BRIDGING THE GAP
RESEARCH and PRACTICALITY
Fitness gains do not occur during exercise. Exercise is a stress that creates the stimulus for adaptations. These adaptations require proper rest and optimal nutrition. Muscular strength and endurance are achieved when adequate nutrients are available for muscle glycogen synthesis and muscle protein synthesis during recovery. Optimal nutrition enhances performance and recovery while combating the muscle mass loss, bone density loss, fatigue, injury, and decreased immune function that are associated with inadequate caloric intake. Post-exercise nutrition should focus on adequate hydration, carbohydrates for glycogen synthesis, and protein for muscle protein synthesis. To maximize the multitude of effects conferred by endurance and resistance exercise, nutrient amount, composition, and timing must be taken into consideration(1).
Glycogen is the storage form of glucose that is stored in the liver and skeletal muscle to be released as glucose into the bloodstream when the body needs energy. Exercise decreases the body’s glycogen stores, which then must be replenished through carbohydrate intake. Thus, inadequate carbohydrate intake post-exercise can result in impaired recovery. Recommended carbohydrate intake for athletes is 6 to 10 grams per kilogram of body weight daily depending on activity level and daily caloric expenditure (ref). Found in whole grains, fruits, and vegetables, carbohydrates are an integral part of the recovery process (1).
Additionally, exercise breaks down skeletal muscle and the subsequent repair, facilitated by nutrition, allows for changes in muscle strength, size, and quality. Resistance training, for example weight-lifting, elicits muscular hypertrophy (changes in muscle size) and changes in muscle strength while endurance training, like biking, causes qualitative muscular improvements. Muscle protein breakdown and muscle protein synthesis, the processes by which muscles adapt, require available amino acids (the building blocks of protein). Therefore, protein ingestion after exercise enhances skeletal muscle adaptations in response to exercise. Increased muscle protein synthesis and decreased muscle protein breakdown require essential amino acids found in food to rebuild and remodel muscle, making proper post-exercise nutrition essential to changes in strength and body composition. Clearly dietary carbohydrates and protein are necessary for post-exercise recovery, but the question of which nutrients should be consumed after different types of exercise, the ideal ratio of nutrients, and the time frame of feeding is more complicated.
Endurance Exercise and Interval Training
Endurance exercise is continuous activity performed over an extended period of time (long distance running), while interval training is high-intensity exercise performed intermittently over a period of time (sprints). Both types of exercise cause muscle glycogen depletion (2). To counteract glycogen depletion and ensure proper recovery, carbohydrates should be consumed within 30 minutes of exercise. Following exercise there is a two hour “window of opportunity” or “metabolic window” during which carbohydrate intake results in higher levels of glycogen than if intake is delayed by 2 hours, so it is important to begin replenishing shortly after the completion of an exercise session (ref). Recommended carbohydrate intake is 1.2 grams per kilogram of bodyweight during the first 30 minutes and, every 2 hours, throughout a 4 to 6 hour period. Additionally, carbohydrates with a high glycemic index (how quickly carbohydrates are converted to glucose in the body) cause higher muscle glycogen levels 24 hours after exercise due to the larger insulin response to high glycemic foods (potatoes, raisins, oatmeal), which helps move glucose to muscle cells for recovery (1). Carbohydrates are integral for glycogen synthesis and the ability to perform optimally in subsequent bouts of exercise, but a mixture of protein and carbohydrates may confer added benefits.
The research findings concerning carbohydrate and protein consumption after exercise is mixed. (2-4). Some studies have shown that a combination of carbohydrate and protein may reduce muscle soreness, enhance subsequent muscle function, and positively impact glycogen synthesis (3). One study suggests that a beverage composed of a combination of carbohydrates and protein resulted in greater glycogen synthesis that carbohydrates alone, however these findings may be because the mixed beverage was higher in overall calories (2).
In contrast, another study that compared the effects of beverages with similar calorie content but different ratios of carbohydrates, protein, and fat (carbohydrate only, high-carbohydrate/low-protein, and low-carbohydrate/high-protein) found no significant difference on muscle recovery and exercise performance (3). The findings suggest that a combination of protein and carbohydrates is beneficial when carb intake is suboptimal following exercise. This finding has positive implications for prolonged exercise after which the consumption of adequate carbohydrates is not feasible due to suppressed appetite and the mere quantity of calories expended (4). Furthermore, protein’s stimulating effect on muscle protein synthesis following muscle breakdown caused by exercise may enhance the qualitative muscular adaptations of endurance exercise (4).
Resistance and strength training employs weighted repetitions to increase lean body mass, muscle strength and size. Since muscle burns calories, resistance exercise can speed up metabolism, aid in weight loss and maintenance, as well as combat age-related muscle loss (sarcopenia). However, an optimal nutrition program for resistance training must include adequate protein intake to decrease muscle protein breakdown and increase muscle protein synthesis to elicit in adaptation. For an athlete, the daily recommendation of protein is 1.2 to 1.7 grams per kilogram of body weight daily. Normally protein requirements can be fulfilled through diet alone and protein supplementation has not been shown to increase athletic performance (1). While protein ingestion is crucial for the regulation of muscle mass, surplus protein stresses the kidneys and is stored as fat. High protein diets do not enhance muscle protein synthesis as long as requirements for protein and calories are fulfilled. Thus, the minimal amount of protein that maximally stimulates muscle protein synthesis is optimal for recovery after exercise. 20 grams of protein is sufficient for maximal muscle protein synthesis during recovery with larger quantities conferring no added benefits (5).The protein obtained from food differs in quality. Soy, whey & casein (milk proteins), and egg whites have the highest quality rating on the Protein Digestibility Corrected Amino Acid Score (PDCAAS). The majority of studies on the effect of protein on muscle protein synthesis and recovery have focused on the milk proteins whey and casein, which seem to have similar overall effects in a six hour period. Whey is a rapidly digested protein that results in high periods of muscle protein synthesis during the first three hours after ingestion. In contrast, casein is digested slowly and causes more sustained muscle protein synthesis and decreased muscle protein breakdown (5). Although whey results in higher levels of leucine (an amino acid that stimulates muscle protein synthesis), essential amino acids, and amino acids for the first three hours, these levels are higher with casein ingestion later on in the recovery period. However, over a six hour period there is no significant difference between the effects of whey and casein on muscle protein synthesis (6).
Whey is beneficial immediately after exercise to elicit rapid muscle protein synthesis while casein allows for a longer period of recovery. This suggests that the best protein type for post-exercise recovery is situation-dependent. For exercise during the day, whey is most beneficial since other meals will likely be ingested within 2 to 3 hours. In contrast for evening exercise with no following meals, casein is more beneficial because its slow-digesting proteins will stimulate muscle protein synthesis throughout the night. Muscle protein synthesis benefits from both the fast-acting effects of whey and more prolonged effects of casein, when ingested in the form of dairy (20% whey and 80% casein).
The abundance of research on the effects of milk proteins on muscle protein synthesis makes milk seem like the best choice. However, dietary restrictions, ethical choices, and health concerns may prevent milk consumption. Soy protein, like casein, has the highest rating of protein quality and soy has been found to behave similarly to whey protein. A study that compared the effects of whey, casein, and soy protein on muscle protein synthesis found that all three proteins caused a rise in the essential amino acids necessary for repair(7). Whey and soy caused a significantly greater increase than casein post exercise, probably because whey and soy are fast-acting proteins. However, whey protein’s stimulation of muscle protein synthesis was significantly higher than that of soy, perhaps due to higher levels of leucine. The study concluded that milk is more effective post workout because it contains both slow and fast acting proteins, while soy only contains a fast-acting protein (7). However, soy protein stimulates muscle protein synthesis and is a good source of post exercise protein.
Although it seems logical that the body would signal for more calories on days with higher energy expenditure, food intake does not usually correspond with increases or decreases in activity. This can cause weight gain during periods of decreased activity and weight loss with increased activity. Appetite regulation is influenced by hormones in addition to other factors like pleasure and social context. Moderately vigorous activity can suppress appetite in the short term, resulting in negative caloric balance (ref). The adherence to a regular exercise program usually elicits the adaptation of appetite through a corresponding partial, though incomplete, appetite increase. Exercise’s ability to suppress hunger has positive implications for those trying to lose weight, but athletes must be aware of the appetite suppressing character of aerobic exercise and compensate accordingly in order to ensure adequate nutrient intake (8).
Hormones sense the digestion and absorption of food to signal fullness. Aerobic exercise may decrease levels of the appetite-inducing hormone ghrelin and increase levels of appetite-suppressing hormones for one to five hours after exercise. Because optimal nutrition quickly following exercise aids in recovery, adaptation, and subsequent exercise, adequate nutrition should be consumed despite suppressed hunger. This phenomenon differs slightly according to gender: females tend to compensate for caloric expenditure more than males, possibly due to the important role adequate fat stores play in reproduction. Additionally, food intake following exercise is influenced by environmental factors. Appetite decreases when performing exercise in higher altitudes than normal and in hot environments while it increases when performing in cold environments. Thus, post-exercise intake should be monitored more carefully to ensure adequacy in these situations (8).
Besides nutrient, food intake post-exercise replacing fluid (water & electrolytes) lost during exercise is critical. Fluid intake after exercise is of the utmost importance because many athletes do not take in enough water during exercise. Inadequate water intake can lead to dehydration and electrolyte imbalance, contributing to muscle cramping. In addition, optimal thermoregulatory control is needed to maintain core temperature and places stress on the CV system and muscle function. Recovery from dehydration after prolonged activity and dehydration is enhanced by drinking 16 to 24 ounces of fluid for every 0.5 pound of weight lost during exercise (ref – ACSM Position Stand). However, a positive hydration balance can usually be achieved through the normal intake of fluids and meals (1).
Humans were intended for physical activity and the adoption of an exercise program confers many benefits including improved body composition, a faster metabolism, elevated mood, and stress relief. However, adequate nutrient intake following exercise is fundamental to allow adaptation and recovery. The information in this article can assist in calculating the amount of carbohydrates and protein needed to enable optimal performance and fitness gains.
Written by Anna Gregor with Eric Sternlicht, Ph.D., Occidental College, Los Angeles, CA.
6. Journal of Applied Physiology, Tang J. et al., Ingestion of Whey Hydrolysate, Casein, or Soy Protein Isolate: Effects on Mixed Muscle Protein Synthesis at Rest and Following Resistance Exercise in Young Men, 2009.