Protein and Physical Activity

Protein utilization is, to a large degree, a function of total energy intake adequacy. An inadequate total energy intake forces athletes to burn protein for energy, making less protein available for other critical functions. Therefore, the protein requirement for athletes (i.e., 12 to 15 percent of total calories or 1.2 to 1.7 grams per kilogram) is based on the assumption that total energy intake is adequate.

A standard tenet in nutrition is that carbohydrate has a protein-sparing effect. This means that if you can supply sufficient carbohydrate to the system for fuel, then protein will be spared from being burned so it can be used for more important functions. Studies have generally found that the maximal rate of protein utilization for nonenergy uses is approximately 1.5 grams of protein per kilogram of body weight. If this amount is exceeded, body tissues must make some decisions about what to do with the excess. The excess can be stored as fat, or some of the excess can be burned as energy. In either case, nitrogen must be removed from the amino acids, and this nitrogenous waste must be removed from the body. Virtually all studies that have looked at the total energy consumption of athletes indicate that athletes consume less total energy than they should to support the combined needs of activity, growth, and tissue maintenance. Since burning protein causes a lot of metabolic waste, it would be better to meet the energy requirement by providing a cleaner-burning fuel-carbohydrate.

A goal for most athletes is to remain in a nitrogen-balanced state in which as much nitrogen is coming into the system as is being excreted. A negative nitrogen balance suggests that more nitrogen is being excreted than is being consumed, a state that will inevitably lead to muscle loss. A positive nitrogen balance suggests that more nitrogen is being retained than is being excreted, a state that suggests that muscle is being gained. The amount of protein required to maintain a nitrogen-balanced state in nonathlete adults has been well studied and established at the level of .8 grams of protein per kilogram of body weight per day, while for athletes an intake of between 1.2 and 1.7 grams of protein per kilogram of body weight per day is needed. Both the athlete and nonathlete recommendations are based on the consumption of a total caloric intake that satisfies energy needs.

The higher protein recommendation for athletes is based on four factors:^{37, 39, 42} (1) Athletes typically have a higher lean body mass that requires more protein to sustain; (2) athletes lose a small amount of protein in the urine, and nonathletes do not (the greater the intensity and duration, the greater the proteinuria); (3) athletes “burn” a small amount of protein (approximately 5 percent of total energy combustion) during physical activity; and (4) athletes require additional protein to recover from the muscle damage that occurs during training .

The Institute of Medicine has stated that additional protein for healthy adults who exercise regularly is not needed because exercise increases protein retention. Nevertheless, both the American College of Sports Medicine and the American Dietetic Association recommend that protein intakes range between 1.2 and 1.7 grams per kilogram of body weight in physically active people. In reality, most athletes consume far more protein than they require, and typically more than the maximum recommended level of 1.7 grams per kilogram. Some strength and power athletes regularly consume 300 to 775 percent of the recommended level for protein (.8 grams per kilogram).^{43, 44} Possible exceptions to high protein intakes may occur among vegetarian athletes and athletes in subjectively judged sports who strive to maintain low body weights (i.e., gymnasts, divers, figure skaters).

Protein oxidation during short-term, intense exercise is insignificant, but protein provides from 3 to 5 percent of total energy needs during endurance exercise.^{45, 46} Protein utilization during exercise will rise to a level greater than 5 percent of total energy needs if glycogen levels are low, blood sugar is low, exercise intensity is high, or exercise duration is long.

There is a common misunderstanding that extra protein intake alone will support a larger muscle mass, and this theory is the main rationale for the large protein intakes seen in many athletes. In fact, additional total calories are required to support a larger muscle mass, and protein should constitute the same relative proportion of the extra calories consumed. For instance, if a 75-kilogram (165 pound) man wishes to increase his muscle mass by 3 kilograms (6.6 pounds), he would need to consume approximately 1.5 additional grams of protein for each kilogram of muscle mass desired. This amounts to only 4.5 grams of additional protein to support the larger muscle mass. By contrast, 30 grams per kilogram of additional carbohydrate, or 90 grams of additional carbohydrate in total, is required to support the larger muscle mass. Here is the total additional caloric requirement represented by the additional muscle:

• 4.5 grams protein × 4 calories per gram = 18 kilocalories from protein
• 90 grams carbohydrate × 4 calories per gram = 360 kilocalories from carbohydrate
• Total additional calories = 378 calories per day above current requirements to support a 3-kilogram increase in muscle mass

Of course, this athlete would also need to stimulate muscle enlargement by undertaking the appropriate strength-building exercises. Otherwise, the extra calories would manifest themselves as stored fat rather than additional muscle. It is likely that the large amount of protein consumed by so many athletes represents the extra calories they require to maintain or enlarge the muscle mass. Although it is certainly possible to use protein as a primary energy source, it is not the most desirable source because of the nitrogenous wastes produced with protein oxidation. In addition, protein can be an expensive source of calories when provided in supplement form. For instance, eggs (an extremely high-quality source of protein) cost approximately 13 cents per 8 grams of protein, while protein capsules cost approximately $1.20 per 8 grams of protein and may be of questionable quality.

High-protein foods have a long gastric emptying time so are not recommended immediately before or during exercise. In addition, there is no evidence that adding protein to a glucose- and sodium-containing sports beverage does anything useful for either endurance or power enhancement. In fact, protein added to a sports beverage that is consumed during competition increases the risk of gastrointestinal distress and may delay the delivery of fluids and carbohydrate to needy muscles. Protein added to a sports beverage reduces the content of what athletes really need: fluid, carbohydrate, and electrolytes. Therefore, the majority of energy in the preexercise meal and during exercise fluid replacement should be from carbohydrate.

An increasing body of evidence suggests that adding small amounts of protein to postexercise food and drink is useful for muscle recovery, although the benefit of protein is reduced if sufficient carbohydrate is ingested postexercise to replenish glycogen stores.⁴⁷

Consuming a drink that contains approximately .1 gram of protein per kilogram (30 calories of protein for a 75-kilogram athlete) after heavy resistance training does appear to improve muscle protein balance.^{48, 49} The general guideline is for endurance athletes to consume carbohydrate at the rate of a minimum of 1.2 grams per kilogram of body weight each hour during the first 3 to 5 hours after prolonged exercise. This strategy ensures muscle glycogen replenishment. The addition of some protein in this carbohydrate mix may be useful for muscle recovery, but the majority of postexercise substrate should clearly be from carbohydrate.