If you are a Gary Taubes fan you may like this one. A link to this fascinating little study was posted by blogger David Evans on his "Healthy Diets and Science" (a very fun blog to dig around in). The name of the study is: "Use of dietary linoleic acid for secondary prevention of coronary heart disease and death: evaluation of recovered data from the Sydney Diet Heart Study and updated meta-analysis" and it was published this month in BMJ. The Sydney Diet Heart Study was conducted in 1966-1973 with 458 males, aged 30-59 years, who already received care for coronary issues . The study was to measure the effectiveness of replacing dietary saturated fat with omega 6 linoleic acid (safflower oil). The researchers assumed that the safflower oil would decrease total serum cholesterol and thereby increase health benefits. And yes, the cholesterol levels did decrease. However, death rates for all causes of mortality rose for those on the "heart healthy" oils. We all know the extent of dietary advice given that we should replace saturated fat with omega-6 rich vegetable oils such as safflower, soybean, and corn oil, and yet most of us are unfamiliar with the lack of evidence to conclusively back this up. Here are some eerie comments from the study:
"Advice to substitute vegetable oils rich in polyunsaturated fatty acids (PUFAs) for animal fats rich in saturated fatty acids (SFAs) has been a cornerstone of worldwide dietary guidelines for the past half century. When this advice originated in the 1960s, PUFAs were regarded as a uniform molecular category with one relevant biological mechanism—the reduction in blood cholesterol. Omega 6 (n-6) linoleic acid (LA) was the best known dietary PUFA at the time. Therefore, the terms “PUFA” and “LA” were often used interchangeably when interpreting clinical trial results and delivering dietary advice...
However, there is currently no clinical trial evidence indicating that replacing SFAs with n-6 LA, without a concurrent increase in n-3 PUFAs, lowers the risk of cardiovascular disease or death. Thus, benefits attributed to PUFAs as a general category might be due to n-3 PUFAs specifically, particularly eicosapentaenoic acid and docosahexaenoic acid. Such benefits are not necessarily generalizable to n-6 LA or other PUFA species. Since n-6 LA is the most abundant dietary PUFA, and edible oil sources with markedly different contents of fatty acids are commercially available it is important to ascertain the benefits and risks specific to n-6 LA."
"Oxidation products of n-6 LA have been implicated in the pathogenesis of cardiovascular disease, and alcohol use and cigarette smoking are major sources of free radical mediated oxidation.Therefore, we hypothesized that alcohol use or smoking at baseline modified the association between longitudinal change in PUFA intake and mortality using likelihood ratio tests (α=0.15). The original SDHS investigators posited that the LA intervention would reduce serum cholesterol as an intermediate for the prevention of cardiovascular death. Thus, to examine whether the magnitude of postrandomization changes in total blood cholesterol were associated with mortality, we calculated hazard ratios for each mortality outcome as a function of time varying change from baseline in total blood cholesterol."
"Among intervention patients (in whom the PUFA increase was n-6 LA from safflower oil), an increase of 5% of food energy from n-6 LA predicted 35% and 29% higher risk of cardiovascular death and all cause mortality, respectively (models adjusted for age, dietary cholesterol, body mass index at baseline, smoking, alcohol use, and marital status)."
"Conclusions Advice to substitute polyunsaturated fats for saturated fats is a key component of worldwide dietary guidelines for coronary heart disease risk reduction. However, clinical benefits of the most abundant polyunsaturated fatty acid, omega 6 linoleic acid, have not been established. In this cohort, substituting dietary linoleic acid in place of saturated fats increased the rates of death from all causes, coronary heart disease, and cardiovascular disease. An updated meta-analysis of linoleic acid intervention trials showed no evidence of cardiovascular benefit. These findings could have important implications for worldwide dietary advice to substitute omega 6 linoleic acid, or polyunsaturated fats in general, for saturated fats."