Protein is what the body is made of. The body needs protein in order to build itself (replacing dead cells, growing, repairing injuries, etc.), and for a variety of other purposes. As such protein is an essential nutrient. Protein can also be burned by the body for energy; as such it contains calories. Foods high in protein include meat, dairy, eggs, nuts, beans, and grains.
Protein is the most expensive essential nutrient, and the diets of impoverished people are often deficient in it. For example, some people in poor countries subsist primarily on cassava root, a food which is high in carbohydrate but low in protein. These people are at risk of developing protein-energy malnutrition, wherein they may be in a state of starvation even if receiving adequate calories. This condition is ultimately fatal.
The DRIs set an RDA of 0.8g protein per kilogram of body weight. So for example a 70kg (154lb) person would have an RDA of 56g protein. However, things are more complicated than this.
Protein is made of molecules called "amino acids." There are 22 different amino acids, and our body needs all of them. Of these, all but nine can be synthesized by the body from other amino acids. These nine amino acids are called the "essential amino acids." It is necessary not only to get enough protein, but also to get enough of each essential amino acid in one's diet. The DRIs establish the following RDAs for each essential amino acid:
- Tryptophan: 0.295g
- Threonine: 1.18g
- Isoleucine: 1.121g
- Leucine: 2.478g
- Lysine: 2.242g
- Methionine + Cysteine : 1.121g
- Valine: 1.416g
- Phenylalanine + Tyrosine: 1.947g
- Histidine: 0.826g
As usual there are extra complications, this time in the columns labeled "methionine + cysteine" and "phenylalanine + tyrosine." Methionine is an essential amino acid, and cysteine is an amino acid which can be synthesized from methionine. The DRIs set an RDA of 1.121g for the sum of the intake of methionine and the intake of cysteine. (Presumably it is also necessary that, for example, not the entirety of that summation is from cysteine. This would never occur in the real world.) Similarly, phenylalanine is an essential amino acid, tyrosine can be synthesized from phenylalanine, and the DRIs define an RDA for the sum of their two intakes.
The DRIs recommend that no more than 35% of calories come from protein.