The fatty mouse on the left has a lot of adipose tissue. In many animals, adipose tissue is used to store energy. For comparison, a mouse with a normal amount of adipose tissue is shown on the right

Fat is one of the three main types of macronutrients. Fats are found in many foods. They are made of macromolecules called lipids. Lipids are based on long-chain fatty acids. Some of these are essential because the body cannot make them.