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Cell membranes were once thought to be relatively static barriers separating a cell’s surroundings from its inner workings. Today we know cell membranes are bustling with activity. Long-chain polyunsaturated fatty acids are vital membrane components that serve in cellular communication. These fatty acids are released from membranes when certain signals are received by a cell and are shuttled along metabolic pathways to form a bewildering array of potent bioactive signaling molecules. The specific agent formed from these polyunsaturated fatty acids depends on the cell that makes it—enzymes expressed in one type of cell differ from those expressed in other cells. The products formed will also depend on the essential fatty acid status of active membrane components and the types of signals received. These mercurial lipid messengers can easily pass through membranes, so there is no mechanism for confining them in vesicular bundles (which is generally the case for nonlipid hormones and neurotransmitters) until the time is right for their release. The lipid messengers from fatty acids are formed instantaneously and migrate only short distances to elicit their actions on neighboring cells or within the cell that forms them. They exist for a brief moment to accomplish their purpose and are rapidly inactivated by enzymes that keep them in check. Their fleeting existence has made it difficult to study these elusive characters, which are often major players in the orchestration of life’s complex processes. The polyunsaturated fatty acids may also undergo spontaneous oxidation that is not dependent on enzymes. This process is known as lipid peroxidation and may destroy cell membranes, which can lead to the ultimate demise of a cell. A variety of potentially toxic and carcinogenic substances that are produced during lipid peroxidation may spread to other cells and cause persistent damage there. Although the polyunsaturated fatty acids are essential to life, it may be possible to get too much of them. 15 Dietary Choices for Comprehensive Health 201 INFLUENCE OF DIET ON OVERALL HEALTH 202 The saturated and monounsaturated fatty acids can be synthesized in the body from other nutrients, typically from excess dietary carbohydrate (sugars and starches). Consequently they are not essential fatty acids. The saturated and monounsaturated fatty acids are resistant to lipid peroxidation, and the messengers formed from them are far fewer and less formidable in their actions. What Fatty Acids Are Better in the Diet? There is widespread opinion that saturated fats in the diet are unhealthy. This belief stems from early studies of the effects of dietary fats on serum cholesterol in humans, done in the 1950s and early 1960s (discussed in chapter 1). The early studies used saturated fat sources that had negligible amounts of polyunsaturated fatty acids and consequently caused serum cholesterol levels to rise. Such diets would not normally be consumed in a free-living population. Today we know that moderate amounts of polyunsaturated fatty acids in the diet regulate expression of specific genes to suppress total serum cholesterol and regulate other metabolic pathways. The level of dietary polyunsaturated oils needed to accomplish a more favorable lipid profile is not certain, because genetic variation is a major influence on serum cholesterol levels in any individual. Recent research has helped to clarify some of these diverse effects of polyunsaturated fatty acids (Bordoni et al. 2006). Some studies of the impact of dietary fats on serum cholesterol are at odds with early studies that established the saturated fat–serum cholesterol association and the relationship of both to cardiovascular disease. Two separate investigations monitored the effects of diets containing different amounts of monounsaturated versus saturated fatty acids on serum cholesterol levels in young, healthy volunteers with normal serum cholesterol levels (Ng et al. 1992; Choudhury, Tan, and Truswell 1995). The aim was to compare diets that had predominantly monounsaturated fatty acids from olive oil with diets that had nearly equal proportions of monounsaturated and saturated fatty acids from palm oil. Each of the diets had about 10 percent polyunsaturated linoleic acid. Both studies found nearly identical levels of total serum cholesterol and LDL-cholesterol in subjects at the end of thirty days or six weeks on each experimental diet. These results refute the notion that a higher ratio of saturated relative to monounsaturated fatty acids in dietary fats produces a less favorable lipid profile that might increase the risk of heart disease. Another study compared diets containing palm oil with high-oleic sunflower oil as the major sources of fats and showed...

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