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We digest proteins from foods into amino acids, but before these smaller molecules can be used as energy sources, they must be deaminated.

Individual plant proteins typically do not provide enough of one or more essential amino acids for adequate nutrition of a person. Combining appropriate plant foods can provide a diversity of dietary amino acids. For example, beans are low in methionine but have enough lysine. Rice lacks lysine but has enough methionine. A meal of beans and rice offers enough of both types of amino acids.

Plants can be genetically modified to make their protein more "complete." For example, genetic instructions for producing the amino acid tryptophan inserted into corn cells, can compensate for the low levels of this nutrient normally found in corn.

Eight essential amino acids (e) cannot be synthesized by human cells and must be provided in the diet. Two additional amino acids (ch) are essential in growing children.

How do cells utilize proteins?

Which foods provide rich sources of protein?

Why are some amino acids called essential?

Distinguish between a complete protein and an incomplete protein.

Nitrogen Balance

In a healthy adult, proteins are continuously built up and broken down. This occurs at different rates in different tissues, but the overall gain of body proteins equals the loss, producing a state of dynamic equilibrium (di-nam'ik e"kwi-lib're-um). Since proteins contain a high percentage of nitrogen, dynamic equilibrium also brings nitrogen balance (ni'tro-jen bal'ans)—a condition in which the amount of nitrogen taken in is equal to the amount excreted.

A person who is starving has a negative nitrogen balance because the amount of nitrogen excreted as a result of amino acid oxidation exceeds the amount the diet replaces. Conversely, a growing child, a pregnant woman, or an athlete in training is likely to have a positive nitrogen balance because more protein is being built into new tissue and less is being used for energy or excreted.

Protein Requirements

In addition to supplying essential amino acids, proteins provide nitrogen and other elements for the synthesis of nonessential amino acids and certain nonprotein nitrogenous substances. The amount of protein individuals require varies according to body size, metabolic rate, and nitrogen balance condition.

For an average adult, nutritionists recommend a daily protein intake of about 0.8 grams per kilogram (0.4 grams per pound) of body weight. For a pregnant woman, who

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