Function

Every cell in the human body contains protein. It is a major part of the skin, muscles, organs, and glands. Protein is also found in all body fluids, except bile and urine.

You need protein in your diet to help your body repair cells and make new ones. Protein is also important for growth and development during childhood, adolescence, and pregnancy.

Food Sources

When proteins are digested, amino acids are left. The human body needs a number of amino acids to break down food. Amino acids need to be eaten in large enough amounts for optimal health.

Amino acids are found in animal sources such as meats, milk, fish, and eggs, as well as in plant sources such as soy, beans, legumes, nut butters, and some grains (such as wheat germ). You do not need to eat animal products to get all the protein you need in your diet.

Amino acids are classified into three groups:

• Essential
• Nonessential
• Conditional

Essential amino acids cannot be made by the body, and must be supplied by food. They do not need to be eaten at one meal. The balance over the whole day is more important. The nine essential amino acids are:

• Histidine
• Isoleucine
• Leucine
• Lycine
• Methionine
• Phenylalanine
• Threonine
• Tryptophan
• Valine

Nonessential amino acids are made by the body from essential amino acids or in the normal breakdown of proteins. They include:

• Alanine
• Asparagine
• Aspartic acid
• Glutamic acid

Conditional amino acids are usually not essential, except in times of illness and stress. They include:

• Arginine
• Cysteine
• Glutamine
• Glycine
• Ornithine
• Proline
• Serine
• Tyrosine

Protein foods are no longer described as being "complete proteins" or "incomplete proteins."

Side Effects

A diet high in meat can contribute to high cholesterol levels or other diseases such as gout. A high-protein diet may also put a strain on the kidneys.

Recommendations

A nutritionally balanced diet provides enough protein. Healthy people rarely need protein supplements.

Vegetarians are able to get enough essential amino by eating a variety of plant proteins.

The amount of recommended daily protein depends upon your age and health. Two to three servings of protein-rich food will meet the daily needs of most adults.

The following are the recommended serving sizes for protein:

• 2 to 3 ounces of cooked lean meat, poultry, or fish (a portion about the size of a deck of playing cards)
• 1/2 cup of cooked dried beans
• 1 egg, 2 tablespoons of peanut butter, or 1 ounce of cheese

For recommended serving sizes of protein for children and adolescents, see age-appropriate diet for children.

Choose:

• Turkey or chicken with the skin removed, or bison (also called buffalo meat)
• Lean cuts of beef or pork, such as round, top sirloin, or tenderloin (trim away any visible fat)
• Fish or shellfish

Other good sources of protein include:

• Pinto beans, black beans, kidney beans, lentils, split peas, or garbanzo beans
• Nuts and seeds, including almonds, hazelnuts, mixed nuts, peanuts, peanut butter, sunflower seeds, or walnuts (just watch how much you eat, because nuts are high in fat)
• Tofu, tempeh, and other soy protein products
• Low-fat dairy products

Do not eat more than four eggs per week. Although they are a good source of protein and are low in saturated fat, eggs are very high in cholesterol. Try recipes with egg whites only.

For more information, see the food guide plate.

Alternative Names

Diet - protein; Complete protein; Incomplete protein

References

Escott-Stump S. Nutrition and Diagnosis-Related Care. 6th ed. Philadelphia, Pa: Lippincott Williams & Wilkins; 2008.

United States Department of Agriculture. Center for Nutrition Policy and Promotion. Dietary Guidelines for Americans. 2010. National Academy Press, Washington, DC, 2010.

National Academy of Sciences. Institute of Medicine. Food and Nutrition Board. Dietary Reference Intakes for Energy, Carbohydrate, Fiber, Fat, Fatty Acids, Cholesterol, Protein, and Amino Acids. National Academy Press. Washington, DC, 2005.

Update Date: 5/5/2012

Reviewed by: A.D.A.M. Editorial Team: David Zieve, MD, MHA, and David R. Eltz. Previously reviewed by Alison Evert, MS, RD, CDE, Nutritionist, University of Washington Medical Center Diabetes Care Center, Seattle, Washington (5/5/2011).