They are the only source of the essential fatty acids linoleic and linolenic acids. They are an important source of calories for many adults and for infants and toddlers, who have the highest energy needs per kilogram of body weight of any age group. Fat provides 9 calories per gram, compared with 4 calories per gram for protein and carbohydrates. Energy is stored in the body mostly in the form of fat. Fat is needed in the diet to supply essential fatty acids, substances essential for growth but not produced by the body itself.
All fatty acids are molecules composed mostly of carbon and hydrogen atoms.
A saturated fatty acid has the maximum possible number of hydrogen atoms attached to every carbon atom. Some fatty acids are missing one pair of hydrogen atoms in the middle of the molecule. Monounsaturated and polyunsaturated fatty acids are mostly found in foods of plant origin and some seafoods. These are byproducts of partial hydrogenation, a process in which some of the missing hydrogen atoms are put back into polyunsaturated fats. Both fat and cholesterol belong to a larger family of chemical compounds called lipids.