Nutrition is a reasonable food for human beings to repair old tissues, proliferate new tissues, produce energy and maintain physiological activities. Substances in food that can be absorbed by the body are called nutrients. Carbohydrates, fats, proteins, vitamins, water and inorganic salts (dietary fiber is called the "seventh nutrient") are the six major nutrients needed by the human body. The first three produce energy after metabolism in the body, so they are also called energy-producing n...