11.7: Summary

We need 20 kinds of amino acids to make protein. Our bodies can make 11 of them; the other 9 must come from the diet. An evaluation of dietary protein focuses on the quality and amount of protein in the diet. Protein quality focuses on the 9 amino acids essential in the diet, and is based on how closely the proportion of these amino acids matches what our body needs. Animal protein is generally the highest in quality, with egg protein as the gold standard. Also, animal foods generally have more protein than do plant foods.

Relative to our needs, a plant protein may be low in one of the essential amino acids. This “low” amino acid is called a limiting amino acid, because its relative scarcity limits our ability to make our own proteins. This limitation can be overcome by complementing proteins, which simply means eating, together, plant proteins that have a different limiting amino acid. For example, strict vegetarians (vegans) can improve the quality of the protein in their diet by eating grains and legumes together.

We also need a certain amount of protein for normal growth and maintenance of body tissues. The amount an individual needs varies, but the most important factors are whether the person is growing and how big he or she is in terms of lean body mass. More protein is needed in the diet when it’s only from plant sources because of its lower quality and because plant foods are a bit less digestible. For these reasons, vegans are advised to add 25% to the “regular” protein RDA.

Extra amino acids aren’t stored by the body. Instead, the amino part comes off, and the rest of the molecule goes for ATP production or for storage as fat. To avoid accumulating the discarded aminos as toxic ammonia, the body makes urea, which is excreted in the urine. A large excess of dietary protein means making lots of urea. This can have a dehydrating effect, since water is lost in getting rid of urea through the urine.

Protein deficiency is most common in infants and children in low-income countries. Compared to adults, they need higher-quality protein and more protein per body weight, because of their fast growth. Animal foods are scarce in these countries. Even when people in these countries get enough calories, they may not get enough protein, simply because their foods are so “dilute” in protein.

Often, protein and calorie deficiencies occur together, which makes protein deficiency even worse. When calorie needs aren’t met, the body uses some of the dietary protein as a source of calories, rather than using it to meet protein needs. The effects of protein-calorie malnutrition include stunted growth, anemia, and impaired immunity.

World health organizations encourage mothers in low-income countries to breast-feed. A malnourished mother makes less milk, but the milk she does make has high-quality protein, and is the best food for her child. For many of these children, breast-feeding is the difference between health and sickness or death. Aggressive marketing of infant formula in these countries has been heavily criticized.

In marked contrast to diets in low-income countries, the typical American diet includes an abundance of high-quality protein, because it’s so rich in animal foods. Such a diet also tends to be high in saturated fat and calories.

Environmentally, the mass production of animals for food uses much more fresh water than does growing a comparable amount of plant food. Also, the huge amount of animal waste can lower the quality of air and drinking water of those living nearby. Ruminants such as cows also produce lots of methane, a much more potent greenhouse gas than carbon dioxide.

A prudent diet is one which lies between the extremes—a diet that includes only moderate amounts of animal foods and an abundance and variety of plant foods.