Do the dietary guidelines have it wrong?

The consequences of replacing saturated fats with polyunsaturated (Ω-6) fats

Not only has the condemnation of saturated fats led to an increased consumption of carbohydrates, it has also led to several dietary guidelines recommending replacement of saturated fats with polyunsaturated fats, without specifying which polyunsaturated fatty acid (ie, Ω-3 vs Ω-6). The recommendation for increasing polyunsaturated fat stems from pooled analyses of data looking at increasing Ω-3 and Ω-6 polyunsaturated fatty acids.19 ,20 However, a meta-analysis of randomised controlled trials showed that replacing a combination of trans-fats and saturated fats with Ω-6 polyunsaturated fats (without simultaneously increasing Ω-3 fatty acids) leads to an increased risk of death.21 These results were corroborated when data were recovered from the Sydney Diet Heart Study and included in an updated meta-analysis.22

Other human trials, not included in the aforementioned meta-analysis, include the Anti-Coronary Club trial, which showed that more people died (overall (26 vs 6) and due to coronary heart disease (8 vs 0)) when saturated fat was replaced with polyunsatured fat.23 The National Diet Heart Trial, a randomised, double-blind study, also showed a higher number of cardiovascular events (n=4) on a diet that was high in the polyunsaturated(P)/saturated(S) fat ratio (2 : 1), than on a diet high in saturated fat (n=1, P/S=0.4).24Thus, advice to replace saturated fats with polyunsaturated fats (ie, Ω-6) may increase the risk of coronary heart disease, cardiovascular events, death due to coronary heart disease and overall mortality.21–24

Reasons for the potential harmful effects of Ω-6 fatty acids may be due to their promotion of cancer, suppression of the immune system, lowering of HDL-C and increasing the susceptibility of LDL to oxidation.25 Further evidence indicates a role of Ω-6 in promoting prostate26–28 and breast cancer.29 This is supported by the Anti-Coronary Club study, where there was 71% increased risk of death from causes other than coronary heart disease among individuals who were placed on a diet designed to increase the P/S ratio in those who had not experienced a new coronary event.30 Moreover, in a controlled clinical trial by Dayton et al,31 there was a greater than threefold increased risk of death due to carcinoma when saturated fat (mainly of animal origin) was substituted for Ω-6 polyunsaturated fat (including corn, soybean, safflower and cottonseed). The potential harms of replacing saturated fat with carbohydrates or Ω-6 polyunsaturated fats are summarised in box 1.

In the Women's Health Initiative (WHI), a randomised controlled trial including 48 835 postmenopausal women, a low-fat diet was not shown to reduce coronary heart disease, stroke or cardiovascular disease,33 despite a significant reduction in LDL-C, nor was there a reduction in cancer.34 ,35 A meta-analysis by Siri-tarino et al36 consisting of 21 prospective epidemiological studies, derived from 347 747 participants, indicated that the intake of saturated fat does not increase coronary heart disease or cardiovascular disease. Moreover, a recent Cochrane meta-analysis indicated that changing dietary fat intake does not affect total mortality or cardiovascular mortality.37 Although reducing saturated fat was associated with a reduced risk of cardiovascular events by 14%, this was not shown with reducing total fat consumption.37 While the WHI study and the Siri-tarino and Cochrane meta-analyses cannot be taken at face value, taken together with “the low-fat diet in myocardial infarction trial”, a compelling argument can be made for the general lack of evidence in support of a low-fat diet. Dietary recommendations based on evidence from the literature are summarised in box 2.

The final nail in the low-fat diet coffin is two randomised trials, one for the primary prevention of cardiovascular disease, PREDIMED38 (Prevención con Dieta Med- iterránea), indicating a reduction in the incidence of major cardiovascular events with a Mediterranean diet compared with a low-fat diet, and the other for the secondary prevention of cardiovascular disease, the Lyon Diet Heart Study39 showing that a Mediterranean diet reduces all-cause and cardiovascular mortality as well as non-fatal myocardial infarction compared with a prudent diet.

Conclusions

In summary, the benefits of a low-fat diet (particularly a diet replacing saturated fats with carbohydrates or Ω-6 polyunsaturated fatty acids) are severely challenged. Dietary guidelines should assess the totality of the evidence and strongly reconsider their recommendations for replacing saturated fats with carbohydrates or Ω-6 polyunsaturated fats.

• Accepted January 7, 2014.

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