Fatty acid synthesis is regulated, both in plants and animals. Excess carbohydrate and protein in the diet are converted into fat. Only a relatively small amount of energy is stored in animals as glycogen or other carbohydrates, and the level of glycogen is closely regulated. Protein storage doesn't take place in animals. Except for the small amount that circulates in the cells, amino acids exist in the body only in muscle or other protein‐containing tissues. If the animal or human needs specific amino acids, they must either be synthesized or obtained from the breakdown of muscle protein. Adipose tissue serves as the major storage area for fats in animals. A normal human weighing 70 kg contains about 160 kcal of usable energy. Less than 1 kcal exists as glycogen, about 24 kcal exist as amino acids in muscle, and the balance—more than 80 percent of the total—exists as fat.
Plants make oils for energy storage in seeds. Because plants must synthesize all their cellular components from simple inorganic compounds, plants—but usually not animals—can use fatty acids from these oils to make carbohydrates and amino acids for later growth after germination.