One gram of protein has 4 calories. The USDA recommends that 10 to 35 percent of a person's calories should come from protein. The body uses protein for growth and repair of tissue, production of hormones and enzymes, immune system functions, protection of muscle mass and energy.
Carbohydrates also contain 4 calories per gram. These nutrients are primarily used as the body's energy source. They area also important to maintain proper function of many body systems, including the digestive, muscular and central nervous systems.
Each gram of fat has 9 calories. Fats help the body grow and develop, absorb certain vitamins, protect organs and sustain cell membranes. Fat also provides energy.
Alcohol has 7 calories per gram. Unlike proteins, carbohydrates and fats, it is not considered an essential nutrient because it is not necessary to survival