What is Omega-3 and Omega-6 fatty acids & their known functions?
There are two major classes of polyunsaturated fatty acids (PUFAs): the omega-3 and the omega-6 fatty acids. They are distinguished by their chemical structure. Only the fatty acids alpha-linolenic acid (ALA) and linoleic acid (LA) must come from the diet because they cannot be made by the body. ALA, an omega-3 fatty acid, is converted in the body to eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA). EPA and DHA also occur naturally in some foods. LA, an omega-6 fatty acid, is converted in the body to arachidonic acid (AA). Both EPA and DHA are metabolized through the same biochemical pathways as AA. Studies show that omega-3 fatty acids in general decrease triglyceride and very-low-density lipoprotein blood levels in hyperlipidemic individuals but may increase or have no effect on low-density lipoprotein (LDL) levels.
Both AA and EPA are further metabolized to produce hormone-like agents called eicosanoids, which include prostaglandins, thromboxanes, and leukotrienes. Eicosanoids regulate fundamental physiological processes such as cell division and growth, blood clotting, muscle activity, secretion of digestive juices and hormones, and movement of substances like calcium into and out of cells. However, AA and EPA lead to the production of different subgroups of eicosanoids with sometimes opposing effects. Eicosanoids formed from AA (particularly the series-2 prostaglandins and series-4 leukotrienes) are released in the body in response to injury, infection, stress, or certain diseases. They increase platelet aggregation and enhance vasoconstriction and the synthesis of substances involved with the inflammatory process. Eicosanoids derived from EPA (particularly the series-3 prostaglandins), in contrast, decrease excessive series-2 prostaglandin production. As a result, adequate production of EPA-derived series-3 prostaglandins may help protect individuals against heart attacks and strokes as well as certain inflammatory diseases such as arthritis, systemic lupus erythematosus, and asthma.
The omega-3 fatty acid DHA, while not involved in eicosanoid formation, is the major polyunsaturated fatty acid found in the brain and is important for brain development and function. Synapses are rich in DHA, which suggests that this fatty acid is involved in signal transmission along neurons. DHA is also required to produce one member of a family of compounds called resolvins that participate in the body’s response to inflammation in the brain. The DHA-derived resolvin in particular helps to reduce inflammation brought about by ischemic insults (reductions in blood flow). (EPA also helps to temper inflammatory responses by decreasing production of proinflammatory compounds such as cytokines.)
Most Western diets provide at least 10 times more omega-6 than omega-3 fatty acids. There is now general scientific agreement that individuals should consume more omega-3 and fewer omega-6 fatty acids for good health. It is not known, however, whether a desirable ratio of omega-6 to omega-3 fatty acids exists for the diet or to what extent high intakes of omega-6 fatty acids interfere with any benefits of omega-3 fatty acid consumption.
What foods contains Omega 3 (ALA – alpha-linolenic acid)?
- Leafy green vegetables
- Vegetable oils such as canola and soy, and especially in flaxseed and flaxseed oil.
- Good sources of EPA and DHA are fish (both finfish and shellfish and their oils and eggs) and organ meats.
What foods contains Omega 6 (LA – linolenic acid)?
- Vegetable oils (e.g., safflower, sunflower, corn, soy)
- Processed foods made with these oils.
The Institute of Medicine has established Adequate Intakes for ALA and LA (1.1-1.6 g/day and 11-17 g/day, respectively, for adults) but not for EPA and DHA.
Omega-3 fatty acids are found in a variety of dietary supplements. For example, products containing flaxseed oil provide ALA, fish-oil supplements provide EPA and DHA, and algal oils provide a vegetarian source of DHA.
Omega-3 fatty acids for cardiovascular health and disease
Epidemiological studies first published in the late 1970s noted relatively low cardiovascular mortality in populations such as Eskimos with high fish consumption. The apparent health benefits of fish are explained, at least in part, by the EPA and DHA they contain.
The three reports by the Tufts EPC focused on different areas of research concerning this relationship between omega-3 fatty acids and cardiovascular health and disease and involved systematic reviews of the available scientific-medical literature.
Cardiovascular risk factors and intermediate markers of CVD
Overall, strong evidence showed that fish-oil supplements had a substantial and beneficial effect on triglycerides that was greater with larger intakes of fish oil; most studies reported a net decrease of about 10-33%. There is also evidence of a very small beneficial effect of fish oils on blood pressure and possible beneficial effects on coronary artery restenosis after angioplasty, exercise capacity in patients with coronary atherosclerosis, and heart rate variability (particularly in patients with recent myocardial infarctions).
Overall, evidence from both the primary and secondary prevention studies supports the hypothesis that consumption of omega-3 fatty acids, fish, and fish oil reduces all-cause mortality and various CVD outcomes such as sudden death, cardiac death, and myocardial infarction. The evidence is strongest for fish or fish oil whereas the potential effects of ALA are largely unknown and the relative effects of ALA versus fish oil are not well defined. In the only RCT that directly compared ALA and fish oil, both treatments reduced CVD outcome.
The lessons to be drawn from all these studies to date regarding use of omega-3 fatty acids for preventing and treating CVD are not completely clear. Because the studies involved a variety of methods of estimating fish or omega-3 fatty acid intake, background diets, background risk for heart disease, settings, and methods for reporting results, the validity of applying the results of studies conducted outside the United States to the U.S. population is uncertain. Furthermore, dietary intervention trials are limited by the multiple and complex dietary changes in the trials that make it difficult to distinguish among components and determine which specific components or combinations of these diets are most beneficial. For example, the different types of fish consumed and the method of food preparation may cause different effects.
Omega-3 fatty acids for asthma
Basic research suggests that omega-3 fatty acids may affect asthma because they influence substances that are part of the inflammatory process involved with asthma, such as the series-2 prostaglandin PGE2.
Whether omega-3 fatty acids are effective in the primary prevention of asthma is unknown. Four observational studies in children support a positive association for dietary patterns that include all fish or oily fish, but a prospective study of adult nurses found no association between asthma onset and dietary fish intake.
Omega-3 fatty acids for other diseases
The RAND EPC conducted a comprehensive search of published and unpublished scientific-medical literature on the health effects of omega-3 fatty acids in type II diabetes and metabolic syndrome, inflammatory bowel disease, rheumatoid arthritis, renal disease, systemic lupus erythematosus, and bone density/osteoporosis.
Type II diabetes and metabolic syndrome
Eighteen of the 34 RCTs whose subjects had type II diabetes or metabolic syndrome provided sufficient statistics to be included in a meta-analysis. The analysis found that omega-3 fatty acids had a favorable effect on triglyceride levels when compared with placebo but had no effect on total, LDL, or HDL cholesterol; fasting blood sugar; or glycosylated hemoglobin. A qualitative analysis of 4 studies concluded that omega-3 fatty acids had no effect on plasma insulin or insulin resistance in subjects with either disorder.
Inflammatory bowel disease (Crohn’s disease and ulcerative colitis)
In the 13 studies that reported outcomes in patients with inflammatory bowel disease, omega-3 fatty acids had variable effects on assessment scores (clinical, sigmoidoscopic, and histologic), induced remission, and relapse rates.
A meta-analysis of nine studies of patients with rheumatoid arthritis concluded that omega-3 fatty acids had no effect on patients’ reports of pain and disease severity, swollen joint count, or erythrocyte sedimentation rate (a measure of disease activity). However, an earlier meta-analysis found that the omega-3 fatty acids produced a statistically significant improvement in tender joint count as compared with placebo. A qualitative analysis of seven studies that assessed the effect of omega-3 fatty acids on anti-inflammatory drug or corticosteroid requirements found that six demonstrated reduced requirements. Overall, omega-3 fatty acids appear to reduce tender joint counts in individuals with rheumatoid arthritis and may reduce requirements for corticosteroids.
Omega-3 fatty acids and cognitive function, dementia, and neurological diseases
Omega-3 fatty acids appear to be important in brain development and function. Their effects on cognitive function in normal aging, incidence and treatment of dementia, incidence of several neurological diseases, and progression of multiple sclerosis were evaluated. A comprehensive search of the published and unpublished scientific-medical literature identified 12 studies that met inclusion criteria.
Multiple sclerosis and other neurological diseases
The quantity and strength of evidence for the effects of omega-3 fatty acids on cognitive function and decline, dementia, and neurological diseases vary greatly. Given the overall small number of studies and generally poor quality of clinical trials, substantive conclusions about the value of these compounds for these conditions cannot be drawn.
Omega-3 fatty acids for organ transplantation
Several laboratory, animal, and human studies suggest that omega-3 fatty acids from fish oil may improve outcomes in organ transplantation (e.g., decrease rejection; reduce hyperlipidemia, hypertension, and blood viscosity; and decrease the toxicity of the immunosuppressive agent cyclosporin A).