Proteins are combinations of amino acids. During digestion, the proteins in food are broken down into separate amino acids, which are then absorbed by the body and used to:

  • Build, repair and maintain body tissues;
  • Synthesize hormones and enzymes;
  • Supply energy, when carbohydrates or fat are not available.

There are twenty different amino acids. Your body can combine amino acids in different ways to make an incredible array of proteins. In just one body cell, for example, thousands of different proteins may be present, each one with a different arrangement of amino acids.

Amino acids are classified as either essential or nonessential. Your body can make nonessential amino acids, but you have to consume the nine essential amino acids through a healthy diet. Essential amino acids include:

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

All plant and animal cells contain protein, but the amount, as well as quality, varies widely among foods. A food is considered to be a "complete protein" if it contains all nine of the essential amino acids needed by humans. Examples of complete protein are roast beef, turkey, and chicken breast. Most plant foods like beans, peas, nuts, seeds, grain products like breads, and vegetables have "incomplete" proteins because they are missing one or more of essential amino acids. Soybeans, which do supply complete protein, are the exception. However, your body can combine amino acids from the foods you eat to make the proteins it needs as along as you eat a variety of plant foods and enough calories during the day.


The current Recommended Dietary Allowance (RDA) for protein is 63 grams a day for adult men and 50 grams a day for adult women. Most people meet this requirement easily and usually exceed it. To keep your body healthy, it is recommended that protein intake be less than 35% of calories or no more than 150 grams a day. A gram of protein supplies 4 calories.


Nutrient

Food Sources What it does
Iron Meat and poultry products, fortified cereal, cooked dried beans (e.g., kidney, navy, lima, etc), spinach An essential component of hemoglobin, which carries oxygen in blood from the lungs to body cells; supports a healthy immune system; helps in brain development
Zinc Meat and poultry products, fortified cereals, wheat bran, oysters, beans, dairy products, nuts Adequate intake is critical for proper growth; works with over 200 enzymes in the body; assists the body in using carbohydrates, proteins, and fats; helps with cell reproduction, tissue growth, and repair
Vitamin B12 Meat, poultry, fish, eggs, milk and dairy products; fortified cereal Assists folate in making red blood cells; helps body use fatty acids and some amino acids; is a critical component of many body chemicals and as a result, is in every body cell
Vitamin B6 Meat, poultry, and fish products, whole grains, fortified cereals, nuts, and cooked dried beans Helps change tryptophan (an amino acid) into niacin and into serotonin; helps the body make nonessential amino acids which are in turn used to make body cells.
Niacin Meat and poultry products, peanut butter, cooked dried beans Helps make energy in body cells; helps with normal functioning of enzymes; helps the body use sugars and fatty acids
Riboflavin Milk and dairy products, organ meats (e.g. liver and kidney), enriched bread and grain products, Helps make energy in body cells; helps change tryptophan (an amino acid) into niacin
Thiamin Whole grain and enriched grain products, pork products, organ meats Helps make energy from carbohydrates in body cells
Selenium Seafood, organ meats, meat products, enriched grain products, Brazil nuts Assists in cell growth; helps with immune function; works with vitamin E
Phosphorus Milk and dairy products, meat and poultry products Is the regulator of energy metabolism in organs; important component of teeth and bones (second only to calcium); component of DNA and RNA which are critical for cell repair and growth; assists in generating energy in body cells

For more information on nutrients, go to http://www.cc.nih.gov/ccc/supplements/intro.html


Some studies indicate that protein intakes above the RDA suggested level may help with weight control by maximizing fat loss while at the same time, minimizing muscle loss.

One recent scientific review of the effect of protein consumption on weight loss concluded that a diet moderate in protein had positive and consistent effects on weight loss. There are several ways which protein may work, including promotion of satiety, maintaining lean body mass, and higher calorie burning.

Two recent studies support these findings. One study at Duke University showed that people following a lower carbohydrate, higher protein diet lost more weight than those eating a low fat, low cholesterol, and low calorie diet over a 6-month period. Another study conducted at the Philadelphia Veterans Affairs Medical Center also showed greater weight loss among those on a low carbohydrate/higher protein diet at the 6-month mark, but found no differences in weight loss between the groups after one year. Both studies indicated that low carbohydrate diets may lower triglyceride levels while increasing HDL levels. Longer and larger studies are needed to determine the long-term safety and effectiveness of lower carbohydrate, higher protein, and higher fat diets.


There are a variety of "vegetarian" eating styles, and estimates vary on how many people can really be described as vegetarians. According to a Time Magazine/CNN survey published in 2002, 4 percent of adults said they considered themselves vegetarian, but more than half of those described themselves as only "semi-vegetarian." Only five percent of the self-described "vegetarians" considered themselved "vegans," who avoid all use of animal products.

A vegetarian diet (see definitions below) is not automatically a healthy diet. Although vegetarian diets certainly can be nutritious, they can also be high in fat and cholesterol, low in fiber, and low in important nutrients, depending on food choices. Nutrients from plant food sources may be less available for absorption. Over time, this may lead to deficiencies in nutrients such as iron, vitamin B12, zinc and riboflavin. Studies of female adolescents have also shown that lacto-ovo vegetarians, semi-vegetarians, and vegans are more at risk for nutrient inadequacies than those who include food from animals in their diets.

"Vegetarian" Definitions

Lacto-Ovo Vegetarian: Does not eat meat (red meat, poultry, fish) but will eat animal products such as milk, eggs and cheese.
Lacto-Vegetarian: Avoids meat, poultry, fish and eggs but eats dairy products.
Semi-Vegetarian: Mostly follows a vegetarian eating style but occasionally eats meat, poultry or fish.
Flexitarian: Vegetarian who occasionally eats meat, poultry or fish or non-vegetarians who sometimes seek out vegetarian meals.
Vegan (strict vegetarian): Eats only plant foods and no animal foods (including eggs, milk or other dairy products).

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Institute of Medicine. Dietary Reference Intakes for Energy, Carbohydrate, Fiber, Fat, Fatty Acids, Cholesterol, Protein, and Amino Acids (Macronutrients)
Washington, D.C., National Academy Press. http://www.iom.edu/file.asp?id=4154, 2002.

Institute of Medicine. Dietary Reference Intakes for Thiamin, Riboflavin, Niacin, Vitamin B-6, Folate, Vitamin B-12, Pantothenic Acid, Biotin and Choline. Washington, D.C., National Academy Press, 2000.

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