Since dietary support and advice by health professionals and self-help materials are effective at reducing cardiovascular risk, at least in the short term, what actual changes to diet are effective in protecting people from cardiovascular disease? Again considering systematic reviews of randomised controlled trials as the best level of evidence, we are lucky that quite a few have been published in the area of diet and cardiovascular disease. The most important studies show that dietary intervention actually makes a difference to health or mortality.
1.4.1 Dietary Interventions That May Reduce Illness and Death
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.1618 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.
1.4.2 Dietary Interventions That May Alter Risk Factors
It is, of course, useful to know which dietary interventions affect which risk factors. The problem is that the answers are not always entirely consistent with effects on health. The reason is that altering one dietary component may affect many other dietary components and risk factors. If all of these effects worked in the same direction (to promote health), then the overall effect on health might be much greater than the estimated effect on the risk factor alone. On the other hand, if some effects on risk factors are positive and others are negative, the result might be no overall gain in health or even an overall loss of health even though the effects on the single risk factor measured looked promising. For this reason, understanding the effects of changes in diet on individual risk factors is interesting but not as helpful as understanding overall effects on health.
Unless otherwise specified, all references in this section are to systematic reviews of randomised controlled trials.
While dietary advice has a role to play in normalizing abnormal serum lipids in people with cardiovascular disease, aspects of diet that clearly protect against death and disease should be given greater emphasis than lipid reduction in this group. Dietary changes are likely to result in reductions of total cholesterol of about 5%,25-27 while statin trials reduce total cholesterol by 18 to 28%; therefore, lipid lowering medication is more effective than dietary advice.25 Metabolic ward studies suggest that replacing 60% of saturated fats by other fats and avoiding 60% of dietary cholesterol would reduce serum total cholesterol by 0.8 mmol/L (about 13%), but it appears difficult to maintain this in a normal lifestyle.25 Replacing saturated fats with unsaturated fats leads to improved lipid levels28 and a reduction in cardiovascular events, but it is not clear whether polyunsaturated or monounsaturated fats are more cardioprotective.29
Daily use of realistic levels of soluble fiber (found in oats, pectin, psyllium, guar gum) will lower total serum cholesterol by about 2%,30,31 while large intakes of purified soy protein will lower total cholesterol levels by about 10%.32 Garlic supplements appear to lower serum cholesterol but trials are of poor quality and may have been biased.33-37
Weight can be altered by dietary change, but this appears to be difficult. Trials are often of very short duration and suffer from high drop-out rates, limiting their validity. Several good quality systematic reviews offer insights. A behavioral component improves weight loss in dietary and exercise programs (including very low calorie diets), as do written meal plans, weekly shopping lists, and group (rather than individual) therapy. Weight maintenance strategies (such as support groups) should be integral parts of all weight loss programs.38 There is little strong evidence that the proportion of dietary fat (as distinct from calories eaten) has an effect on body weight.38-40 In terms of managing cardiovascular risk, people with hyperlipidemia should receive dietary lipid lowering advice in addition to weight management advice.41
Elevated homocysteine levels can be reduced by supplementation with folic acid, alone or with vitamins B6 and B12,22,42,43 but it is not yet clear whether the reduction will reduce cardiovascular risk.
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If you’re overweight, you are not a bad person. You’re simply overweight. But it’s important to lose the extra pounds so you’ll look good, feel healthier and develop a sense of pride and self-esteem. Once you’ve lost the fat, you’ll need to maintain your weight.