There are 4 calories in a gram of protein. Calories are measured as the potential amount of energy that a type of food provides the body. The USDA recommends that 10 to 35 percent of daily caloric intake should come from proteins.
Proteins are broken down by the body into amino acids. Essential amino acids are introduced to the body by foods that are consumed, and nonessential amino acids are naturally produced by the body. Proteins from animal sources contain all essential amino acids the human body requires for proper function, while proteins from strictly plant-based sources lack certain essential amino acids.