The human body needs fatty acids; most are easily supplied in sufficient amounts through our diet, and if not we can make them from excess carbohydrates and proteins. There are two fatty acids that are considered essential fatty acids, because they cannot be synthesized by humans, and must be supplied by diet. These are linoleic acid and alpha-linolenic acid (commonly called linolenic acid, for short). Both linoleic and linolenic acid are long-chain polyunsaturated fatty acids with 18 carbon molecules. Linoleic acid is an example of an omega-6 fatty acid, so called because the first double bond of the carbon chain is placed in the sixth position, counting from the omega end of the fatty acid. It’s needed for normal growth and for synthesis of important hormonelike compounds called eicosanoids. By far, linoleic acid is the most abundant polyunsaturated fatty acid in our diet. The primary sources are cooking oils, salad dressings, nuts, and seeds.

Linolenic acid is an omega-3 fatty acid—with the first double bond of the carbon chain located in the third position. Like phospholipids and sterols, it’s a structural component of cell membranes. Linolenic acid is found in walnuts, flax seeds, soy, canola oil, and chia seeds. In relation to linoleic acid, intake of linolenic is low in the diets of most Americans, who consume only about one-tenth of our essential fatty acids in this omega-3 form.

One of the most important functions of essential fatty acids is to provide parent compounds to produce the hormonelike eicosanoids, compounds that regulate blood pressure, inflammation, and even pain. However, the effect of an eicosanoid on body functions depends on the fatty acid from which it is made. Two polyunsaturated fatty acids that are directly used for eicosanoid synthesis are arachidonic acid and EPA, which are made from linoleic and linolenic acids, respectively.

Blood clotting and inflammation is promoted when there is excess production of eicosanoids from arachidonic acid (an omega-6 fatty acid), compared with those produced from EPA (an omega-3 fatty acid). Blood clotting and inflammation are decreased when the production of eicosanoids from EPA increases. Because of these opposing effects, it is desirable to have a proper balance of omega-6 and omega-3 fatty acids in the diet. Although neither EPA nor DHA are essential fatty acids—in fact DHA can be made from EPA—we do not effectively convert linolenic acid to EPA or DHA. We can more efficiently boost EPA and DHA concentrations in our body by regularly eating fish, particularly fatty fish like salmon, albacore tuna, trout, and sardines, which are excellent sources of these fatty acids.(INFOGRAPHIC 6.8)

Question 6.8

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EPA and DHA in the bodies of the Inuit stemmed from their high intakes of fish, and marine mammals that eat lots of fish (such as seals). To better understand this diet, the scientists asked the Inuit to surrender their lunches so they could analyze the food for fatty acid content. Some would hand over plastic bags dripping with seal intestines and kidneys; one fisherman submitted raw liver and an eye. “The eye was a delicacy,” says Dyerberg. “We got one, and he ate the other one.” Even raw liver and intestine—if turned inside out and cleaned—is tasty,he recalls. “I’ve done that with the seal intestine many times. I can tell you it’s good.”

Sure enough, these lunch samples were packed full of EPA and DHA. But what about them was protecting the Inuit’s heart and blood vessels from cardiovascular disease? Were EPA and DHA more protective than other polyunsaturated fatty acids?