The body is constantly recycling proteins - breaking them down (catabolizing) and synthesizing them (anabolizing). About 20% of the Resting Enegy Expenditure is required for this process. Catabolism releases nitrogen and anabolism requires nitrogen so nitrogen balance is achieved when these processes are in equilibrium.
Proteins are made up of 20 different building blocks, called amino acids, all of which except the indispensable (formerly, essential) amino acids can be synthesized by the liver. Proteins are used for many different things other than building tissue - for example, transportation, signaling (hormones), enzymes (catalysts), the immune system and acid balance.
Catabolism (also a necessary step before proteins can be metabolized) produces ammonia which is toxic to the body and secreted in urine. For this reason, it is thought that more than 2.5g/kg/d of protein intake could be harmful. Large amounts can also result in dehydration.
One to two hours after training is the best time to ingest protein (in the anabolic window) - 10-20g of high quality protein is recommended. According to Mettler & Meyer 2010, the recommended daily amount of protein for athletes is 1.5g/kg/d for 0 hours training, 1.6g/kg/d for 1 hour, 1.7g/kg/d for 2 hours and 1.8g/kg/d for 3 or more hours training.
In the same paper, based on the goal of replenishing glycogen stores after exercise, the recommended daily intake of carbohydrates for athletes is 3.5g/kg/d for 0 hours training, 4.5g/kg/d for 1 hours, 6.2g/kg/d for 2 hours, 7.5g/kg/d for 3 hours and 8.5g/kg/d for 4 hours training. The highest rates of muscle glycogen synthesis were seen if 1.5g/kg were consumed in the first hour after exercise. Some studies show that adding protein may increase this yet further.
There is a limit on the absorption rate of carbohydrates and this is why sports drinks have a concentration of 6-8%. It's estimated that 60g can be absorbed per hour but that it is possible to increase this to about 90g per hour by mixing glucose, fructose and sucrose, which have different transport mechanisms. However, indigested fructose ferments in the digestive tract, leading to bloating and gases.
If carbohydrates are consumed at least 1-2 hours before exercise, or as little as 15 minutes before, high GI (Glycemic Index) carbs have not been demonstrated to have a detrimental effect (e.g., blood sugar rebound) in all but a minority of cases. After exercise it is more effective to consume high GI carbs to restore glycogen while insulin favours storage.
Carbohydrate loading can result in almost a doubling of stored glycogen but does not require a severe depletion phase (Sherman 1981).
Ingestion of fats does not necessarily lead to storage of fat; excess calorie intake, however, does. Ironically, many "fat free" product have more calories from sugar!
Fats are needed by the body, for example, to digest certain vitamins (A, D, E & F). Chronic fat restriction can impact testosterone levels and HDL (which helps transport cholesterol from the arteries).
Trans fatty acids increase the risk of heart disease. The Recommended Daily Intake (RDI) is less than 2g but any amount less than 0.5g may be quoted as 0g on the label; look instead for hydrogenated oils on the ingredients.
The essential fatty acids omega 6 and omega 3 have opposite inflammatory effects but compete for the same enzymes. The recommended ratio for omega 6 to 3 is less than 4:1 but is more like 15:1 in the typical American diet. Omega 9 (found in olive oil) cab reduce the risk of heart disease.
Indigested fat in the stomach delays gastric emptying so it is best to restrict fat in pre-competition meals. It takes about 3-6 hours for fat to enter the bloodstream after consumption.
When exercising, it takes about 10-20 minutes to reach the maximum fat oxidation rate. Even very lean individuals have enough fat to run for 100 hours. Fat oxidation is inhibited when insulin is high (after meals) or when lactate levels in the blood are high but is encouraged 3-4 hours after eating.
A famous study was conducted by Phinney et al (1983) in which athletes followed an extremely high fat diet (85%) for four weeks. Although their rate of fat oxidation actually doubled, the time to exhaustion in a sub-maximal test was not significantly different. The price for conserving carbohydrate stores appears to be the intensity at which exercise can be performed.
No evidence for the effectiveness of supplementation with either glutamine or glucosuamine.
Creatine has been shown to be effective for weight lifters but most non-vegetarians ingest enough creative in their diets.
Caffeine has not been demonstrate to have either an impact on electrolyte imbalance or to cause dehydration but it can delay the sensation of fatigue. The optimal dose is considered to be 2-3g/kg.
Supplementation with BCAA (Branched Chain Amino Acids) shows promise for immune system support and helps reduce post exercise fatigue.