A nutrient is either a chemical element or compound used in an organism's metabolism or physiology. A nutrient is essential to an organism if it cannot be produced by the organism and must be obtained from a food source. This article deals with nutrition in animals, especially humans. Nutrition in plants, fungi, bacteria and archaea is not discussed here. Substances that provide energy Carbohydrates are compounds made of sugars. Carbohydrates are named by the number of sugar units they have. Monosaccharides (such as glucose and fructose) have one. Disaccharides (such as sucrose and lactose) have two. Oligosaccharides, and polysaccharides (such as starch, glycogen, and cellulose) have more. Proteins are organic compounds that are amino acids joined by peptide bonds. The body cannot make some of these; so they must come from food. In the body, proteins are broken down through digestion back into free amino acids. Fats are a glycerin molecule with three fatty acids attached. Fats are needed to keep cell membranes functioning...