Fish, health and omega-3 fatty acids

Fish and cardiovascular disease

Enthusiasm for a role for fish in the prevention of coronary heart disease (CHD) emerged in the 1970's. This was stimulated by observations of the low incidence of CHD in Greenland Eskimos, despite the consumption of a diet rich in fat – seal fat. They had low blood lipids and prolonged bleeding times compared with Danes living on a Western diet. Further research showed the Japanese, who have traditionally consumed high quantities of fish, also had a low incidence of CHD. Although scientists were unsure which component in fish was responsible for the apparent benefit, the research focus narrowed on the omega-3 fatty acids. 

Fish and seafood are a good source of omega-3 fatty acids EPA and DHA. Studies show that these fatty acids in fish and seafood have a valuable antiarrhythmic role. They reduce myocardial irritation and stabilise the heart muscle to prevent sudden cardiac death and other future cardiac events, which is consistent with data that they have an anti-arrhythmic effect. They also appear to improve blood lipid profiles, have anti-inflammatory and anti-clotting effects. Overall, epidemiological studies provide evidence of an inverse association between regular fish consumption (1-2 meals/week) and death from CHD, especially non-sudden death from myocardial infarction (Daviglus et al. NEJM 1997; 336: 1046-53).

Several large prospective studies have suggested an apparent cardio-protective effect of relatively small amounts of fish eaten regularly (Katan , NEJM 1995; 332: 1024-5). 

People who eat omega-3 fats as seafood have a lower risk of primary cardiac arrest (Siscoviccket al.JAMA1995;274:1363-7). 

However, the very large Health Professionals Follow-up study published in 1995 failed to confirm an association with non-fatal coronary disease (Ascherio et al. NEJM 1995; 332:997-82), but the risk of death from CHD was 25% lower in fish eaters compared with non-fish eaters. Most of the men in this study had greatly increased their intake of fish over the 10 years prior to the start of the study which may have obscured a ‘dose’ related response.

The Dart Trial from Wales (Burr et al. Lancet 1989; 2: 757-62) with 2033 men who had recovered from a myocardial infarction, found that men who ate equivalent to ONE fish meal per week had a 39% reduction in mortality, although the risk of myocardial infarction was not affected, similar to the study by Hu et al (NEJM 1999; 69: 890-7). 

Long-term 20 year data from 852 men in the Zutphen study in the Netherlands showed that eating 200g of fish a week (equivalent to one serving) gave protection against heart disease in the long term – mortality was 50% lower in the fish eaters compared with the non-fish eaters. The Boston study has also shown that one fish meal a week significantly reduces the risk of CHD, although they were unable to show that eating more fish had any extra advantage. Another study from the Netherlands showed that the risk of coronary mortality in fish eating elderly was half that of non-fish eaters (Kromhout et al. Int J Epidem 1995; 24: 340-5). 

The data from these studies suggest that a small amount of fish is protective against coronary mortality and the benefits are particularly evident for those in whom consumption was initially low. The cardio-protection occurs with 1-2 servings of fish per week (beyond this level of intake further benefit may be marginal) and the elderly can also benefit from increased fish intake. Fish seems to protect against heart arrhythmia, and ventricular fibrillation (the main cause of sudden death from acute heart attacks). 

Omega-3 marine fats may also relieve rheumatoid arthritis (Kremer et al. Lipids 1996; 31: 243-7; Shapiro et al. Epidemiology 1996; 7: 3:256 -263), the skin condition psoriasis and lessen the risk of childhood asthma (more evidence needed). The cardio-protective protective effect of fish is probably due to the omega-3 content, however, there may be another component in fish that is yet to be identified which is also providing protection.

 

Last Updated: March 27, 2001.