Protein, along with sugar and fat, belong to the three major categories of nutrients for the human body. Protein, like sugar and fat, can also provide calories to the body. The calories produced by protein are 4kcal from the oxidation of 1g of protein in the body, or 4 calories, which is the same as the calories produced by 1g of sugar, and much lower than the calories produced by fat, which is 9 calories from 1g of fat. 100g of protein, or 2 taels of protein, can produce 400 calories; 1 pound of protein, or 500g, can produce 2000 calories. Protein is digested and absorbed into the human body, and it can also provide calories to the body. Providing calories to the body is not the main function of protein, but usually sugar is the main source of calories for the body. Fat becomes the main source of calories for the body during hunger, and protein can also provide some of the calories.