Water | Carbohydrates | Proteins | Fats | Vitamins & Minerals
Dietary proteins are powerful compounds that build and repair body tissues, from hair and fingernails to muscles. In addition to maintaining the body’s structure, proteins speed up chemical reactions in the body, serve as chemical messengers, fight infection, and transport oxygen from the lungs to the body’s tissues. Although protein provides 4 calories of energy per gram, the body uses protein for energy only if carbohydrate and fat intake is insufficient. When tapped as an energy source, protein is diverted from the many critical functions it performs for our bodies.
Proteins are made of smaller units called amino acids. Of the more than 20 amino acids our bodies require, eight (nine in some older adults and young children) cannot be made by the body in sufficient quantities to maintain health. These amino acids are considered essential and must be obtained from food.
When we eat food high in proteins, the digestive tract breaks this dietary protein into amino acids. Absorbed into the bloodstream and sent to the cells that need them, amino acids then recombine into the functional proteins our bodies need.
Animal proteins, found in such food as eggs, milk, meat, fish, and poultry, are considered complete proteins because they contain all of the essential amino acids our bodies need. Plant proteins, found in vegetables, grains, and beans, lack one or more of the essential amino acids.
However, plant proteins can be combined in the diet to provide all of the essential amino acids. A good example is rice and beans. Each of these foods lacks one or more essential amino acids, but the amino acids missing in rice are found in the beans, and vice versa. So when eaten together, these foods provide a complete source of protein. Thus, people who do not eat animal products (see Vegetarianism) can meet their protein needs with diets rich in grains, dried peas and beans, rice, nuts, and tofu, a soybean product.
Experts recommend that protein intake make up only 10 percent of our daily calorie intake. Some people, especially in the United States and other developed countries, consume more protein than the body needs. Because extra amino acids cannot be stored for later use, the body destroys these amino acids and excretes their by-products.
Alternatively, deficiencies in protein consumption, seen in the diets of people in some developing nations, may result in health problems. Marasmus and kwashiorkor, both life-threatening conditions, are the two most common forms of protein malnutrition.
Some health conditions, such as illness, stress, and pregnancy and breast-feeding in women, place an enormous demand on the body as it builds tissue or fights infection, and these conditions require an increase in protein consumption.
For example, a healthy woman normally needs 45 grams of protein each day. Experts recommend that a pregnant woman consume 55 grams of protein per day, and that a breast-feeding mother consume 65 grams to maintain health.
A man of average size should eat 57 grams of protein daily. To support their rapid development, infants and young children require relatively more protein than do adults. A three-month-old infant requires about 13 grams of protein daily, and a four-year-old child requires about 22 grams.
Once in adolescence, sex hormone differences cause boys to develop more muscle and bone than girls; as a result, the protein needs of adolescent boys are higher than those of girls.
"Human Nutrition."Microsoft® Encarta® Encyclopedia 2001. © 1993-2000 Microsoft Corporation. All rights reserved.
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