Protein is made from different combinations of amino acids and is good for keeping the body running.
However, 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.
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.
Eating lots of protein, such as the amounts recommended in the low-carb or no-carb diets, takes up lots of calcium from the body. 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, however, could weaken bone. Although more research is clearly needed to define the optimal amount of daily protein, long-term high-protein diets should be practiced with adequate knowledge of the effects and amounts of protein ingested.
Some of the protein you eat contains all the amino acids needed to build new proteins. This kind is called complete protein. Animal sources of protein tend to be complete. Other protein sources lack 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.
Adults should get 45 percent to 65 percent of their calories from carbohydrates, 20 percent to 35 percent from fat, and 10 to 35 percent from protein. Acceptable ranges for children are similar to those for adults, except that infants and younger children need a slightly higher proportion of fat (25 to 40 percent).