Fat, also called lipid, is a compound made by chemically bonding fatty acids to glycerol to form glycerides. When three fatty acids are hooked to glycerol, the fat compound is a triglyceride. Almost 95% of fat stored in the body is triglyceride, with the remaining 5% consisting of other glycerides and cholesterol. Scientific literature usually refers to triglycerides when it discusses fat. The fatty acids that make up triglycerides can be saturated, monounsaturated, or polyunsaturated.

Chemically, fats are chains of carbon atoms strung together with hydrogen atoms. If it is a saturated fat, the carbon chain carries all the hydrogen atoms it can. If it is unsaturated, there is room in the carbon chain for more hydrogen. If the chain is monounsaturated, there is room for two hydrogen atoms. If it is polyunsaturated, there is room for four hydrogen atoms. If it is highly polyunsaturated, there is room for many more hydrogen atoms.

Many people mistakenly assume that the word "polyunsaturated" on a food label means that the fat in the food is not saturated. This may not be true because of food processing techniques. If the words "hydrogenated" or "partially hydrogenated" are on the food label, the food contains varying amounts of saturated fats. Because fats are less stable, they are prone to spoilage. Consequently, for many foods, manufacturers use a chemical process called hydrogenation, in which hydrogen atoms are added to the unsaturated or polyunsaturated fats to make them more saturated and more resistant to spoilage. If hydrogen is added to a monounsaturated fatty acid, it becomes a saturated fat. If hydrogen is added to only one pair of carbon atoms in a polyunsaturated fat, it becomes a monounsaturated fat. This process of hydrogenating food yields a new type of fat not found in nature called transfatty acids. Transfatty acids are saturated fats commonly found in margarine, fried fast foods, cookies, cakes, and many other foods made with shortening. Some scientists believe that transfatty acids, even those originating from a polyunsaturated food source, are as detrimental to health as saturated animal fat. High levels of these fats are less effective in lowering total and LDL cholesterol than the liquid oils from which they are made. Also they seem to depress HDL cholesterol.

How much trans fatty acid is in food? To answer this question it is necessary to have a complete breakdown of fat content. For example, a popular low-saturated margarine contains 10 grams of total fat with 2 grams of saturated fat, 1.5 grams of polyunsaturated fat, and 2 grams of monounsaturated fat.

Adding the amounts of these three fats and subtracting them from the total leaves about 4.5 grams unaccounted for. It is probably 4.5 grams of transfatty acids. Because food labels don't usually provide a complete breakdown of fat content, it is difficult to assess the amount of transfatty acids contained in a particular product.

Saturated and unsaturated fats can be differentiated by their appearance. Saturated fat is typically solid at room temperature. Lard, fat marbled in meat, and hardened grease from a skillet are good examples. Polyunsaturated fats are usually liquid at room temperature. Examples include safflower and corn oils. Solid vegetable shortenings are partially hydrogenated and have a soft consistency. Coconut oil, palm kernel oil, and palm oil are exceptions. They are vegetable oils and are liquid at room temperature, but they are among the most saturated of fats and the worst in terms of causing adverse changes in fatty cholesterol levels.

Fish oils are among the most unsaturated fats available. They are roughly twice as unsaturated as vegetable oils. They do not harden, even at low temperatures. Their unsaturation has created special interest in relation to heart disease. Fatty acids in coldwater seafood, such as salmon, mackerel, sardines. herring, anchovies, whitefish, bluefish, swordfish. rainbow trout, striped bass, Pacific oysters, and squid, consist of omega-3 fatty acids, which are thought to be effective at lowering cholesterol and triglyceride levels and reducing clot-forming rates, thereby reducing the risk of heart disease. Studies of eskimos have been cited to demonstrate the relationship between fish oils and heart disease. Eskimos consume more fat than americans but have significantly fewer heart attacks. The unexpected relationship is attributed to their primary source of fat: fish oil. Will americans benefit similarly? The results of recent studies have dampened some of the early enthusiasm for fish oil. A 6-year harvard study of 44,895 professional men, 40 to 75 years of age, did not show a decreased risk of heart disease resulting from the amount of fish consumed. One possible reason is that the fish consumed by americans was mainly fried commercially in cholesterol raising fat. While the fish oil-heart disease relationship is debated, the consensus recommendation is for americans to increase their consumption of cold-water seafood.

Cholesterol, a waxy substance that is technically a steroid alcohol found only in animal foods, is probably the most researched blood lipid. High levels of cholesterol are usually discussed as one of the major risk factors of cardiovascular disease.