8.6: Summary

Dietary fat is important in many ways:

• It contains the essential fatty acids linoleic acid (an omega-6 fatty acid) and linolenic acid (an omega-3 fatty acid). These are the only fats known to be required in our diet, and deficiencies are rare.
• It carries the fat soluble vitamins A, D, E, and K.
• It makes food more pleasurable—carrying fat-soluble flavors and making foods more tender, smooth, and juicy.
• Fat is a very concentrated source of calories— compared to carbohydrate or protein, it doesn’t have much oxygen in its structure, and excludes water.

Fat, by definition, doesn’t dissolve in water, but dissolves in organic solvents such as ether or chloroform. Triglycerides, phospholipids, and cholesterol are three different types of fat, but when we speak of fat or fatty foods or salad oil, we are generally referring to triglycerides.

Triglycerides make up 98% of the fat in our food, and is the storage form of fat in our body. Phospholipids and cholesterol generally are referred to specifically by name.

A triglyceride is made up of three fatty acids attached to a backbone of glycerol. Since the glycerol portion is a constant, and there are always three fatty acids, triglycerides differ only in having different fatty acids. When one speaks of saturated or unsaturated fats, one is referring to the kinds of fatty acids in the triglyceride.

A fatty acid can be thought of as a chain of hydrogen-carrying carbons. It can vary in the number of carbons in its chain (fatty acids with 16-18 carbons predominate in our diet) and also in the number and location of double bonds in that chain. The terms omega-3 and omega-6 refer to the location of the first double bond in the chain.

Fatty acids that don’t have any double bonds in their carbon chain are called saturated fatty acids. Fatty acids with one double bond are called monounsaturated, and those with more than one double bond are called polyunsaturated.

Saturated means that a fatty acid is holding all the hydrogens it can hold—it’s saturated with hydrogen. In contrast, an unsaturated fatty acid can take on more hydrogens wherever there is a double bond between two of its carbons.

A predominance of saturated fatty acids makes a fat solid at room temperature, whereas a predominance of unsaturated fatty acids makes a fat liquid. Fats from land animals generally predominate in saturated fatty acids, whereas fats from plants and sea animals generally predominate in unsaturated fatty acids and are called oils.

By a process called hydrogenation (adding hydrogen to unsaturated fatty acids), food companies can take vegetable oils and make them more solid. The extent of hydrogenation can be adjusted to get the desired solidity. Oils treated this way are described on food labels as “partially hydrogenated oils.” A by-product of partially hydrogenating oils is trans fatty acid, where the arrangement around a remaining double bond changes to where it is more like a saturated fatty acid in shape and function. Artificial trans fat (created by manufacturers) is now banned in processed food sold in the U.S. because trans fat increases the risk of heart disease.

Phospholipids are similar in structure to triglycerides, except that one of the positions on the glycerol backbone holds a phosphorus-containing substance rather than a third fatty acid. Phospholipids (e.g., lecithin) are used as an emulsifier in foods. In the body, phospholipids form the basic structure of our cell membranes.

Cholesterol is quite unlike the structures of triglycerides and phospholipids. It can’t be broken down in the body, and therefore excessive amounts can be a problem. It’s used in our body in cell membranes and to make essential substances. Phospholipids and cholesterol are made in our body and aren’t essential in the diet.