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. 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.

Can you get too much protein? Digesting it 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 low-carb or no-carb diets, takes 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). Although more research is clearly needed to define the optimal amount of daily protein, these results suggest that long-term high-protein diets should be used with caution, if at all.

The protein package

Animal protein and vegetable protein probably have the same effects on health. It's the protein package that's 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. 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. A cup of cooked lentils has 18 grams of protein, but under 1 gram of fat.

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 allergies. Proteins in food and the environment are responsible for these 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.

Why do high-protein, low-carb diets seem to work more quickly than low-fat, high-carbohydrate diets? First, chicken, beef, fish, beans, or other high-protein foods slow the movement of food from the stomach to the intestine. Slower stomach emptying means you feel full for longer and get hungrier later. Second, protein's gentle, steady effect on blood sugar avoids the quick, steep rise in blood sugar and just as quick hunger-bell-ringing fall that occurs after eating a rapidly digested carbohydrate, like white bread or baked potato. Third, the body uses more energy to digest protein than it does to digest fat or carbohydrate.

There's no need to go overboard on protein and eat it to the exclusion of everything else. Avoiding fruits, vegetables, and whole grains means missing out on healthful fiber, vitamins, minerals, and other phytonutrients. It's also important to pay attention to what accompanies protein. Choosing high-protein foods that are low in saturated fat will help the heart even as it helps the waistline.

Nuts for the Heart

Many people think of nuts as just another junk food snack. In reality, nuts are excellent sources of protein and other healthful nutrients.

One surprising finding from nutrition research is that people who regularly eat nuts are less likely to have heart attacks or die from heart disease than those who rarely eat them. Several of the largest cohort studies, including the Adventist Study, the Iowa Women's Health Study, the Nurses' Health Study, and the Physicians' Health Study have shown a consistent 30 percent to 50 percent lower risk of myocardial infarction, sudden cardiac death, or cardiovascular disease associated with eating nuts several times a week.

There are several ways that nuts could have such an effect. The unsaturated fats they contain help lower LDL (bad) cholesterol and raise HDL (good) cholesterol. One group of unsaturated fat found in walnuts, the omega-3 fatty acids, appears to prevent the development of erratic heart rhythms. Omega-3 fatty acids (which are also found in fatty fish such as salmon and bluefish) may also prevent blood clots, much as aspirin does. Nuts are rich in arginine, an amino acid needed to make a molecule called nitric oxide that relaxes constricted blood vessels and eases blood flow. They also contain vitamin E, folic acid, potassium, fiber, and other healthful nutrients.

Eating nuts won't do much good if you gobble them in addition to your usual snacks and meals. At 185 calories per ounce, a handful of walnuts a day could add 10 pounds or more in a year if you don't cut back on something else. This weight gain would tip the scales toward heart disease, not away from it. Instead, eat nuts instead of chips or other, less healthy snacks. Or try using them instead of meat in main dishes, or as a healthful crunch in salads.

This research has prompted scads of media reports touting the joys of soy. It also has food makers churning out new soy products that are beginning to move into the mainstream. In Boston, for example, soymilk is now advertised on the radio during Boston Red Sox games, alongside donuts, oil additives, and beer.