Harvard School of Public Health
Surprisingly little is known about protein and health. We know that adults need a minimum of 1 gram of protein for every kilogram of body weight per day to keep from slowly breaking down their own tissues. That's about 9 grams of protein for every 20 pounds. Beyond that, there's relatively little solid information on the ideal amount of protein in the diet, a healthy target for calories contributed by protein, or the best kinds of protein.
Around the world, millions of people don't get enough protein. This protein malnutrition leads to the condition known as kwashiorkor. Lack of protein can cause growth failure, loss of muscle mass, decreased immunity, weakening of the heart and respiratory system, and death.
In the United States and other developed countries, it's easy to get the minimum daily requirement of protein. Cereal with milk for breakfast, a peanut butter and jelly sandwich for lunch, and a piece of fish with a side of beans for dinner adds up to about 70 grams of protein, plenty for the average adult.
Too little protein is clearly a problem. What about too much? The digestion of protein releases acids that the body usually neutralizes with calcium and other buffering agents in the blood. Eating lots of protein, such as the amounts recommended in the so-called no-carb diets, requires lots of calcium. Some of this may be pulled from bone. Following a high-protein diet for a few weeks probably won't have much effect on bone strength. Doing it for a long time, though, could weaken bone. In the Nurses' Health Study, for example, women who ate more than 95 grams of protein a day were 20 percent more likely to have broken a wrist over a 12-year period when compared to those who ate an average amount of protein (less than 68 grams a day).(2) While more research is clearly needed to define the optimal amount of daily protein, these results suggest that high-protein diets should be used with caution, if at all.
All protein isn't alike
Some of the protein you eat contains all the amino acids needed to construct new proteins. This kind is called complete protein. Animal sources of protein tend to be complete. Other protein lacks one or more amino acids that the body can't make from scratch or create by modifying another amino acid. Called incomplete proteins, these usually come from fruits, vegetables, grains, and nuts.
Vegetarians need to be aware of this difference. To get all the amino acids needed to make new protein–and thus to keep the body's systems in good shape–people who don't eat meat, fish, poultry, eggs, or dairy products should eat a variety of protein-containing foods each day.
The protein package
Animal protein and vegetable protein probably have the same effects on health. It's the protein package that is likely to make a difference. A 6-ounce broiled Porterhouse steak is a great source of complete protein–38 grams worth. But it also delivers 44 grams of fat, 16 of them saturated. (3) That's almost three-fourths of the recommended daily intake for saturated fat. The same amount of salmon gives you 34 grams of protein and 18 grams of fat, 4 of them saturated. (3) A cup of cooked lentils has 18 grams of protein, but under 1 gram of fat.(3) The bottom line is that it's important to pay attention to what comes along with the protein in your food choices. If you are partial to beef, stick with the leanest cuts. Fish or poultry are excellent alternatives. Even better options are vegetable sources of protein, such as beans, nuts, and whole grains.
Protein and chronic disease
The most solid connection between proteins and health has to do with a common disorder of the immune system. Proteins in food and the environment are responsible for a variety of allergies. These are basically overreactions of the immune system to what should be harmless proteins. Beyond that, relatively little evidence has been gathered regarding the effect of protein on the development of chronic diseases.
Cardiovascular disease: Cardiovascular disease: To date, only one large, prospective study has investigated the association between dietary protein and heart disease or stroke. In the Nurses' Health Study, women who ate the most protein (about 110 grams per day) were 25 percent less likely to have had a heart attack or to have died of heart disease than the women who ate the least protein (about 68 grams per day) over a 14-year period. (4) Whether the protein came from animals or vegetables–or whether it was part of low-fat or higher-fat diets–didn't seem to matter. These results offer reassurance that eating a lot of protein doesn't harm the heart. In fact, it is possible that eating more protein while cutting back on easily digested carbohydrates may be benefit the heart.
Diabetes: Diabetes Proteins found in cow's milk may play a role in the development of type 1 diabetes (formerly called juvenile or insulin-dependent diabetes). That's one reason why cow's milk isn't recommended for infants.(5) Later in life, the amount of protein in the diet doesn't seem to adversely affect the development of type 2 (adult-onset) diabetes, although research in this area is ongoing.
Weight control: Weight control: In short-term studies, a lower-calorie diet that includes more protein and less carbohydrate is more effective for losing weight or keeping weight steady than a lower-calorie, high-carbohydrate diet.
Eating high-protein foods such as beef, chicken, fish, or beans makes you feel full longer because they slow the movement of food from the stomach to the intestine. This strategy may also delay hunger signals. The digestion of protein, when compared to that of carbohydrates, results in smaller, steadier increases in blood sugar. This helps avoid the steep climbs and drops in blood sugar–which trigger hunger pangs and occur after eating rapidly digested carbohydrates. Unfortunately, few data have been collected on the long-term effects of a high-protein diet on weight control.
Cancer: There's no good evidence that eating a little protein or a lot of it influences cancer risk.