Originally published in The Ottawa Citizen October 05, 2005
The omega-3 (linolenic) and omega-6 (linoleic) fatty acids discussed in detail last week are the focus of many clinical trials. Have the clinical studies supported the health claims?
Several well-designed cardiovascular health studies support the hypothesis that omega-3 fatty acids like eicosapentaenoic acid (EPA) and decosahexanoic acid (DHA) have a protective effect on the heart. In the Zutphen study, men who ate fish more than once a week had significantly less coronary artery disease rates than men who rarely consumed fish.
The diet and reinfarction trial found that men with a previous history of heart attack who ate two or more fatty fish servings per week over two years had a 29 per cent reduction in all-cause mortality (the number of deaths expected per year in a population). They were compared to men with similar heart attack histories who did not eat fish.
The Lyon diet heart health trial looked at the effect of two diets in people who had recent heart attacks. One group was randomized into a Mediterranean diet rich in omega-3 fatty acids and the control group participated in the American Heart Association step-1 diet.
Although there was no improvement in cholesterol levels or body mass index in the two groups, the death rates were different. Fourteen people in the Mediterranean diet group died, whereas 59 died in the control group; a 76 per cent relative reduction in mortality.
Several studies suggest that omega-3 fatty acids improve lipid profiles. In one study, four grams of omega-3 per day reduced LDL-cholesterol levels by five to 10 per cent, increased HDL one to three per cent, and reduced triglycerides by 20 to 30 per cent. HDL is the “good” cholesterol that helps eliminate cholesterol from the body while LDL does the reverse.
Several reviews indicate modest reductions in blood pressure with increasing fish oil consumption. It had no effect on people with normal blood pressure.
There are some small studies that point to a reduction of morning stiffness and joint swelling in people with rheumatoid arthritis. These effects were observed in people taking greater daily dosages of fish oil for at least 12 weeks. Further reducing omega-6 intake seemed to reduce the chemical mediators that cause inflammation. Some patients were able to reduce their dose of anti-inflammatory medications.
In the March 23 issue of the Journal of Neuroscience, a study out of UCLA looked at DHA’s effect on Alzheimer’s disease progression.
Mice were genetically altered to develop Alzheimer’s disease. One group was fed DHA-fortified feed and the control group had a normal diet, or a DHA-depleted diet.
The DHA-fed mice had 70 per cent less buildup of amyloid protein in the brain over a three- to five-month interval (equivalent to several human years).
Another study by the same group published in Neuron in the fall of 2004 showed that DHA protected the brain cells communication areas (synapses). These mice performed better on memory tests.
People eating fatty fish should be aware of the contamination risk from polychlorinated biphenyls (PCBs), methylmercury, dioxins, and other environmental contaminants. They can accumulate in certain fish such as king mackerel, shark, swordfish and tilefish. Health Canada advises Canadians to “limit consumption of these fish and fresh and frozen tuna, to one meal per week.
“Pregnant women, women of child-bearing age and young children should eat no more than one meal per month.”
Shrimp, canned light tuna, salmon, pollock and catfish are low in mercury. Albacore (white tuna) has more mercury than canned light tuna and should be limited to less than six ounces per week.
A survey published in 2004 in the journal Science (Hites RA, Foran JA, Carpenter DO, Hamilton MC, Knuth BA, Schwager SJ. Global assessment of organic contaminants in farmed salmon. Science 2004;303:226-9) found that farmed salmon has significantly higher levels of polychlorinated biphenyls and other organochlorine contaminants than wild salmon. Researchers disagree about how much farmed salmon consumption is safe. These contaminants are usually not found in high-quality fish oil supplements.
It is always preferable to get our nutrients from food. Foods other than oily fish that are a good source of omega-3 fatty acids include oils (canola, walnut, fish and flaxseed), green leafy vegetables, nuts, seeds and beans.
Although well-tolerated, some common side effects of fish oil supplements are dose dependent; a fishy aftertaste and gastrointestinal effects like nausea, bloating and belching.
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© Dr. Barry Dworkin 2005