M Richter MSc, RD(SA), Prof CM Smuts PhD, RNT(SA), Centre of Excellence for Nutrition, North-West University
Early studies showed that dietary saturated fat and cholesterol intake is associated with coronary heart disease (CHD)1. More recent epidemiological studies, however, have shown positive2, inverse3 or no associations4 of dietary saturated fat with CHD mortality and/or morbidity. It should be noted, however, that individual dietary saturated fatty acids do not necessarily affect CHD risk factors equally and that food sources of these fatty acids may also play a role. For example, a diet high in stearic acid (C18:0) does not raise serum cholesterol levels, however, it lowers LDL-C when compared to diets enriched with palmitic (C16:0) or myristic acid (C14:0) and lauric acid(C12:0)5, 6. The hypercholesterolemic effect of palmitic acid is more than that of lauric acid 7. Myristic acid is more hypercholesterolemic than palmitic acid, however, part of the effect may be attributed to an increase in HDL-C7.Additionally, trans fatty acids have clear adverse effects on CHD and should be avoided as far as possible4, 8.
A recent Meta-analysis of prospective cohort studies, evaluating the association of saturated fat with cardiovascular disease,found no significant evidence for concluding that dietary saturated fat is associated with increased risk of CHD9. This study further suggested that the historically assumed beneficial effects of diets with reduced saturated fat on CVD risk may be dependent on a significant increase in polyunsaturated fat in these diets due to substitution to keep energy constant between groups in studies9. The WHO/FAO report of an expert consultation even concluded that there is convincing evidence that replacing saturated with polyunsaturated fat decreases the risk of CHD10. Studies often only measure the total amount of saturated fat, monounsaturated fat or polyunsaturated fat in the diet. One should keep in mind, however, that it is becoming clearer in science that even within classification-groups of fat such as polyunsaturated fatty acids (PUFA), sub-groups (i.e.omega-3 PUFA or omega-6 PUFA), specific individual fatty acids or even ratios can cause different effects with regards to coronary heart disease.A recent evaluation of recovered data from the Sydney Diet Heart Study, for example, found that substituting dietary saturated fat with linoleic acid increased the rates of death from coronary heart disease and cardiovascular disease11.Modest increased intakes of long-chain omega-3 PUFA, on the other hand,have been shown to result in pronounced cardiovascular benefits, however, a decreased risk in cardiovascular mortality is probably due to the beneficial effect of omega-3 PUFA on thrombosis or on cardiac arrhythmias rather than on lipoprotein profile12.A meta-analysis of randomised controlled trials onCHD, that considered effects ofspecifically omega-6 fatty acids aswell as a combination of omega-3 and omega-6fatty acids, criticised studies and meta-analyses that advise to substitute saturated fatty acids for omega-6 PUFA rich vegetable oils13. Critique includes oversight of relevant trials with unfavourable outcomes; inclusion of trials with weak design and dominant confounders; failure to distinguish between trials that selectively increased omega-6 PUFA from trials that substantially increased n-3 PUFA; failure to acknowledge that omega-6 and omega-3 PUFA replaced large quantities of trans fatty acids, in addition to saturated fatty acids13.
The message regarding fatty acids and CHD has changed a lot as research evolved. It is important to consider the new research, which has yet again changedthe perspective on fat and fatty acids and coronary heart disease, and to convey the correct message as nutrition experts to the public. It is important to keep in mind that even though saturated fat was not conclusively proven to increase risk, substitution with polyunsaturated fat decreases the risk of CHD, indicating that saturated fat is still a less favourable choice with regards to CHD. One should also acknowledge the differences effects of subtypes of fat as well as individual fatty acids, when making recommendations. Although fat is an important source of energy in the diet, the main message should be to balance energy intake with energy expenditure in an effort to reach and maintain a normal body weight and to ensure that the type of fat consumed promotes health.
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