Some Problems with Proteins
Proteins from different sources differ in how well they can be digested and absorbed by the human digestive tract. In some foods, the amino acids are structured in a way that resists the digestive action of enzymes in the gastrointestinal tract. The protein in cereals, legumes, and nuts is less digestible than that in meat and eggs, and the amino acids from vegetable proteins are less well absorbed than those from animal proteins.
Since the RDA takes into account variations in the digestibility of different proteins for people eating a diet containing both animal and vegetable proteins, the lower usability of vegetable proteins is not likely to result in a shortage of protein for most people. However, vegetarians who eat little or no animal protein must be careful to consume their full RDA for protein each day to assure that they are getting adequate amounts of usable protein.
Some circumstances disturb the body’s need for and ability to use dietary protein. Even when the amount of protein in your diet is adequate, inactivity, illness, injury, and emotional stress cause your body to lose more protein than it synthesizes. A healthy person who spends his days in bed loses nitrogen (protein) rather rapidly-12 to 18 grams a day. This is the result of muscle tissue wasting away. Have you ever noticed how thin your arms and legs look after you have spent a week in bed with flu? That’s not just lost water or fat-it’s also lost protein. Fever, severe pain, diarrhea, surgery, wounds, and burns also result in an excessive loss of body protein.
The protein RDA contains an allowance to cover differences in individual needs for protein due to ordinary life stresses like minor infections or injuries, loss of sleep, or psychological strain. However, for persons who are seriously injured or very ill, some doctors now recommend feeding extra protein to compensate for their excessive protein loss and to enhance their ability to recover. Extra protein fed by tube into the stomach or directly into a vein can speed the healing of wounds and burns and enhance natural defenses against infectious organisms and cancer cells.
The usefulness of various proteins can also be affected by the way they are processed or cooked. Refining grains to make white flour
lowers the usable protein because much of the lysine, already in short supply in grains, is lost when the germ is removed from the grain. Thus, products made from whole wheat provide more protein than those prepared from refined white flour.
Heat can also adversely affect protein availability. Puffing of wheat at high temperatures destroys some of the lysine. Overheated pork is digested more slowly and less completely than meat that is properly cooked (to an internal temperature of 140 to 160 degrees). With soybeans, moderate heating increases the amount of amino acids the body can absorb, but overheating diminishes it. However, the digestibility of the protein in dried milk is improved by heat used to evaporate the water. The browning reaction used in preparing certain breakfast cereals (for example, Wheat Chex and Corn Flakes) renders about a third of the protein unusable.
Certain protein-rich foods, such as peanuts, should not be eaten raw because they contain toxic substances or enzymes that must first be destroyed by heat before the food can be used by the body. However, raw (unroasted) peanuts can be used if, in preparing the food, they will be cooked for at least a few minutes at a high temperature.