It was one of the largest, most rigorous experiments ever conducted on an important diet question: How do fatty foods affect our health? Yet it took more than 40 years — that is, until today — for a clear picture of the results to reach the public.
The fuller results appeared Tuesday in BMJ, a medical journal, featuring some never-before-published data. Collectively, the fuller results undermine the conventional wisdom regarding dietary fat that has persisted for decades and is still enshrined in influential publications such as the U.S. government’s Dietary Guidelines for Americans. But the long-belated saga of the Minnesota Coronary Experiment may also make a broader point about how science gets done: it suggests just how difficult it can be for new evidence to see the light of day when it contradicts widely held theories.
The story begins in the late 1960s and early ’70s, when researchers in Minnesota engaged thousands of institutionalized mental patients to compare the effects of two diets. One group of patients was fed a diet intended to lower blood cholesterol and reduce heart disease. It contained less saturated fat, less cholesterol and more vegetable oil. The other group was fed a more typical American diet.
Just as researchers expected, the special diet reduced blood cholesterol in patients. And while the special diet didn’t seem to have any effect on heart disease, researchers said they suspected that a benefit would have appeared if the experiment had gone on longer.
There was “a favorable trend,” they wrote, for younger patients.
Today, the principles of that special diet — less saturated fat, more vegetable oils — are recommended by the Dietary Guidelines for Americans, the government’s official diet advice book. Yet the fuller accounting of the Minnesota data indicates that the advice is, at best, unsupported by the massive trial. In fact, it appears to show just the opposite: Patients who lowered their cholesterol, presumably because of the special diet, actually suffered more heart-related deaths than those who did not.
The higher rate of mortality for patients on the special diet was most apparent among patients older than 64.
The new researchers, led by investigators from the National Institutes of Health and the University of North Carolina, conclude that the absence of the data over the past 40 years or so may have led to a misunderstanding of this key dietary issue.
“Incomplete publication has contributed to the overestimation of benefits and underestimation of potential risks” of the special diet, they wrote.
“Had this research been published 40 years ago, it might have changed the trajectory of diet-heart research and recommendations” said Daisy Zamora, a researcher at UNC and a lead author of the study.
The new research drew quick criticism, however, especially from experts who have been prominent in the campaign against saturated fats.
“The bottom line is that this report adds no useful new information and is irrelevant to current dietary recommendations that emphasize replacing saturated fat with polyunsaturated fat,” Walter Willett, chair of the nutrition department at Harvard University, said in a blog post from the school. “Many lines of evidence support this conclusion.”
He characterized the new analysis of the old experiment as “an interesting historical footnote.”