Fats and oils

Nutrients that provide the body with its most concentrated form of energy. Fats, which are also called lipids, are compounds containing chains of carbon and hydrogen with very little oxygen. Chemically, fats consist mostly of fatty acids combined with glycerol. They are divided into 2 main groups, saturated and unsaturated, depending on the proportion of hydrogen atoms. If the fatty acids contain the maximum possible quantity of hydrogen, the fats are saturated. If some sites on the carbon chain are unoccupied by hydrogen, they are unsaturated; when many sites are vacant, they are polyunsaturated. Monounsaturated fats are unsaturated fats with only one site that could take an extra hydrogen. Animal fats, such as those in meat and dairy products, are largely saturated, whereas vegetable fats tend to be unsaturated. Fats are usually solid at room temperature; oils are liquid. The amount and types of fat in the diet have important implications for health. A diet containing a large amount of fat, particularly saturated fat, is linked to an increased risk of atherosclerosis and subsequent heart disease and stroke. Some dietary fats, mainly triglycerides (combinations of glycerol and 3 fatty acids), are sources of the fat-soluble vitamins A, D, E, and K and of essential fatty acids. Triglycerides are the main form of fat stored in the body. These stores act as an energy reserve and also provide insulation and a protective layer for delicate organs. Phospholipids are structural fats found in cell membranes. Sterols, such as cholesterol, are found in animal and plant tissues; they have a variety of functions, often being converted into hormones or vitamins. Dietary fats are first emulsified by bile salts before being broken down by lipase, a pancreatic enzyme. They are absorbed via the lymphatic system before entering the bloodstream. Lipids are carried in the blood bound to protein; in this state they are known as lipoproteins. There are 4 classes of lipoprotein: very low-density lipoproteins (VLDLs), low-density lipoproteins (LDLs), high-density lipoproteins (HDLs), and chylomicrons. LDLs and VLDLs contain large amounts of cholesterol, which they carry through the bloodstream and deposit in tissues. HDLs pick up cholesterol and carry it back to the liver for processing and excretion. High levels of LDLs are associated with atherosclerosis, whereas HDLs have a protective effect. (See also nutrition.)