It is widely accepted that diets rich in polyunsaturated fats protect against heart disease. Recently, the Global Burden of Disease team reported that each year insufficient intake of omega-6 polyunsaturated fats, the most common subgroup of polyunsaturated fats, results in over 700 000 deaths from coronary heart disease.1 Or does it? A linked study by Ramsden and colleagues (doi:10.1136/bmj.i1246) adds to the doubts around the health benefits of replacing saturated fat with polyunsaturated fats.2
This new study re-examines recovered data from a double blind randomised controlled trial that took place 45 years ago. The Minnesota Coronary Experiment (MCE) followed 9423 participants from state mental hospitals and a nursing home for 4.5 years. The trial tested whether replacement of saturated fat with vegetable oil rich in linoleic acid (an omega-6 polyunsaturated fat) reduces the risk of coronary heart disease and death through a reduction in serum cholesterol concentration.
As expected, the diet enriched with linoleic acid lowered serum cholesterol concentration. But it did not reduce mortality: in fact participants in the intervention group had a higher mortality than controls. The pooled results of the MCE and four similar trials failed to find any reduction in mortality from coronary heart disease.3 4 5 6
These unexpected results proved difficult to stomach for researchers at the time. The trial ended in 1973, but it took until 1989 for the results to be published.7 The authors reported no differences between the treatment and control groups for cardiovascular events, cardiovascular deaths, or total mortality, but immediately added that “a favorable trend for all these end-points occurred in some younger age groups.” In contrast, Ramsden and colleagues now suggest the possibility of increased risk of death in older adults among the participants given more linoleic acid.2 The findings of the two teams of authors do not differ fundamentally, but their interpretation does.