Research Review by Dr. Shawn Thistle©

Date:

2004

Study Title:

Science, Medicine, and the Future Omega 3 fatty acids and cardiovascular disease - fishing for a natural treatment

Authors:

Din JN, Newby DE, Flapan AD

Publication Information:

British Medical Journal 2004; 328: 30-35.

Summary:

This is an important clinical review discussing the evidence for the role of omega 3 fatty acids (from fish oils) in the treatment of cardiovascular disease. This paper is summarized below. The interested reader can access the full paper online at the link listed above.

In the 1970s Danish researchers first suggested that high intakes of omega 3 fatty acids might be protective against cardiovascular disease. This arose from observational studies of the Greenland Inuit, who have a low mortality from coronary heart disease and consume large amounts of omega 3 fatty acids from fish, seal and whale in their diets.

Since that time, "…most studies have shown an inverse association between fish consumption and the risk of coronary heart disease." [p. 31] It is important to note that those studies not showing this association were done mainly on populations who already had a high fish intake, thereby potentially masking any relation.

Several prospective trials have been performed to assess this relationship. Most of these studies have been done on patients after myocardial infarction (heart attack) - and therefore assess the secondary preventive capabilities of fish oils.

For example, the diet and reinfarction trial (DART) randomized 2033 men with a recent heart attack into three dietary interventions. Patients who received advice on fish had a relative reduction in total mortality of 29% during the two-year follow up (mainly because of a reduction in deaths from coronary heart disease).

In a larger Italian study (over 11 000 patients), those who were given fish oil capsules had a reduction in relative risk of cardiovascular death of 30% and of sudden death by 45% (these results were apparent within only 4 months of randomization).

Despite the abundant evidence to support the use of fish oil (either dietary or via supplement) in patients who have suffered a heart attack, the knowledge of the exact mechanism behind this association remains sparse.

Proposed mechanisms include:
  • antiarrhythmic effect
  • prevention of thrombus (clot) formation
  • prevention of atherosclerosis
  • anti-inflammatory effect
  • improved endothelial function
  • blood pressure reduction
  • triglyceride lowering effect
Initially, it was thought that the primary mechanism of action was anti-thrombotic, although recently the effect of omega 3 fatty acids on platelet function has become controversial. Recent evidence suggests that omega 3 fats have inconsistent effects on fibrinolysis and little effect on blood coagulability. Omega 3 fatty acids have been shown to have a direct positive effect on vasomotor activity (artery dilation), suppress pro-inflammatory metabolites in the endothelial lining and produce modest reductions in blood pressure.

The most recent work in this area actually suggests that the predominant effect of omega 3 fats may be anti-arrhythmic. More work is needed to elucidate the mechanism of effect and determine appropriate recommendations for dosing. Further, the primary preventive role of omega 3 fats should be investigated (i.e. can supplementation prevent heart attacks), although this presents many study design difficulties.

Conclusions & Practical Application:

Current intake of dietary omega 3 fatty acids from marine sources is low (0.1-0.2 g/day) - despite recommendations from various health care organizations to consume 0.65-1.2 g/day. Secondary prevention recommendations after a heart attack indicate that consuming 0.5-1.8 g/day may be beneficial. This can come in the form of dietary supplements, or consumption of oily fish such as mackerel, salmon, herring, tuna, trout and sardines.

Current recommendations from the American Heart Association are:
  • Patients without documented coronary heart disease: Eat a variety of (preferably oily) fish at least twice weekly. Include oils and foods rich in linolenic acid.
  • Patients with documented coronary heart disease: Consume 1 g of eicosapentanoic and docosahexanoic acid daily, preferably from oily fish. Supplements could be considered in consultation with a doctor.
  • Patients with hypertriglyceridaemia: Take 2-4 g of eicosapentanoic acid and docosahexanoic acid daily, provided as capsules under a doctor's care.
As mentioned earlier, no trial has assessed the effects of fish oils in the prevention of coronary heart disease; therefore explicit recommendations for this group cannot be made yet. The consumption of fish twice weekly as part of a balanced diet is still a prudent recommendation. As always, discuss any dietary supplement intake with your primary health care professional.