The protein found in animal products contains all of the essential amino acids and is called therefore complete protein. These amino acids are also contained in the proper proportions necessary for synthesizing proteins within the body. Protein is found in lesser amounts in plant materials but may be lower in three essential amino acids. Vegetables can be combined to create a complete supply of essential amino acids. Legumes, such as dried beans and peas, provide almost as complete a source of protein as animal foods.
Protein is found in every cell in the body. It has many functions such as the formation of new tissues and the replacement of worn out ones. It also regulates the balance of water, the balance of acids and bases and transports nutrients in and out of cells. Protein forms antibodies, hormones and enzymes. Protein transports nutrients and oxygen in the blood and is essential for blood clotting.
The amount of protein necessary in the diet varies with different life stages with the growth phase requiring the most. The need stabilizes in early adulthood. Throughout the life cycle the protein requirement is based on the weight of the individual. The body needs a new supply of protein every day since excess protein is stored not as amino acids but as fat. Excess intake of protein can lead to increased fat stores and can strain the kidneys in an effort to rid the body of it.
Protein is not a major energy source, but excess protein may be converted to carbohydrate or fat. During periods of starvation or semistarvation when adequate amounts of fat and CHO are not available, protein can be utilized for energy. The demand for energy in the body takes precedence over tissue building. The active individual who desires to maintain lean body mass must have adequate fat and CHO calories to spare protein to be used for its more important functions. The dietary goals of 12% allow for adequate protein intake. Many of the common sources of protein are high in fat and calories. Look for animal protein with less fat such as low fat dairy products, poultry and fish, combine small amounts of animal protein with plant sources. Protein insufficiency is one of the major nutritional problems in the world, but the average American consumes about twice as much as is needed per day.
During exercise, protein usually contributes only 1-2% of the energy. Recent research has shown that protein may contribute about 4% of energy demand during prolonged exercise with normal glycogen stores. The percentage may rise to 10% if you are depleted of glycogen, as in the last part of the marathon. CHO loading may have a protein sparing effect for distance runners. The research on whether or not there is destruction of muscular protein during exercise is inconclusive. The amino acid alanine has been found to be released by exercising muscles, but the muscles are also absorbing other amino acids from the blood. Whether there is a net loss of cellular tissue has not been determined. There is little data to support the recommendations of some individuals to increase protein intake during physical training. The overall results show that prolonged low protein intake may have adverse effects on physical performance, but intakes above what is considered normal have not been shown to improve performance.