Dietary fats and plasma lipids

Triacylglycerols (TAG) are quantitatively the most important class of dietary fats. The influence of TAG on blood lipids is determined by the chemical nature of their constituent fatty acids. The absence or presence and number of double bonds (saturated versus mono- and polyunsaturated), the location of the double bonds (ω-6 versus ω-3), and the cis versus trans configuration of the unsaturated fatty acids are the most important structural features that influence blood lipids.
1. Saturated fats: TAG composed primarily of fatty acids whose hydrocarbon chains do not contain any double bonds are referred to as saturated fats. Consumption of saturated fats is positively associated with high levels of total plasma cholesterol and LDL-C and an increased risk of CHD. The main sources of saturated fatty acids are dairy and meat products and some vegetable oils, such as coconut and palm oils (a major source of fat in Latin America and Asia, although not in the United States). Many experts strongly advise limiting intake of saturated fats to <10% of total caloric intake and replacing them with unsaturated fats (and whole grains).
Saturated fatty acids with carbon chain lengths of 14 (myristic) and 16 (palmitic) are most potent in increasing the plasma cholesterol level. Stearic acid (18 carbons, found in many foods including chocolate) has little effect on blood cholesterol.
2. Monounsaturated fats: TAG containing primarily fatty acids with one double bond are referred to as monounsaturated fats. Monounsaturated fatty acids (MUFA) are generally obtained from plant-based oils. When substituted for saturated fatty acids in the diet, MUFA lower both total plasma cholesterol and LDL-C and maintain or increase HDL-C. This ability of MUFA to favorably modify lipoprotein levels may explain, in part, the observation that Mediterranean cultures, with diets rich in olive oil (high in monounsaturated oleic acid), show a low incidence of CHD.
[Note: Although there is no AMDR for MUFA, a common recommendation is 10%–20% of caloric intake.]
a. The Mediterranean diet: The Mediterranean diet is an example of a diet rich in MUFA (from olive oil) and polyunsaturated fatty acids or PUFA (from fish oils, plant oils, and some nuts) but low in saturated fat. For example, Figure 1 shows the composition of the Mediterranean diet in comparison with both a Western diet similar to that consumed in the United States and a typical low-fat diet. The Mediterranean diet contains seasonally fresh food, with an abundance of plant material, low amounts of red meat, and olive oil as the principal source of fat. The Mediterranean diet is associated with decreased plasma total cholesterol and LDL-C, decreased TAG, and increased HDL-C when compared with a typical Western diet higher in saturated fats.

Figure 1: Composition of typical Mediterranean, Western, and low-fat diets.
3. Polyunsaturated fats: TAG containing primarily fatty acids with more than one double bond are referred to as polyunsaturated fats. The effects of PUFA on cardiovascular disease are influenced by the location of the double bonds within the molecule.
a. ω-6 Fatty acids: These are long-chain PUFA, with the first double bond beginning at the sixth bond position when starting from the methyl (ω) end of the fatty acid molecule. [Note: They are also called n-6 fatty acids .] Consumption of fats containing ω-6 PUFA, principally linoleic acid (18:2 [9,12]), obtained from vegetable oils, lowers plasma cholesterol when substituted for saturated fats. Plasma LDL-C is lowered, but HDL-C, which protects against CHD, is also lowered, partially offsetting the benefits of lowering LDL-C. Nuts, avocados, olives, soybeans, and various oils, including sunflower and corn oil, are common sources of these fatty acids. The AMDR for linoleic acid is 5%–10%. [Note: The lower recommendation for intake of PUFA relative to MUFA is because of concern that free radical–mediated oxidation (peroxidation) of PUFA may lead to deleterious products.]

b. ω-3 Fatty acids: These are long-chain PUFA, with the first double bond beginning at the third bond position from the methyl (ω) end. Dietary ω-3 PUFA suppress cardiac arrhythmias, reduce plasma TAG, decrease the tendency for thrombosis, lower blood pressure, and substantially reduce risk of cardiovascular mortality (Fig. 2), but they have little effect on LDL-C or HDL-C levels. Evidence suggests that they have anti-inflammatory effects. The ω-3 PUFA, principally α-linolenic acid, 18:3(9,12,15), are found in plant oils, such as flaxseed and canola, and some nuts, such as walnuts. The AMDR for α-linolenic acid is 0.6%–1.2%. Fish oil contains the long-chain ω-3 docosahexaenoic acid (DHA, 22:6) and eicosapentaenoic acid (EPA, 20:5). Two fatty fish (for example, salmon) meals per week are recommended. For patients with documented CHD, 1 g/day of fish oils is recommended, while 2–4 g/day is prescribed to lower TAG. [Note: DHA is included in infant formulas to promote brain development.] Linoleic and α-linolenic acids are essential fatty acids (EFA) requiredfor membrane fluidity and synthesis of eicosanoids . EFA deficiency, caused primarily by fat malabsorption, is characterized by scaly dermatitis as a result of the depletion of skin ceramides with long-chain fatty acids .

Figure 2: Dose responses of physiologic effects of fish oil (ω-3) intake. EPA = eicosapentaenoic acid (20:5); DHA = docosahexaenoic acid (22:6).
4. Trans fatty acids: Trans fatty acids (Fig. 3) are chemically classified as unsaturated fatty acids but behave more like saturated fatty acids in the body because they elevate LDL-C and lower HDL-C, thereby increasing the risk of CHD. Trans fatty acids do not occur naturally in plants but occur in small amounts in animals. However, trans fatty acids are formed during the hydrogenation of vegetable oils (for example, in the manufacture of margarine and partially hydrogenated vegetable oil).
Trans fatty acids are a major component of many commercial baked goods, such as cookies, and most deep-fried foods. Many manufacturers have reformulated their products to be free of trans fats. In 2006, the U.S. Food and Drug Administration required that Nutrition Facts labels  portray the trans fat content of packaged food. By 2018, virtually no industrial trans fatty acids will be permitted in food.

Figure 3: Structure of cis and trans fatty acids.
5. Dietary cholesterol: Cholesterol is found only in animal products. The effect of dietary cholesterol on plasma cholesterol (Fig. 4) is less important than the amount and types of fatty acids consumed. Many experts recommend ≤300 mg/day. However, having an upper limit has become controversial.

Figure 4: Response of plasma low-density lipoprotein (LDL) cholesterol concentrations to an increase in dietary cholesterol intake.