At one end of the carbon chain is the acid group which is able to combine with glycerol. The nature of the fat depends on the types of fatty acids which make up the triglycerides. All fats contain both saturated and unsaturated fatty acids but are sometimes described as saturated or unsaturated according to the proportions of fatty acids present. For example, butter is often described as a saturated fat because it has more saturated fatty acids than unsaturated fatty acids, while most vegetable oils are described as unsaturated fats as they have more mono- or polyunsaturated fatty acids. As a rough guide, saturated fats are solid at room temperature and tend to be from animal sources.
Most unsaturated fats are liquid at room temperature and are usually vegetable fats.
This is known as hydrogenation . In this way unsaturated fats are made more saturated, and some trans fatty acids may be formed. Hydrogenated oils are frequently used in the manufacture of margarine and cooking fats. These are important in the formation of cell membranes particularly in nerve tissue. A high fat intake, and in particular a high intake of saturated fatty acids, has been associated with a raised blood cholesterol level, which is one of the risk factors for coronary heart disease.