What is protein?
The word protein comes from the Greek protos meaning "of prime importance," which indicates its role in nutrition. Protein is the body's basic building material and is essential to all metabolism. There are many popular misconceptions about protein. For example, many people believe that protein is not fattening. In reality, a gram of protein contains 4 calories (the same number found in sugar and complex carbohydrates), and excess protein in the diet is converted to fat and stored that way.
Why is protein important to nutrition?
The body uses 20 amino acids — the building blocks of protein — in its metabolic activity. Nine of these amino acids have to be obtained from the diet, and are known as the essential amino acids. The other 11 can be manufactured by the body, and do not need to be consumed in the diet.
Proteins from the diet are necessary for repair and maintenance of body tissue, growth and development, production of mother's milk, growth of hair and nails, and production of certain hormones and enzymes. They are also important components of the immune system, and they help transport nutrients in the blood. Muscles and other body tissue are composed largely of proteins, which constitute approximately 75 percent of the body's solid material.
Advice about protein
The average American diet provides more than adequate protein to meet all human needs. Even a strict vegetarian can fulfill his or her protein requirements by balancing complementary vegetable proteins from grains, nuts, seeds, and legumes.
How much protein do I need?
The recommended dietary allowance (RDA) for protein is calculated in grams per kilogram of body weight. For infants, this is 0.9 to 1 gram
per kilogram (2.2 pounds) of weight; for children, it ranges from 0.8 grams per kilogram for 1- to 3-year olds to 0.4 grams for older children and adolescents. Adults need about 0.36 grams per kilogram; pregnant women and nursing mothers need twice this amount.
In practical terms, all the protein that is needed can be obtained from an ordinary diet that contains a variety of vegetables and grains and a small amount of animal products. For example, 3 ounces of roast beef contains about 25 grams of protein, which is half of the RDA for a 150-pound adult man. Even a strict vegetarian diet that avoids all animal products provides adequate protein if complementary vegetable proteins and grains are consumed together. (See box on good sources of protein.)
What happens if I don't get enough protein?
Protein deficiency causes a variety of disorders, including poor growth and intellectual deficits in children, problems in development of the fetus, and a compromised immune system.
What happens if I get too much protein?
Most Americans eat an excess of protein-rich foods. Excessive protein intake makes the kidneys work harder to clear
Nuts and grains that are consumed with lentils, beans, and other legumes form complete protein.
the body of the harmful byproducts of protein metabolism. Many high-protein foods (meats milk, cheese, and nuts, for example) are also high in fats. Thus a diet high in animal foods is likely to be high in fats and calories, and excesses of these components have been implicated in many diseases, especially obesity, hardening of the arteries, and heart disease.
Low-protein diets are recommended for people with kidney disorders, Parkinson's disease, and certain metabolic diseases in which the body cannot metabolize protein.
Should I take a protein supplement?
A healthy person does not need to take protein supplements. Furthermore, concoctions of amino acids marketed to athletes may cause nutritional imbalances.
Good sources of protein
Foods containing all 9 of the essential amino acids include:
- Meat, poultry, and fish.
- Egg whites.
- Milk, yogurt, and other dairy products.
"Incomplete" proteins are found in vegetables, rice, lentils, dried beans, peas, and other legumes. To make complete proteins, these foods should be eaten in combinations that complement each other. Such combinations include:
- Lentils, beans, and other legumes with rice, wheat, corn, and other grains; for example, Mexican tortillas and beans or Indian rice and lentils.
- Rice or other grains with tofu and vegetables, for example, stir-fried Chinese vegetables with rice and tofu.
- Peanuts or peanut butter with wheat, corn, or other grains; for example, a peanut butter sandwich made with whole-grain bread.
Adding milk, cheese, or eggs to a grain or legume dish also makes complete protein.