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Most American diets contain protein from several animal and plant sources, says Lynne Ausman. Photo: iStock
Ask The Professor
How much protein do I need, and what are the best sources? And what is the difference between soy and whey protein?
This month’s expert, Lynne Ausman, a Friedman School professor and a scientist at the Jean Mayer USDA Human Nutrition Research Center on Aging, responds:
Protein requirements are based on body weight. Men and women in good health, ages 19 and older, need 0.8 grams of protein per kilogram of body weight per day. (Children need somewhat more because they are still growing.) For a woman of about 57 kg (126 pounds), the Recommended Daily Allowance works out to about 46 grams of protein per day; a man of 70 kg (155 pounds) would need about 56 grams.
What does this look like at mealtime? The amounts of protein found in some typical foods are:
1 cup milk = 8 grams
3 ounces skinless chicken breast = 27 grams
2 tablespoons peanut butter = 8 grams
1 ounce almonds (about 23 nuts) = 6 grams
3 ounces canned tuna in water = 20 grams
¾ cup baked beans = 10 grams
2 cups cooked brown rice = 10 grams
According to the National Health and Nutrition Examination Survey from 2003 to 2004, adult males averaged about 105 grams of protein per day, and adult females averaged 70 grams, more than most healthy people need.
As for sources, you should take amino acids into account. Proteins from animal sources, such as meat, eggs and dairy, have a slightly more balanced mixture of the essential amino acids (those that the body cannot make on its own) than do plant proteins. If you got all your protein from one plant source, such as white rice, you would have to eat 16 cups per day to get enough of the essential amino acids to keep you in good health.
In actual practice, most American diets contain protein from several animal and plant sources, which helps with protein complementation. The brown rice on the list above has extra amounts of the amino acid methionine, while the beans are higher in lysine. Eat them together as rice and beans, and the resulting amino acid mixture is comparable in nutritional value to consuming 3 ounces of tuna in water.
Cow’s milk has two major proteins: lactalbumin (sometimes called whey protein) and casein. In the process of making cheese, the casein is precipitated into a curd and the remaining solution containing the lactalbumin protein is removed. It is sometimes used to fortify other food products. The whey protein has an excellent amino acid mixture comparable to other animal proteins. On the other hand, soy protein is somewhat limiting in one of the essential amino acids, methionine, but for the purposes of supplementing a food and contained in a diverse diet, either protein source would be fine.
