Fats exist in foods—and are usually stored in the body—as triglycerides. Recent research relating levels of triglycerides in the blood stream to heart attacks in human presents a sometime confusing picture but a mounting level evidence suggests that, along with other indicators, triglyceride levels can be used to predict heart attack risk, especially in women and diabetics.
Although the exact mechanisms are not fully known, elevated triglycerides allow increased blood clot formation and may slow the natural deterioration of clots once formed.
Fat molecules are generally made up of four parts: a molecule of glycerol and three molecules of fatty acids. Each fatty acid consists of a hydrocarbon chain with a carboxyl group at one end. The glycerol molecule has three hydroxyl groups, each able to interact with the carboxyl group of a fatty acid. Removal of a water molecule at each of the three positions forms a triglyceride. The three fatty acids in a single fat molecule may be all alike or they may be different. They may contain as few as four carbonatoms or as many as 24. Because fatty acids are synthesized from fragments containing two carbon atoms, the number of carbon atoms in the chain is almost always an even number. In animal fats, 16-carbon, for example, palmitic acid and 18-carbon, for example, stearic acid fatty acids are the most common.
Some fatty acids comprising a given triglyceride have one or more double bonds between their carbon atoms. They are then said to be unsaturated because they can hold more hydrogen atoms than they do. Mono-unsaturated fats have a single double bond in their fatty acids while polyunsaturated fats, such as trilinolein, have two or more. Additionally, there are trans-fats, which are only partially hydrogenated having fewer double bonds in a trans (as opposed to the usual cis) chemical configuration, and also omega-3 fats, which have at least one double bond, three carbon atoms in from the end of the fatty acid molecule. Linolenic acid is an example and fish oils are generally a rich source of omega-3 fatty acids.
Double bonds are rigid and those in natural fats introduce a kink into the molecule. This prevents the fatty acids from packing close together and as a result, unsaturated fats have a lower melting point than saturated fats. Because most of them are liquid at room temperature, they are called oils. Corn oil, canola oil, cottonseed oil, peanut oil, and olive oil are common examples. As this list suggests, plant fats tend to be unsaturated while fats from such animals as cattle tend to be saturated.
Ingested fats provide the precursors from which fat as well as cholesterol and various phospholipids are created (synthesized). In humans, fat provides the concentrated form of energy. The energy content of fat (9 kcal/gram) is more than twice as great as carbohydrates and proteins (4 kcal/gram).
Humans can synthesize fat from carbohydrates. However, there are two essential fatty acids that cannot be synthesized this way and must be incorporated into the diet. These are linoleic acid (an omega-6 fat, with the endmost double bond six carbons from the methyl end) and alpha-linolenic acid (an omega-3 fat, with the endmost double bond three carbons from the methyl end). Many studies have examined the relationship between fat in the diet and cardiovascular disease. There is still no consensus, but the evidence seems to indicate that a diet high in fat is harmful and that mono- and poly-unsaturated fats are less harmful than saturated fats, with the exception of trans unsaturated fats which, according to some, are more harmful than saturated fats. It is also been suggested that ingestion of omega-3 unsaturated fats may be protective for the human body.