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 is 1g of protein oxidized in the body to produce 4kcal of calories, or 4 calories, which is the same as the calories produced by 1g of sugar, and is much lower than the calories produced by fat, which is 9 calories for 1g of fat. Protein can also provide calories to the body after digestion and absorption into the body. Providing calories to the body is not the main function of protein, and usually sugar is the main source of calories for the body. Fat becomes the main source of calories for the body when hungry, and protein can also provide some of the calories.