Omega-3 Fatty Acids: Metabolism Matters

Sharon Palmer, R.D. Contributing Editor Comments

Omega-3 fatty acids are definitely on consumers’ radar. In the 2007 “Food & Health” survey from the International Food Information Council, Washington D.C., 71% of consumers reported they were aware of omega-3 fatty acids, up from 63% in 2006. But the omega-3 fatty acids’ effect on health is far more complicated than meets the eye.

“The metabolism of omega-3 fatty acids is underappreciated,” says Joyce Nettleton, D.Sc., editor, “Fats of Life” newsletter and “PUFA Newsletter.” Indeed, consumers may have a hard time grappling with the complexities of omega-3 fatty acids actions and the differences between individual omega-3 fatty acids, especially when combined with the many overlapping concerns that muddy the waters, including fat vilification, fish toxicity, endangered fish species and vegetarianism.

Omega-3s up close

“It is very important to look at what type of omega-3s people are getting,” says Nettleton. After all, not all omega-3s are created equally, structurally and functionally. Classified as polyunsaturated fatty acids (PUFAs), all omega-3 fatty acids have their first double bond located between the third and fourth carbon atom, counting from the methyl end of the fatty acid (n-3). Omega-6 fatty acids’ first double bond is positioned between the sixth and seventh carbon atom from the methyl end of the fatty acid (n-6). Because humans lack the enzyme required to insert a cis double bond at the n-6 or n-3 position of a fatty acid, consuming omega-3 and omega-6 fatty acids is essential.

The 18-carbon omega-3 fatty acid, alpha-linolenic acid (ALA), is a parent fatty acid within the omega-3 fatty acid group—humans can synthesize important long-chain (at least 20 carbons) omega-3 fatty acids, eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) from it. The conversion of ALA, commonly found in plant sources like flax seed, soy, walnuts and canola oil, to the important long-chain omega-3 fatty acids, EPA and DHA, is modest.

The long-chain gang

A growing body of evidence is linking increased EPA and DHA intakes to decreased risk of cardiovascular disease, arrhythmias that can lead to sudden cardiac death, and thrombosis that can prompt myocardial infarction or stroke, as well as decreased levels of triglycerides, slower growth of atherosclerotic plaque, improved vascular endothelial function, modest blood pressure lowering, and decreased inflammation. FDA has concluded there is sufficient evidence for a qualified health claim for omega-3 fatty acids, EPA and DHA, and the risk of coronary heart disease. In addition, long-chain omega-3 fatty acids have shown success in treating symptoms of rheumatoid arthritis and psychiatric disorders, including schizophrenia and major depressive disorder.

DHA, in particular, might play an important role in vision and central nervous function, since it is part of retinal cell and postsynaptic neuronal cell membranes. Brain gray matter phospholipids contain high levels of DHA. Especially critical is the knowledge that DHA is accumulated in the fetal brain and retina during the last trimester of pregnancy. “Because it is crucial for fetal brain development and function, it is recommended that pregnant and nursing women consume at least 200 mg per day of DHA,” says Nettleton. “There is growing evidence that this long-chain omega-3 is essential throughout most of life. Later in life, this fatty acid may reduce the risk of Alzheimer’s disease. It also helps protect neurons.”

EPA has its own important functions related to anti-inflammatory and anti-thrombotic actions. During an inflammatory response, the omega-6 fatty acid, arachidonic acid (AA), and EPA in cell membranes can be metabolized by enzymes to form prostaglandins and leukotrienes. If more AA than EPA is found in the diet, as is typical in the United States, the formation of more eicosanoids derived from AA occurs rather than those derived from EPA. The body responds to these eicosanoids differently. “Eicosanoids derived from AA tend to promote platelet aggregation and thrombosis, which doesn’t help the heart. Eicosanoids derived from EPA dampen down these pro-inflammatory effects, because they compete for the same enzymes and yield products with only weak inflammatory effects,” says Nettleton.

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