Lipids are required for every physiological system in the body and are thus essential nutrients. Because of the inherent difficulties related to the hydrophobic nature of lipids, the process of lipid circulation in the body is more complex than it is for other macronutrients. Triglycerides make up most of the lipids in foods and in the body, and are usually what is referred to when the term "fat" is used. Phospholipids are another class of lipids. In addition to triglycerides and phospholipids, the lipids include the sterols, compounds with a multiple-ring structure. Lipoproteins help transport both dietary lipids from the small intestine and stored or newly synthesized lipids from the liver. Since most body cells cannot effectively break down cholesterol, it must be returned to the liver to be eliminated from the body. Recommendations for a healthy diet suggest that we consume a diet moderate in fat and low in saturated fat, trans fat, and cholesterol.