To date, the most effective dietary intervention for people who already have cardiovascular disease is omega-3-rich fish oil. Evidence for this comes from a high-quality systematic review of randomised controlled trials.14 Advice to increase intakes of long chain omega-3 fats for people with some cardiovascular disease (compared with no such advice) appears to reduce the risk of fatal myocardial infarction (relative risk 0.7, 95% CI 0.6 to 0.8), sudden death (relative risk 0.7, 95% CI 0.6 to 0.9), and overall death (relative risk 0.8, 95% CI 0.7 to 0.9), but not nonfatal myocardial infarction (relative risk 0.8, 95% CI 0.5 to 1.2). The effects of these cardioprotective doses of omega-3 fats appear consistent whether the advice is dietary (eating more oily fish, usually 2 to 3 large portions weekly) or supplemental (taking the equivalent of 0.5 to 1.0 g of a mixture of eicosapentanoic acid (EPA) and docosahexanoic acid (DHA) fatty acids daily).
A further systematic review examined the effects of omega-3 fats in diabetics.15 Unfortunately, no studies or large subgroups of published studies assess the effects of omega-3 fats on disease endpoints in diabetics. There is no evidence of detrimental effects of cardioprotective doses of omega-3 fats on glycemic control or LDL cholesterol levels (higher levels of supplementation have been used to reduce triglyceride levels; the smaller cardioprotective doses mentioned above may well save lives of diabetics but do not alter triglycerides significantly). More evidence would be useful to clarify this issue.
Several systematic reviews have assessed the effect on morbidity and mortality of reductions in dietary fats.16-18 A systematic review including 27 studies and over 30,000 person-years of follow-up revealed that a reduction in saturated fat, if followed for at least 2 years, produced a small but potentially important reduction in risk of cardiovascular events.19 Most of the included studies aimed to replace saturated fats with unsaturated fats, rather than achieving big reductions in total fat intake. This alteration of dietary fat intake had a minimal effect on total mortality (rate ratio 0.98, 95% CI 0.86 to 1.12). Cardiovascular mortality was (nonsignifi-cantly) reduced by 9% (rate ratio 0.91, 95% CI 0.77 to 1.07) and cardiovascular events significantly reduced by 16% (rate ratio 0.84, 95% CI 0.72 to 0.99). Trials with at least 2 years' follow-up provided stronger evidence of protection from cardiovascular events (rate ratio 0.76, 95% CI 0.65 to 0.90).
Although no studies compare the effect of reducing saturated fats to that of increasing omega-3 fats, an indirect comparison suggests that the effect of reducing saturated fats is smaller than the effect of increasing omega-3 fats, takes longer to be seen, but may increase in importance over periods longer than 2 years.
Other systematic reviews suggest no evidence of protective effects of dietary supplements of antioxidant vitamins20-22 and no evidence of effects of garlic capsules on peripheral arterial occlusive disease.23
Evidence for a Mediterranean diet high in omega-3 fats, fruits, and vegetables and low in saturated fats and processed foods comes from only one trial in men who had recovered from myocardial infarctions.24 While the effects of increasing fruits and vegetables and reducing processed foods appear promising, it is not clear how much of the protective effect seen in this study was due to the rapeseed (canola) margarine supplied to the intervention group (high in omega-3 fats), how much was due to reductions in saturated fats, and how much (if any) was due to fruits and vegetables.
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