This study 40 years ago could have reshaped the American diet. But it
was never fully published.
By Peter
Whoriskey April 12
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 (British 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 blue line represents the mortality rate of
patients on the special cholesterol-lowering diet. The dashed red line
represents the mortality rate of patients on the conventional diet. The
take-away: people on the special diet were dying at a higher rate.
(Source: BMJ)
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."
The new research will agitate the
debate over one of the
most controversial questions in all of nutrition: Does the
consumption of saturated fats - the ones characteristic of meat and dairy
products - contribute to heart disease?
It is, without doubt, an
important question. Heart disease is the leading cause of mortality in the
United States, and Americans eat a lot of red meat and dairy foods.
The federal government has long
blamed saturated fats for health troubles, and it continues — through the
Dietary Guidelines for Americans — to recommend that people limit their intake.
Indeed, the Dietary Guidelines
continue to embrace the principles advocated by the Minnesota researchers from
40 years ago. The book advises Americans to limit their intake of
saturated fats and to replace them at least in part with oils, just as the
Minnesota experimenters did 40 years ago. More specifically, it advises Americans
to consume about five teaspoons (27 grams) of oils per day,
mentioning canola, corn, olive, peanut, safflower, soybean and sunflower
oils.
“Oils should replace solid fats
rather than being added to the diet,” it advises.
But the idea that spurning
saturated fat will, by itself, make people healthier has never been fully
proved, and in recent years repeated clinical trials and large-scale
observational studies have produced evidence to the contrary. Whether cutting
saturated fats out of your diet will make you healthier depends, of course, on
what you replace them with.
“What this research implies is
that there is not enough evidence to draw strong conclusions about the health
effects of vegetable oils” Christopher Ramsden, a medical investigator at NIH
and a lead author of the study, said in an interview. While urging caution in
drawing conclusions about the new analysis, he said the research suggested
saturated fats "may not be as bad as originally thought."
Ramsden and colleagues discovered
the missing data during their research examining the potentially harmful
effects of linoleic acid - a key constituent of most vegetable oils - on human
health. Preliminary research suggests a link between linoleic acid and
diseases such as chronic pain, Ramsden said, and humans have been consuming it
in larger quantities than their bodies may be prepared for. Before the
advent of agriculture, humans got 2 to 3 percent of their calories
from linoleic acid, according to the new paper; today most Americans, awash in
cooking oils and oils added to snack foods, get much more.
It's not exactly clear why the
full set of data from the Minnesota experiment was never published.
As research efforts on diets go,
the study was rigorous. Funded by the U.S. Public Health Service and the
National Heart Institute, it involved more than 9,000 patients who were
randomly assigned to one of the two diets. Detailed measurements of blood
cholesterol and other indexes of health were recorded.
Willett, the Harvard
nutritionist, faulted the experiment because many of the patients were on the
special diets for relatively brief periods - many were being released from the
mental institutions. But about a quarter of the patients remained on the diet
for a year or longer, and why such an apparently well-done study received
so little fanfare is mystifying to some.
The results of the study were
never touted by the investigators. Partial results were presented at an
American Heart Association conference in 1975, and it wasn't until 1989 that
some of the results were published, appearing in a medical journal known as
Arteriosclerosis.
The lead investigators of the
trial, noted scientists Ancel Keys and Ivan Frantz, are deceased.
Steven Broste, now a retired
biostatistician, was then a student at the University of Minnesota and used the
full set of data for his master's thesis in 1981. He interacted with the
researchers. Part of the problem, Broste suggested in an interview,
may have been limits on statistical methods at the time. Computer software
for statistics wasn't as readily available as it is today. So, at the time of
the study, it wasn't as easy to know how significant the data was. Broste
completed his thesis several years after the last patients had left the trial,
but it was not published in a journal.
Broste also suggested that at
least part of the reason for the incomplete publication of the data might have
been human nature. The Minnesota investigators had a theory that they believed
in - that reducing blood cholesterol would make people healthier. Indeed, the
idea was widespread and would soon be adopted by the federal government in the
first dietary recommendations. So when the data they collected from the mental
patients conflicted with this theory, the scientists may have been reluctant to
believe what their experiment had turned up.
“The results flew in the face of
what people believed at the time,” said Broste. “Everyone thought cholesterol
was the culprit. This theory was so widely held and so firmly believed - and
then it wasn’t borne out by the data. The question then became: Was it a bad
theory? Or was it bad data? ... My perception was they were hung up trying to
understand the results.”
Peter Whoriskey is a staff writer for The Washington Post
handling projects in business, healthcare and health. You can email him at peter.whoriskey@washpost.com
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