Protein is made up of smaller units called amino acids. There are 20 different amino acids. The body can only make 13 of them; the other 9 must come from food. These 9 are called "essential" amino acids.

Protein provides 4 calories per gram. It is chemically unique from fat and carbohydrate because it contains nitrogen.

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The amount of protein needed by healthy people is small. The amount of protein an individual needs is dependent on their body weight. Extra protein in the diet is broken down and either leaves the body in urine or feces (stool) or is converted to other forms of energy.

The best sources of "complete" protein come from the middle of the food guide pyramid. Foods from the meat group have about 7 grams of protein per 1 ounce serving. Examples include:

Dairy foods have about 8 grams of protein per serving. Examples include an 8-ounce serving of any of the following: skim, 2%, or whole milk, buttermilk or yogurt.

Some plant foods are high in protein. These include:

A 1/2-cup serving of cooked dry beans provides about 7 grams of protein. Most adults need 2 to 3 servings every day of milk and 2 to 3 servings of lean meat or meat substitute. A serving can be 2 to 3 ounces.

Protein needs are relatively low; roughly 15% to 20% of the day's calories. It is easy to get enough protein from most diets. Even vegetarians can get enough protein, especially if they plan carefully. In fact, most Americans eat too much protein. Healthy adolescents and adults need about 0.8 grams of protein per kilogram of body weight, with the exception of 0.9 grams per kilogram for boys 15 to 18 years old. Infants and growing children need slightly more.

It is easy to figure out how much protein a person needs. Divide body weight by 2.2, this will give weight in kilograms (kg). Multiply this number by 0.8. Here is an example for a 120-pound person: (120 / 2.2) = 54.5 kg x 0.8 = 43.6 kg. This person needs about 44 grams of protein per day.

Too much protein may be harmful to the kidneys. The kidneys filter the by-products that come from the breakdown of protein. These by-products can be toxic to the body. Eating a very high protein diet puts stress on the kidneys because they have to work much harder. Also since complete protein comes from mostly animal products, diets high in protein are also high in fat, cholesterol and saturated fat. Too little protein in the diet generally is not a concern in the United States. However, it is still a problem in many parts of the world, where it results in malnutrition.