Any of many long lipid-carboxylic acid chains found in fats, oils, and as a component of phospholipids and glycolipids in animal cell membranes. When insulin levels are too low or there is not enough glucose in the blood to use for energy, the body burns fatty acids for energy. This causes ketone bodies to be created as waste products, and they can cause the acid level in the blood to become too high. This in turn may lead to ketoacidosis, a serious illness.