March 1998 -- Nutrition Notes
By: Andrea Platzman, R.D.
Though athletes may work their bodies harder than the average American, many of their dietary needs are similar to the population at large. But due to their level of activity, their caloric, protein, vitamin and mineral requirements will differ.
Hoping to gain an edge over their opponents, many athletes have turned to ergogenic aids. The term "ergogenic" means "tending to increase work." In the context of sports, it includes techniques used to increase energy production and performance. The latest ergogenic aids being used today include creatine monohydrate, chromium and carnitine.
To enhance glycogen storage, the athlete's diet should consist of 60% to 70% of calories from carbohydrates mainly complex. A low-fat diet of 20% to 25% (with an even distribution of saturated, polyunsaturated and monounsaturated fats) will help in the muscle aesthetics as well as "making the weight" for some sports. Protein needs are slightly higher than the typical American diet, consisting of 15% to 20% of calories, but not to exceed 2 grams per kilogram.
Athletes should make sure they consume adequate amounts of iron, calcium and zinc, since these minerals are depleted readily through endurance- and muscle-training. "A multivitamin containing extra calcium, iron and folic acid is extremely important for the female athlete," says Elizabeth Kunkel, Ph.D., R.D., professor and chair, department of food science, Clemson University, Clemson, SC.
Gaining an edge
Creatine. Creatine is a nitrogenous substance synthesized by the liver, pancreas and kidneys from the amino acids glycine, methionine and arginine. This synthesis occurs at a rate of approximately 1 gram per day. Most people consume about 1 to 2 grams of creatine daily from eating meat and fish. Creatine is usually supplemented in a monohydrate form. Some researchers claim creatine supplementation can: increase energy and muscular strength; promote greater and faster muscle gains beyond natural capabilities; help burn fat; and improve endurance and delay fatigue.
Skeletal muscle contains nearly all of the body's creatine pool (95%). In the muscle, creatine is synthesized into phosphocreatine (PCr), which plays a major role in the energy metabolism during physical exercise. PCr also is responsible, in part, for the adenosine triphosphate (ATP) resynthesis from adenosine diphosphate during short-term exercise via the creatine kinase reaction.
During exercise, ATP levels decline very little until the stores of creatine phosphate are consumed. Studies have found that supplementation of 20 to 25 grams of creatine per day increases muscle content of creatine by 20% to 30%, with at least 20% in the phosphocreatine form. Therefore, intracellular ATP levels may be maintained at optimal levels for longer periods of time during intense exercise.
Creatine supplementation has been shown to significantly improve performance during short bouts of activities, which require high levels of strength and power. In addition, muscle mass appears to increase. However, the jury is out on whether the weight gain is due to water retention or muscle mass.
Chromium. Chromium is a trace element that functions in carbohydrate, fat and protein metabolism by serving as a key constituent of the body's "glucose tolerance factor." It works closely with insulin in facilitating the uptake of glucose into cells. Chromium appears to bind insulin to receptor tissues, and is necessary for the proper functioning of insulin. A daily amount of 50 to 200 micrograms is required to properly use glucose, and to regulate protein metabolism.
Daily chromium supplementation of 200 to 600 micrograms, in the form of chromium picolinate, has been shown to provide benefits for chromium-deficient humans. Its use results in increased glycogen synthesis, improved glucose tolerance and regulated lipid deposition. Chromium supplementation has been used to treat diabetes, hypoglycemia, acne, and elevated blood cholesterol and triglyceride levels, as well as to promote weight loss. For athletes, it is purported to increase lean body mass, and enhance body fat reduction.
Carnitine. Carnitine is a short-chain carboxylic acid containing nitrogen. It is synthesized in the body from the amino acids lysine and methionine; vitamins C, B3 and B6; and iron. It is found in meat, specifically sheep and lamb, and in dairy products. Limited amounts of carnitine are found in fruits, vegetables, grains and eggs. In the human body, 95% of carnitine is located in muscle. Carnitine is not considered an essential nutrient, because the body compensates for decreased intakes by increasing synthesis and reducing renal clearance for carnitine.
Carnitine is essential in the breakdown of fats into energy, and therefore, weight loss. It is critical in the transport of long-chain fatty acids to the mitochondria, which are the site of beta oxidation, and which play an important role in energy metabolism.
Supplementation of carnitine should only be in the L-form, as the D-form can be toxic. Studies show that L-carnitine supplementation aids cardiac function by fueling the heart with energy, raising high-density lipoprotein levels, lowering triglycerides, and reducing hypertension.
Optimal utilization of fuel for ATP generation by skeletal muscle during exercise depends on adequate carnitine levels. Therefore, many athletes take L-carnitine supplements to enhance aerobic performance and allow the body to exercise longer without fatigue. Studies have found that L-carnitine supplementation (usually 2 grams two to three times daily) results in significant improvements in cardiovascular function in athletes and normal subjects.
Though some ergogenic aids may optimize athletic performance, the best recommendations for meeting the nutritional needs of the athlete are similar to those of the general population.
"Food works best; the athlete should capitalize on food for its benefits," says Nancy Clark, M.S., R.D., a sports nutrition expert at Sports Medicine Brookline, Brookline, MA. "Supplements require an individual prescription." Top performance can only be achieved through a vigorous training schedule, sufficient energy needs provided from a variety of foods, and adequate fluids.
Andrea D. Platzman is a registered dietitian who is a consultant to the food industry, and regularly writes for nutrition publications. She earned a master's degree in nutrition from New York University, and has a culinary and business background.