11.8: Lipids

The body can convert the lipids contained in foods into almost all the other types of lipids it needs. It can also synthesize most types of lipids from carbohydrates and proteins. For example, the liver can manufacture cholesterol.

Though lipids are a diverse group of substances, most dietary lipids fall into only a few categories. We will examine glycerides and cholesterol here (Chapter 2).

Tri-, Di-, and Monoglycerides and Fatty Acids

The most abundant dietary lipids are triglycerides, which consist of three fatty acid molecules attached to a glycerol molecule. Triglycerides are also called fat on food labels and may be either solid or liquid (oils) at room temperature. Many foods contain monoglycerides and diglycerides, which contain one or two fatty acids, respectively. Monoglycerides and diglycerides are not called fat. Therefore, they are not included as part of the fat content listed on food labels even though they contain and give the body the same components as do triglycerides.

The fatty acids in fat may be saturated or unsaturated. Unsaturated fatty acids may be monounsaturated or polyunsaturated. Unsaturated fatty acids differ from one another not only in chain length and number of double bonds but also in the locations and orientations of those bonds. Foods from plants have higher proportions of monounsaturated and polyunsaturated fatty acids, while foods of animal origin have higher proportions of saturated fatty acids.

Naturally occurring unsaturated fatty acids in foods, including those in fat, may have hydrogen added during processing. Fatty acids and fat treated in this way are said to be partially hydrogenated or hydrogenated. Hydrogenation makes the fatty acids and fat more solid and thus improves the texture of some foods.

Two fatty acids are of special importance because they are essential parts of body molecules and must be obtained in the diet since they cannot be manufactured by the body. Therefore, these two fatty acids—linoleic acid and alpha-linoleic acid—are referred to as essential fatty acids. These fatty acids are also called omega-3 fatty acids because of the location of a double bond. Fat in plant oils and fish oils have high levels of these fatty acids.

Cholesterol

The other important dietary lipid is cholesterol. This substance, which has a molecular structure resembling chicken wire, is found only in foods from animals. Foods high in cholesterol include those containing egg yolks, cream, and fat from meats.

Uses

Dietary lipids have several functions. They make some foods more pleasant by improving flavor and texture and producing a sense of fullness and satisfaction. They also aid the absorption of vitamins A, D, E, and K. Lipids that have been absorbed are used to produce bile, supply energy, and build body components and chemicals (e.g., fat tissue, cell membranes, vitamin D, steroid hormones). The two essential fatty acids are used in building cell membranes and producing a variety of chemicals that help regulate diverse processes, including blood clotting, stomach secretion, and immune system functioning.

Recommended Dietary Intakes

Since diets in the United States typically contain more lipids than are needed, many dietary recommendations for adults provide upper rather than lower limits for dietary lipids. In general, fat should account for less than 30 percent of total daily energy intake, and saturated fat and polyunsaturated fat should each account for less than 10 percent. Cholesterol intake should be below 300 mg per day. Many steps can be taken to keep dietary lipid intakes below these limits, and the absorption of dietary cholesterol can be reduced somewhat by diets high in soluble fiber.

Lipid Deficiencies

Individuals with lipid-deficient diets may have low energy levels and deficiencies in fat-soluble vitamins. Diets lacking sufficient plant or fish oils do not provide enough essential fatty acids, resulting in varied disorders, including abnormalities in blood clotting, blood pressure, and immune system functions.

Lipid Excesses

Diets containing excess lipids promote several problems, including indigestion, obesity, colon cancer, atherosclerosis, and possibly breast cancer.

See Limiting Intake of Fat, Saturated Fat, and Cholesterol.

For tips on limiting fat intake, see also https://www.biologyofhumanaging.com/tblfats - true.htm .

When fat and cholesterol are absorbed by the small intestine, they combine with other lipids (phospholipids) and blood proteins to form droplets of lipoprotein. Lipoproteins are also formed using fat and cholesterol made by the liver in cases of positive energy imbalance, especially when it is severe enough to cause obesity. Since all these lipoproteins contain much fat and are relatively very light, they are called very low-density lipoproteins (VLDLs). Much of the fat in VLDLs is removed and used by body cells. The remaining particles are not as light and are called low-density lipoproteins (LDLs).

LDLs are removed from the blood by liver cells and other cells that have receptors for them. Liver cells eliminate the cholesterol from the LDLs they receive by excreting it into bile or converting it into other useful materials. Other body cells store much of the cholesterol in the LDLs they receive. When cells lining blood vessels store LDL cholesterol, it initiates the formation of atherosclerotic plaques.

Since diets low in saturated fat help liver cells remove LDLs from the blood, less LDL enters other cells and less plaque formation occurs. Conversely, diets high in saturated fat inhibit liver cells from removing LDLs from the blood, and cells in vessels remove the LDLs and form plaques. Therefore, plaque formation is kept low by low-saturated-fat diets and is raised by high-saturated-fat diets.

The number of double bonds and their locations in unsaturated fatty acids influences their amount of risk for atherosclerosis. Monounsaturated fatty acids are less likely to produce lipid peroxides (LPs). Polyunsaturated fatty acids (PUFAs) help keep blood LDLs low, but when they have double bonds at a location called the omega-6 position, they promote LP formation. PUFAs with a double bond in the omega-3 position produce less LPs and help keep blood clots and blood pressure low.

Plaque formation is also kept low by lipoproteins called high-density lipoproteins (HDLs), which have relatively little cholesterol or other lipids. HDLs carry cholesterol from body cells to the liver for elimination. This explains why a high HDL/LDL ratio partially counters the adverse effects of high blood cholesterol and high LDL levels and thus helps reduce the risk of atherosclerosis. One way exercise lowers the risk of atherosclerosis is by increasing HDLs and thus lowering blood cholesterol and LDL levels.

A total blood lipoprotein level (total cholesterol level) of approximately 200 mg per 100 milliliters of blood (200 mg/dl) is used by many people to mark the boundary between acceptable and unacceptable blood lipid levels. Individuals with total cholesterol levels substantially above 200 mg/dl have a much higher risk of developing atherosclerosis, while individuals with levels well below 200 mg/dl have a rather low risk. Having LDL levels above 165 mg/dl or HDL levels below 35 mg/dl also implies a substantial risk.

Many individuals can maintain healthful cholesterol and lipoprotein values or improve poor values by eating foods with much soluble fiber, avoiding dietary cholesterol and saturated fat, sustaining the recommended weight through energy balance, and exercising. These steps can substantially reduce the risk of developing atherosclerosis. (Suggestion 247.02.04)