Feature
What to Do When Exercise is Through: A Guide to the Best Cures for Tired Muscles
By Edmund R. Burke, Ph.D.
Runners want to know, "What is the best way to rebuild muscle glycogen stores?" Weightlifters ask, "What should I eat to help my muscles recover?" An exhausted soccer player wonders, "Will a sports drink help rebuild energy stores?" Most athletes feed their bodies correctly before and during training and important competitions, but what about when the exercise is over? A workout doesn't stop when an athlete runs off the field or leaves the gym—it's over when the body is rehydrated and refueled.
Proper Rehydration
After a hard exercise session, it is important to replace the water and sodium lost in sweat. By weighing themselves before and after exercise, athletes can determine the amount of water lost during a strenuous session. It is not uncommon for someone exercising in a hot and humid environment to lose 8 to 12 pounds after four hours of exercise. That's the equivalent of one to 1.5 gallons of sweat. Even a marathon runner may metabolize about three-quarters of a pound of fat, but lose up to 10 pounds of water weight.
| The quicker, the better: The sooner an athlete
eats carbohydrates after exercise, the faster
muscle glycogen will be restored. |
|
The best choices for replacing water are sports drinks that contain sodium and carbohydrates, both of which increase fluid uptake from the intestines into the bloodstream. Research shows that the body absorbs more fluid when electrolytes such as sodium are added. In one study, six volunteers underwent two exposures to heat and then engaged in an exercise regimen that caused mild dehydration, resulting in a 2 to 3 percent decrease in body weight. Each volunteer then rehydrated with either water or a water-sodium solution. During the three-hour rehydration period, subjects rehydrating with water alone restored 68 percent of the fluid they lost, whereas subjects rehydrating with the sodium solution replaced 82 percent of their lost fluids.1
According to the American College of Sports Medicine, carbohydrates should also be included in rehydration drinks. Research shows that carbohydrates increase water absorption by interacting with sodium in the intestinal wall. The glucose molecules that make up a carbohydrate stimulate sodium absorption, and sodium, in turn, is necessary for glucose absorption. When the intestines absorb these two substances, they pull water with them, thus increasing absorption into the bloodstream.2
Replenish Muscle Glycogen
Someone who exercises for more than 90 minutes and wants to be ready for the next day's workout must replenish carbohydrate reserves. The body stores extra carbohydrates as muscle glycogen, which is itself composed of long glucose-molecule chains. Glycogen metabolizes into these glucose molecules and fuels the body during moderate- to high-intensity exercise. To train hard on consecutive days, an athlete must replenish muscle glycogen.
The timing, amount and type of postexercise carbohydrate feedings influence the rate of muscle glycogen resynthesis. Studies repeatedly show that when carbohydrates aren't eaten after exercise, very little muscle glycogen synthesis occurs. Obviously, fasting after a workout is not a good idea.
On the other hand, a series of studies at the University of Texas at Austin demonstrated that properly timed carbohydrate feedings optimized the restoration of muscle glycogen stores. In one experiment, researchers gave subjects placebo, 1 g of glucose/kg of body weight immediately after and two hours after exercise, or 1 g of glucose/kg of body weight two hours after exercise. Muscle biopsies taken immediately after exercise and four hours later revealed that glycogen synthesis was highest when subjects were given glucose immediately after exercise. When no carbohydrates were eaten until two hours after exercise the rate of glycogen synthesis was greater than during the placebo trial but significantly lower than when carbohydrates were eaten immediately after exercise.3
The moral of the story is the sooner one can ingest carbohydrates after exercise, the quicker glycogen will be restored. This is especially true if an individual works out morning and evening—there isn't much time between sessions to replenish glycogen stores. Exercise scientists recommend athletes eat 1-1.5 g of carbohydrates/kg of body weight within 30 minutes after exercise, followed by additional feedings every two to four hours thereafter. In practice, this requires ingesting high-carbohydrate snacks or supplements at intervals between training sessions.
The type of carbohydrate eaten after exercise can affect the rate of glycogen synthesis as well. Recent studies indicate that meals or powder sports supplements made of high-glycemic-index foods actually induce greater glycogen resynthesis than do low-glycemic-index foods.4 A high-glycemic-index food is one that rapidly raises blood sugar levels after it is eaten. For example, sucrose or glucose, both high-glycemic-index sources, resynthesize muscle glycogen twice as fast as fructose, a low-glycemic-index nutrient.
The Role of Insulin Enhancers
Insulin is a hormone released by the pancreas in response to carbohydrate consumption. One of insulin's main functions is helping transport glucose into liver and muscle tissues, where it is stored as glycogen. Insulin also stimulates release of the enzyme glycogen synthase, which helps convert glucose to glycogen.
Timing is key when it comes to muscle glycogen replenishment. Muscle cells are most sensitive to insulin two hours after exercise. Assuming enough carbohydrates are available, elevated blood-insulin levels after exercise expedite the transport of glucose into muscle cells, where it is converted to glycogen. After two hours, muscle cells become more resistant to insulin and remain so for several hours.
Because insulin plays such a vital role in replenishing glycogen stores after exercise, researchers have focused on enhancing insulin release during recovery. Studies show that protein, when combined with carbohydrates, almost doubles the insulin response.5-7 So it seems logical to include protein in a recovery sports drink.
However, more is less in this case. Protein also stimulates a peptide called cholecystokinin, or CCK, which slows stomach emptying. Too much protein, therefore, slows fluid and electrolyte replenishment during recovery because fluids must exit the stomach before they can enter the intestines and be absorbed into the bloodstream.
The challenge, then, is how to gain the insulin-increasing benefits of supplemental protein while avoiding its effect on stomach emptying. In my opinion, this is achieved by carefully balancing carbohydrates and protein according to a critical ratio, which I call the optimum recovery ratio. When the ratio of carbohydrates to protein is 4 to 1, protein does not seem to interfere with rehydration. For instance, if someone eats 56 g of carbohydrates after exercise, he should also eat 14 g of protein to enhance the insulin response without slowing gastric emptying.
The amino acid arginine also affects insulin uptake and postexercise recovery. According to John Ivy, Ph.D., of the University of Texas at Austin, arginine stimulates insulin release from the pancreas and is important for muscle metabolism. Ivy studied the effects of carbohydrate supplements that contained arginine on the rate of muscle glycogen synthesis after exercise. Carbohydrate-arginine supplementation increased muscle glycogen replenishment by 50 percent more than carbohydrate alone. Ivy concluded that arginine, when added to a carbohydrate supplement, makes more glucose available for glycogen production. Arginine does this by increasing the use of fat, rather than glucose, as an energy source after exercise. Simply put, arginine makes the glycogen replenishment process more efficient.8
These studies have important implications for anyone who exercises. To replenish glycogen stores, maximum insulin stimulation is essential immediately following exercise. In the proper proportions, protein, carbohydrates and arginine promote faster recovery and improve performance during the next workout.
The Finish Line
Whether a person trains or competes once or several times a day, muscle and liver glycogen stores must be rebuilt quickly, and the body must be rehydrated between sessions. How quickly the body synthesizes glycogen determines the amount it will ultimately be able to store. Waiting too long blunts the response. And the more glycogen an athlete stores in the liver and muscles and the more she rehydrates, the more energy she'll have during a later training session. Insulin increases the transport of glucose into muscles and stimulates glycogen synthesis, thus enabling the body to store more energy faster.
There is a narrow window during which the glycogen replenishment process is most efficient. Athletes should balance postexercise protein and carbohydrate intake according to the optimum recovery ratio, which is 1 g of protein for every 4 g of carbohydrate. This enhances the insulin response without adversely affecting gastric emptying and rehydration.
Sidebars:
Does Ribose Matter?
24 Hours of Recovery
Edmund R. Burke, Ph.D., is an exercise physiologist at the University of Colorado at Colorado Springs and author of Optimal Muscle Recovery (Avery Publishing, 1999).
References
1. Nose H, et al. Role of osmolality and plasma volume during rehydration in humans. J Appl Physiol 1980;48:657-64.
2. Maughan RJ. Fluid and electrolyte loss and replacement in exercise. J Sports Sci 1991;9:117-42.
3. Ivy JL, et al. Muscle glycogen synthesis after exercise: effect of time of carbohydrate ingestion. J Appl Physiol 1988; 64:1480-5.
4. Burke LM, et al. Muscle glycogen storage after prolonged exercise: effect of the glycemic index of carbohydrate feedings. J Appl Physiol 1993;75:1019-23.
5. Ivy JL, et al. Muscle glycogen storage after different amounts of carbohydrate ingestion. J Appl Physiol 1988;65:2018-23.
6. Yarasheski KE, et al. Acute effect of resistance exercise on muscle protein synthesis rate in young and elderly men and women. Amer J Physiol 1993;265:E210-14.
7. Zawadzki KM, et al. Carbohydrate-protein complex increases the rate of muscle glycogen storage after exercise. J Appl Physiol 1992;72:1854-9.
8. Ivy JL. The effects of arginine on glycogen resynthesis. In review, Med and Sci in Sports and Exer.
