007).
In addition, protein plus amino acid supplementation can up-regulate muscle
protein synthesis in conjunction with resistance training (Willoughby
et al., 2007),
but it seems unnecessary to combine protein and amino acids in an attempt
to further stimulate muscle protein synthesis if an adequate amount of
protein (20 g) is ingested (Tipton et al., 2009)
immediately before or after a resistance exercise bout (Tipton et al.,
2007).
Up
to this point, several conclusions can be determined from the previous
studies: 1) Resistance training increases protein synthesis as well as
protein degradation, 2) This increase in protein synthesis is overshadowed
by a corresponding elevation in protein degradation, resulting in a net
negative protein balance, 3) The intake of dietary protein and/or amino
acids after completion of a resistance training bout augments a net positive
protein balance, resulting in the potential for skeletal muscle hypertrophy
over time (Cribb and Hayes, 2006;
Cribb et al., 2007;
Hayes and Cribb, 2008;
Kerksick et al., 2006;
Willoughby et al., 2007),
and 4) The intake of dietary protein and/or amino acids immediately following
resistance training is more effective in inducing hypertrophy than if
nutrient intake is postponed. Figure 2 illustrates the information discussed up to this
point regarding the possible nitrogen balance states.
Carbohydrate/protein
intake and resistance training
Researchers have additionally defined the functions that carbohydrates
carry out in relation to post-exercise nutrition, specifically pertaining
to resistance training and protein synthesis. Studies have looked at the
effectiveness of carbohydrate consumption alone following exercise, as
well as in combination with protein and/or amino acid supplementation.
Roy and colleagues (1997)
investigated the effect of a glucose supplement administered immediately
and one hour after resistance training on anabolic and catabolic markers
in resistance trained men. Subjects completed four sets of approximately
8-10 repetitions each of unilateral leg press and knee extension exercises
at 85% of 1RM, and either received a glucose supplement (1g·kg-1)
or placebo following the exercise bout. Fractional muscle protein synthesis
rates in the exercised leg muscle increased 36%, where only a 6% increase
was observed in the placebo condition. Urinary urea excretion and
3-methlyhistidine (markers of muscle protein degradation) were lower in
the treatment group (urea excretion: 8.6 g·g-1 creatinine,
3-methlyhistidine: 110.4 μmol·g-1 creatinine) compared
to placebo (urea excretion: 12.3 g·g-1 creatinine, 3-methlyhistidine:
120.1 μmol·g-1 creatinine), signifying a reduction in
protein degradation. The overall effect of the glucose supplement caused
a suppressed protein degradation rate compared to the placebo group, resulting
in a more positive protein balance. These results were supported by a
later analysis that concluded 100 g of carbohydrates improves overall
protein balance when ingested one hour following a resistance exercise
bout (Borsheim et al., 2004b).
Borsheim and colleagues (2004b)
determined if an amino acid, protein, and carbohydrate solution elicited
a greater anabolic response following resistance training than carbohydrates
alone. Eight recreationally active subjects completed two trials of 10
sets X 8 repetitions of leg extensions, and ingested either a solution
containing 77.4g carbohydrates, 17.5g of whey protein, and 4.9g of amino
acids or 100g of carbohydrates for each trial, one hour upon cessation
of exercise. Arterial phenylalanine concentration increased rapidly in
the protein/amino acid/carbohydrate group and remained elevated until
210 minutes after the completion of exercise, causing a net positive muscle
phenylalanine balance for a short period. Phenylalanine concentrations
remained close to baseline levels in the carbohydrate group, inhibiting
net muscle phenylalanine balance from reaching a positive state. Therefore,
the addition of protein and amino acids to a carbohydrate solution increases
net muscle protein synthesis to a higher degree than carbohydrates alone
and shifted the overall balance of muscle protein metabolism to a positive
state. A more recent study (Tang et al., 2007)
elicited similar results to the Borsheim investigation after eight resistance
trained males completed two unilateral trials in random order and ingested
either a whey mixture (10g of whey and 21g of fructose) or carbohydrate
mixture (21g of fructose and 10g of maltodextrin) following the completion
of each trial. In both nutritional conditions, muscle protein synthesis
after the exercise bout was elevated in the exercised leg when compared
to their respective resting legs. Moreover, fractional synthesis rates
were significantly higher when whey protein was ingested when contrasted
with only carbohydrate ingestion. These studies provide solid evidence
that carbohydrates only have a minimal effect on protein synthesis in
the absence of protein ingestion, but can be depended upon as a nutrition
source to minimize protein breakdown when ingested alone. With this being
said, a small amount of whey protein in addition to carbohydrate consumption
in the recovery phase of exercise is a more sufficient means of increasing
protein synthesis. Koopman and colleagues (2007)
found supporting evidence, as they examined the effects of ingesting differing
amount of carbohydrates with adequate protein intake on post-exercise
protein synthesis. Healthy volunteers completed three resistance training
bouts, separated by one week of rest, and consumed protein (3 g·kg-1·hour-1)
with 0, 0.15, or 0.6 grams of carbohydrate/kg/hour respectively for each
trial during a six hour time period following exercise. Protein synthesis,
protein degradation, and net muscle protein synthesis values were constant
across all groups. This suggests that carbohydrates, when supplemented
with adequate quantities of dietary protein, do not heighten the anabolic
response when consumed during the post-exercise period. The interested
reader is encouraged to read these additional studies pertaining to the
effects of combining protein with carbohydrates following resistance training
on muscle protein synthesis (Koopman et al., 2005;
Rasmussen et al., 2000;
Tipton et al., 2001).
This group of studies has given insight on several important components
related to anabolism in the post-exercise state. Carbohydrates alone seem
to have a minimal effect on the net protein balance following exercise.
Whether they marginally reduce protein degradation or slightly increase
protein synthesis, carbohydrates unaccompanied by protein are unable to
generate a positive protein balance and stimulate skeletal muscle hypertrophy.
Different forms, sources and/or quantities of protein supplemented with
carbohydrates can interact to create a greater anabolic environment in
the post-exercise state by elevating protein synthesis levels far greater
than carbohydrates alone could initiate. If a positive protein balance
and subsequently muscle hypertrophy is desired, protein must be added
to carbohydrate supplementation in order to fuel these processes. The
combined effects of carbohydrate and amino acid/protein supplementation
on protein synthesis are equivalent to their independent effects (Miller
et al., 2003).
Glycogen
synthesis
Glycogen is a vital fuel source for high intensity and prolonged exercise,
and the dependence of this energy substrate increases as exercise intensity
rises (Bergstrom and Hultman, 1966;
1967).
Consequently, glycogen synthesis during the post-exercise time period
is essential for replenishing energy stores and aiding the body in the
recovery process. It has been determined that glycogen synthesis following
exercise occurs in two distinct phases. The rapid phase lasts approximately
30-60 minutes and does not require the presence of insulin (Maehlum et
al., 1977).
This phase likely transpires when glycogen reserves have been depleted
to extremely low levels (Maehlum et al., 1977),
or if carbohydrates are ingested immediately following the exercise bout
(Ivy et al., 1988a).
The other phase of glycogen synthesis is the slow phase, which can last
up to several hours if carbohydrate availability is high and insulin levels
remain elevated (Ivy, 1991).
Timing
of nutrient intake
The timing of post-exercise nutrition is an important factor to consider
when attempting to replenish glycogen stores that may have been depleted
from exercise. Ivy and colleagues (1988a)
demonstrated this phenomenon by evaluating the effectiveness of a 25%
carbohydrate solution given to cyclists immediately or two hours after
70 minutes of nonstop exercise on a cycle ergometer. During the initial
two hours of recovery, glycogen synthesis was much higher in the individuals
who consumed the solution immediately after exercise. The cyclists who
ingested the solution two hours after the exercise bout saw an increase
in glycogen synthesis during hours three and four, but this elevation
still remained below those who ingested carbohydrate at the earlier time
point. Ivy concluded that delaying nutrient (carbohydrate) intake by two
hours after a prolonged exercise bout decreases muscle glycogen synthesis
by 45% when measured four hours after the completion of exercise. Parkin
and colleagues (1997)
evaluated the effects of delaying nutrient intake on muscle glycogen synthesis
following a strenuous exercise bout in endurance trained men. A total
of five high glycemic meals were fed to the subjects over a 24 hour period
in a manner that allowed one treatment group to receive nutrients roughly
two hours after the other at all time points. At eight and 24 hours after
exercise, both treatment groups displayed similar muscle glycogen stores.
These findings imply that delaying nutrient intake by two hours after
a prolonged exercise regimen has no effect on the rate of muscle glycogen
synthesis. This is an area in the literature where some conflict and gray
areas have presented themselves. Ivy et al. 's work measured glycogen
synthesis rates up to four hours post-exercise, where the work by Parkin
and colleagues determined glycogen synthesis rates over an eight hour
period but changed the amount of carbohydrates ingested in the immediate
feeding group from 0.8 g·kg-1·hour-1 in the first
four hours to none thereafter. It is not unreasonable to believe that
if the feedings remained constant, glycogen stores might have been higher
at the eight hour time point. Other investigations have demonstrated mixed
results as well. Additional work by Ivy and colleagues (1988b)
showed delayed glycogen synthesis rates of 22% and 24% from hours two
to four during exercise recovery in contrast with the immediate two hour
window following exercise and carbohydrate ingestion. On the contrary
and in agreement with the Parkin et al. study, no differences in glycogen
synthesis rates have been reported between the first 60-120 minutes after
exercise and the 60-120 minutes thereafter (Jentjens et al., 2001;
Reed, et al., 1989).
The dissimilarities observed in these studies are possibly attributable
to the amount and composition of the carbohydrates ingested along with
the degree to which the individuals participating in these investigations
were glycogen depleted. Also, a participant's fitness level may play a
factor, as endurance trained individuals have shown an ability to replenish
glycogen stores more rapidly than untrained counterparts (Hickner et al.,
1997).
Due to the inconsistency in the Parkin study and possible limitations
of others, it can be assumed that athletes should intake nutrients immediately
or soon after the completion of a prolonged or high intensity exercise
bout, especially if quick replenishment of glycogen stores is required.
If fast glycogen recovery is unnecessary and the goal is long-term maintenance
of carbohydrate stores, a daily carbohydrate intake greater than 8 g·kg-1·day-1
is recommended to effectively maintain glycogen stores during repeated
days of endurance training (Kirwan et al., 1988;
Sherman et al., 1993).
Type
and form of nutrient intake
Different types of carbohydrates initiate different outcomes on glycogen
synthesis and ultimately muscle and liver glycogen storage. The Glycemic
Index (GI) was created to distinguish the blood glucose response, and
corresponding insulin response, of a specific food after its digestion
in comparison with the glucose response of a standard amount of glucose/white
bread; the GI is intended to resemble the rate at which a particular food
is digested and absorbed into circulation (Wolever et al., 1991).
Researchers have scrutinized different GI foods in relation to their ability
to accelerate glycogen synthesis. Blom and colleagues (1987)
evaluated muscle glycogen synthesis rates when glucose, sucrose, and fructose
were ingested at zero, two, and four hours after an exhaustive cycling
bout. Glucose and sucrose supplementation initiated a greater increase
in glycogen synthesis when compared to fructose ingestion. Fructose must
be catabolized in the liver before it can enter circulation through the
blood and contribute to glycogen synthesis within skeletal muscle. It
would appear that fructose reduces the availability of circulating glucose
compared to other sugars even though contradicting evidence exists (Wallis
et al., 2008),
and in one case, sucrose replenished glycogen stores to a lesser extent
when contrasted with a glucose polymer solution (Bowtell et al., 2000).
Kiens and colleagues (1990)
explored the effects of ingesting a high or low GI meal, containing 70%
of calories from carbohydrates, following exercise on glycogen synthesis
rates. Subjects who consumed the high GI meal experienced a 61% larger
increase in muscle glycogen synthesis rates. These studies conclude that
high GI foods/carbohydrates are more promising in replenishing glycogen
stores in the early hours following exercise, and in addition, the mode
of nutrient application (oral or IV) does not seem to matter (Blom, 1989).
Other research (Keizer et al., 1987;
Reed, et al., 1989)
has focused on the effects of ingesting a solid or liquid meal following
exercise on the rate of glycogen synthesis. Both of these inquires reached
similar conclusions by determining that carbohydrates in liquid and solid
form are equally effective in replenishing glycogen stores after exhaustive
bouts on a cycle ergometer and that gastric emptying does not impede the
process of glycogen synthesis following exercise.
Amount
of nutrient ingestion
Another factor that is of upmost importance in determining the rate of
glycogen synthesis after exercise is the amount of carbohydrates (determined
by body weight) ingested. The typical rate of muscle glycogen storage
after carbohydrate supplementation immediately upon cessation of exercise
is 20-50 mmol/kg dw/h (Blom, 1989;
Blom et al., 1987;
Ivy et al., 1988a;
Jentjens and Jeukendrup, 2003;
Maehlum et al., 1977;
1978;
Piehl Aulin et al., 2000;
Reed, et al., 1989;
Tarnopolsky et al., 1997;
Zachwieja et al., 1991).
Little research has focused on the direct rates of carbohydrate supplementation
and its effects on muscle glycogen synthesis. Blom and colleagues (Blom,
et al., 1987)
first demonstrated this phenomenon by increasing the rate of muscle glycogen
storage approximately 150% when increasing the amount of carbohydrate
intake from 0.18 to 0.35 g·kg-1·hour-1. More recent
studies have analyzed the effects of higher quantities of carbohydrate
consumption on glycogen storage rates. Ivy and associates (Ivy et al.,
1988b)
utilized dosages of 0.75 and 1.5 g·kg-1·hour-1 over
a four hour period following a 120 minute cycling bout. The results indicate
similar rates of glycogen synthesis for both treatment dosages which alludes
to a possible cap or maximum rate of nutrient consumption that can effectively
increase glycogen storage. Nonetheless, it has been found that increasing
post-exercise carbohydrate intake to 0.8 to 1.2 g·kg-1·hour-1
results in higher rates of glycogen synthesis (van Loon, et al., 2000),
and it seems that a carbohydrate dosage of 1.2 g·kg-1·hour-1
is optimal for reaching maximal post-exercise muscle glycogen synthesis
rates (Jentjens and Jeukendrup, 2003;
Jentjens, et al., 2001;
van Loon, et al., 2000).
Many studies display similar findings and support the notion that increasing
the amount of carbohydrate intake above 0.35 g·kg-1·hour-1
can further stimulate glycogen synthesis (Casey et al., 1995;
McCoy et al., 1996;
Piehl Aulin, et al., 2000;
Tarnopolsky, et al., 1997;
van Hall et al., 2000).
Intervention
of protein
The addition of protein to carbohydrate consumption in the post-exercise
period has led to mixed results. Zawadzki and colleagues (1992)
investigated the effects of carbohydrate, protein, and carbohydrate plus
protein supplements on muscle glycogen synthesis after two hours of cycling.
Participants ingested either 112g of carbohydrates, 41 grams of protein,
or 112 grams of carbohydrate and 41 grams of protein immediately, and
two hours after three separate exercise bouts. Supplementing carbohydrates
and protein together resulted in higher glycogen stores than the carbohydrate
and protein groups. Original research in this area concluded that the
increase in glycogen synthesis is directly related to the upregulatory
effect that certain amino acids have on insulin (Floyd et al., 1966;
Knopf et al., 1966).
Therefore, it is reasonable to believe that carbohydrate plus protein
intake following exhaustive exercise will further enhance glycogen synthesis
over carbohydrates alone. On the contrary, the study design of Zawadzki
limits the generalization of the findings due to the variation in caloric
values provided to the treatment groups. Glycogen stores may have been
further replenished in the carbohydrate + protein group, because additional
calories were consumed. A more recent study took a second look at the
possible impact of protein + carbohydrate supplement on glycogen synthesis
when compared to a carbohydrate solutions of equal caloric value and equal
carbohydrate content (Ivy, et al., 2002).
Cyclists completed two hours of exercise on three different occasions
to analyze all treatments. Supplements were ingested immediately and two
hours following exercise. Four hours after the completion of the exercise
bout, 47% of glycogen depleted during exercise bout was restored in the
carbohydrate + protein group. The equal caloric value and carbohydrate
content groups experienced 31% and 28% glycogen restorations respectively.
The conclusions of the Ivy study help to support the inferences made earlier
by Zawadzki and colleagues. It is apparent that protein can further augment
glycogen synthesis when ingested with an adequate amount of carbohydrates.
However, conflicting evidence does exist. van Loon and colleagues (2000)
concluded that the ingestion of ample carbohydrates is the limiting factor
in determining the magnitude of glycogen synthesis after exercise. When
protein was added to a sufficient carbohydrate solution, glycogen synthesis
was not further stimulated. Other investigations have seen enhanced rates
of glycogen synthesis during the post-exercise period (Berardi et al.,
2006;
Bowtell et al., 1999;
Tarnopolsky, et al., 1997)
whereas others disagree (Carrithers et al., 2000;
Jentjens, et al., 2001;
Yaspelkis and Ivy, 1999).
In conclusion, it looks as if glycogen synthesis can increase with the
addition of protein under certain circumstances, although some evidence
lacks in supporting this claim. A summary of factors affecting glycogen
synthesis immediately after exercise are displayed in Figure
3.
|
|
,
Lem Taylor2 and Chad Kerksick1
