New insights into the health effects of dietary saturated and omega 6 and omega 3 polyunsaturated fatty acids.

Cardiovascular diseases and cancers are leading causes of morbidity and mortality. Reducing dietary saturated fat and replacing it with polyunsaturated fat is still the main dietary strategy to prevent cardiovascular diseases, although major flaws have been reported in the analyses supporting this approach. Recent studies introducing the concept of myocardial preconditioning have opened new avenues to understand the complex interplay between the various lipids and the risk of cardiovascular diseases. The optimal dietary fat profile includes a low intake of both saturated and omega 6 fatty acids and a moderate intake of omega 3 fatty acids. This profile is quite similar to the Mediterranean diet. On the other hand, recent studies have found a
positive association between omega 6 and breast cancer risk. In contrast, omega 3 fatty acids do have anticancer properties. It has been shown that certain (Mediterranean) polyphenols significantly increase the endogenous synthesis of omega 3 whereas high intake of omega 6 decreases it. Finally, epidemiological studies suggest that a high omega 3 to omega 6 ratio may be the optimal strategy to decrease breast cancer risk. Thus, the present high intake of omega 6 in many countries is definitely not the optimal strategy to prevent cardiovascular disease
and cancers. A moderate intake of plant and marine omega 3 in the context of the traditional
Mediterranean diet (low in saturated and omega 6 fatty acids but high in plant monounsaturated fat) appears to be the best approach to reduce the risk of both cardiovascular diseases and cancers, in particular breast cancer. (Authors abstract)

Cardiovascular disease (CVD) is a leading cause of death in most countries. Reducing saturated fatty acid (SFA) intakes is still at the heart of dietary recommendations to reduce CVD, mainly because of its effect on blood cholesterol. This view has recently been challenged. Epidemiological studies failed to conclude that SFAs are associated with an increased risk of CVD. The validity of meta-analyses of clinical trials showing that CVD can be prevented by replacing SFAs with polyunsaturated fatty acids (PUFAs) has been questioned because they omitted relevant trials with unfavorable outcomes (selection bias) and included others that were poorly designed (no randomization). It has been claimed that the effect of diet on a single biomarker (such as plasma cholesterol) is insufficient evidence to assess CVD risk. The hypothetical protective effect of omega 6 PUFAs has been said to be considerably exaggerated And clinical and epidemiological studies exploring the dietary fat issue failed to provide a clear biological understanding of the effect of the various dietary fats on the risk of CVD. There is one exception: the Mediterranean diet, which is a complex interplay between the different series of dietary lipids, including conjugated or non-conjugated (animal or industrial) trans fatty acids, short, medium and long-chain SFAs, monounsaturated fatty acids, and the various series of PUFAs, including omega 3 and omega 6. The authors argue that all these lipids and their interactions should be taken into account when analyzing the effect of dietary fat on CVD complications and mortality. They identify research that suggests that maintaining high intake of omega 6 in lieu of SFAs is definitely not the optimal strategy to prevent CVD complications.

Further they provide evidence that omega 6 PUFAs have a strong mammary tumor-enhancing effect. Several recent epidemiologic studies have found a positive association between dietary omega 6 PUFAs and breast cancer risk. Other factors, such as the obesity status, were shown to affect the association between dietary PUFAs and breast cancer risk. The food sources of omega 3 and omega 6 PUFAs, as well as their relative amounts in the diet of individuals, appear to be very important for breast cancer risk. Research is described whereby omega 3 PUFAs were shown to have chemopreventive properties against various cancers and their complications, including colon and breast cancer. Both alpha-linolenic acid and the polyphenol anthocyanins are present in quite large amounts in the traditional Mediterranean diet, also poor in omega 6, which may at least partly explain the remarkable protection this diet provides against cancers. In contrast, the main dietary omega 6 linoleic acid inhibits synthesis and cell incorporation of long chain omega 3 PUFAs.

From the most recent experimental and epidemiological studies, the authors conclude that the optimal dietary fat pattern to reduce the risk of both CVD and most cancers should include a low intake of SFAs and omega 6 PUFAs. Small amounts (1% to 2% of energy intake) of the essential linoleic acid are sufficient to prevent omega 6 deficiency. The amounts of omega 6 in most Western foods are so high that it could be difficult to obtain an intake of omega 6 lower than 4% of energy, which would probably be the optimal level. The high average intake of omega 6 PUFAs in Western countries may explain the persistently high rate of CVD complications and the increased incidence of certain cancers, including breast cancer. The intake of omega 3 PUFAs, from plant and marine sources, should be moderate (a minimum of 3 g/day in average for an adult with at least 2 g/day of the essential alpha-linolenic acid), which is far from the case at present in many populations.