Fats, or lipids, are the third main class of the macronutrients needed in human nutrition. The lipids are found primarily in meats and dairy foods, at least, these are the most visible sources, but most foods contain some fat. Some of the richer vegetable sources of dietary fat are nuts and seeds, soybeans, olives, peanuts, and avocados, and these contain the needed or essential fatty acids (EFAs). Fats are an important component of our diet, and at least a minimum intake is essential. However, many problems are associated with excessive intake of dietary fat, including obesity, cardiovascular disease, and some forms of cancer.

Primarily a form of energy reserves and insulation in the body, fats can be burned to make energy when we need energy and are not getting enough from our diet. Fats are important in transporting other nutrients, such as vitamins A, D, E, and K—the “fat-soluble vitamins.” Fats are also an essential component of the cell membrane, and internal fatty tissues protect the vital organs from trauma and temperature change by providing padding and insulation. Fatty tissue, in fact, even helps regulate body temperature.

An important component of lipids is the fatty acids. Three essential fatty acids are needed biochemically by our bodies and are available to us only from our diet: linoleic acid (LA), arachidonic acid, and linolenic acid (LNA). All are commonly contained in plant oils. Some sources describe linoleic acid as the only true essential fatty acid, as the others may be made from it. With more recent research it appears that this is true in plants but not in humans. We can make arachidonic acid from linoleic acid but not the important linolenic acid. Actually humans need higher amounts of alpha-linolenic acid than the other fatty acids, and luckily, these three fatty acids are commonly found together in food sources anyway. Most of our dietary intake of fat is in the form of triglycerides, which are composed of three fatty acids and a glycerol molecule. A less prevalent form of dietary fat is the phospholipids, such as lecithin, which is important to cell membranes and the brain and nerves. Cholesterol, a member of the sterol family, is both found in foods and manufactured by the body. It is essential to many body functions but has been implicated as a primary factor in heart and blood vessel disease.

Levels of fat intake are highly correlated with weight. High consumption of dietary fat is associated with both increased body fat and obesity. Fats are the most concentrated source of food energy, supplying nine calories per gram, more than double the calorie content of the proteins and carbohydrates. They provide about 42 percent of the calories in the average American diet. A diet that derives closer to 20–25 percent of total calories from fat is probably healthier. A range of 10–20 percent is also acceptable and may be helpful in reducing the incidence and progression of cardiovascular disease by lowering blood levels of triglycerides and cholesterol. Reducing fat intake to this level means cutting down greatly on consumption of red meats and dairy products such as milk, cheese, and butter. Restricting dietary fat (which usually reduces calorie intake as well) while maintaining adequate protein and complex carbohydrate intake is probably the best long-range approach to weight loss, maintenance of optimal body weight, and general good health.