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Despite these issues, however, the Oslo experiment is remembered only for the success of its cholesterol-lowering diet.

January 1, 1966


Oslo study

The fourth frequently cited diet trial “proving” the diet-heart hypothesis was known as the Oslo study, conducted in the early 1960s.

Paul Leren, a medical doctor in Oslo, Norway, selected 412 middle-aged men who had suffered a first heart attack (rates of heart disease among men in Oslo had skyrocketed from 1945 to 1961) and divided his subjects into two groups. One group followed a traditional Norwegian diet, which Leren describes as high in cheese, milk, meat, and bread as well as vegetables and fruit in season—altogether, 40 percent fat. The second group undertook a “cholesterol-lowering” diet featuring lots of fish and soybean oil but very little meat and no whole milk or cream. In all, the diets contained about the same amount of fat, but in the “cholesterol-lowering” diet most of the fat was polyunsaturated.

Leren chose to study men who had already had a heart attack, in part because such men tend to be highly motivated to stick to a doctor-prescribed diet. This was especially valuable since, as Leren acknowledged, the special high-vegetable-oil diet was received “not with enthusiasm,” and some of the men felt weakened and nauseated by it. The other advantage of working with such a population and why post-heart-attack men are so often chosen for these types of trials is that these men are more likely to have another heart attack soon, and so researchers will have enough “events” to generate statistically significant results.

The experiment lasted five years, and in 1966, Leren published his findings. Like all these other large trials, his diet had successfully lowered the men’s serum cholesterol, in this case by about 13 percent more than the controls. Fatal heart attacks were definitely down in the dieting group: ten versus twenty-three among the controls, which was an impressive result. However, a major wrench in the experiment, and one that has gone unnoticed because until recently no one was looking for it, was that in addition to saturated animal fats, the control group was eating a great deal of hard margarine and hydrogenated fish oils, then staples of the Norwegian diet, amounting to nearly half a cup of trans fats per day. This was many times more than the average American was eating when the Food and Drug Administration deemed trans fats dangerous enough to put them on food labels. The experimental diet, which sought to maximize polyunsaturated soybean oil, did not contain trans fats, and this was a significant difference that might easily have affected the outcome. Also, the experimental group, following a public health campaign of the time, cut out tobacco use by 45 percent more than the control group, a large difference that the investigators could not explain but which alone could have accounted for most of the difference in heart attack numbers. Despite these issues, however, the Oslo experiment is remembered only for the success of its cholesterol-lowering diet.

-Nina Teicholz - page 78