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Diet and Fat: A Severe Case of Mistaken Consensus

In 1988, the surgeon general, C. Everett Koop, proclaimed ice cream
to a be public-health menace right up there with cigarettes. Alluding
to his office’s famous 1964 report on the perils of smoking, Dr. Koop
announced that the American diet was a problem of “comparable”
magnitude, chiefly because of the high-fat foods that were causing
coronary heart disease and other deadly ailments.

He introduced his report with these words: “The depth of the science
base underlying its findings is even more impressive than that for
tobacco and health in 1964.”

That was a ludicrous statement, as Gary Taubes demonstrates in his
new book meticulously debunking diet myths, “Good Calories, Bad
Calories” (Knopf, 2007). The notion that fatty foods shorten your
life began as a hypothesis based on dubious assumptions and data;
when scientists tried to confirm it they failed repeatedly. The
evidence against Häagen-Dazs was nothing like the evidence against

It may seem bizarre that a surgeon general could go so wrong. After
all, wasn’t it his job to express the scientific consensus? But that
was the problem. Dr. Koop was expressing the consensus. He, like the
architects of the federal “food pyramid” telling Americans what to
eat, went wrong by listening to everyone else. He was caught in what
social scientists call a cascade.

We like to think that people improve their judgment by putting their
minds together, and sometimes they do. The studio audience at “Who
Wants to Be a Millionaire” usually votes for the right answer. But
suppose, instead of the audience members voting silently in unison,
they voted out loud one after another. And suppose the first person
gets it wrong.

If the second person isn’t sure of the answer, he’s liable to go
along with the first person’s guess. By then, even if the third
person suspects another answer is right, she’s more liable to go
along just because she assumes the first two together know more than
she does. Thus begins an “informational cascade” as one person after
another assumes that the rest can’t all be wrong.

Because of this effect, groups are surprisingly prone to reach
mistaken conclusions even when most of the people started out knowing
better, according to the economists Sushil Bikhchandani, David
Hirshleifer and Ivo Welch. If, say, 60 percent of a group’s members
have been given information pointing them to the right answer (while
the rest have information pointing to the wrong answer), there is
still about a one-in-three chance that the group will cascade to a
mistaken consensus.

Cascades are especially common in medicine as doctors take their cues
from others, leading them to overdiagnose some faddish ailments
(called bandwagon diseases) and overprescribe certain treatments
(like the tonsillectomies once popular for children). Unable to keep
up with the volume of research, doctors look for guidance from an
expert — or at least someone who sounds confident.

In the case of fatty foods, that confident voice belonged to Ancel
Keys, a prominent diet researcher a half-century ago (the K-rations
in World War II were said to be named after him). He became convinced
in the 1950s that Americans were suffering from a new epidemic of
heart disease because they were eating more fat than their ancestors.

There were two glaring problems with this theory, as Mr. Taubes, a
correspondent for Science magazine, explains in his book. First, it
wasn’t clear that traditional diets were especially lean. Nineteenth-
century Americans consumed huge amounts of meat; the percentage of
fat in the diet of ancient hunter-gatherers, according to the best
estimate today, was as high or higher than the ratio in the modern
Western diet.

Second, there wasn’t really a new epidemic of heart disease. Yes,
more cases were being reported, but not because people were in worse
health. It was mainly because they were living longer and were more
likely to see a doctor who diagnosed the symptoms.

To bolster his theory, Dr. Keys in 1953 compared diets and heart
disease rates in the United States, Japan and four other countries.
Sure enough, more fat correlated with more disease (America topped
the list). But critics at the time noted that if Dr. Keys had
analyzed all 22 countries for which data were available, he would not
have found a correlation. (And, as Mr. Taubes notes, no one would
have puzzled over the so-called French Paradox of foie-gras
connoisseurs with healthy hearts.)

The evidence that dietary fat correlates with heart disease “does not
stand up to critical examination,” the American Heart Association
concluded in 1957. But three years later the association changed
position — not because of new data, Mr. Taubes writes, but because
Dr. Keys and an ally were on the committee issuing the new report. It
asserted that “the best scientific evidence of the time” warranted a
lower-fat diet for people at high risk of heart disease.

The association’s report was big news and put Dr. Keys, who died in
2004, on the cover of Time magazine. The magazine devoted four pages
to the topic — and just one paragraph noting that Dr. Keys’s diet
advice was “still questioned by some researchers.” That set the tone
for decades of news media coverage. Journalists and their audiences
were looking for clear guidance, not scientific ambiguity.

After the fat-is-bad theory became popular wisdom, the cascade
accelerated in the 1970s when a committee led by Senator George
McGovern issued a report advising Americans to lower their risk of
heart disease by eating less fat. “McGovern’s staff were virtually
unaware of the existence of any scientific controversy,” Mr. Taubes
writes, and the committee’s report was written by a nonscientist
“relying almost exclusively on a single Harvard nutritionist, Mark

That report impressed another nonscientist, Carol Tucker Foreman, an
assistant agriculture secretary, who hired Dr. Hegsted to draw up a
set of national dietary guidelines. The Department of Agriculture’s
advice against eating too much fat was issued in 1980 and would later
be incorporated in its “food pyramid.”

Meanwhile, there still wasn’t good evidence to warrant recommending a
low-fat diet for all Americans, as the National Academy of Sciences
noted in a report shortly after the U.S.D.A. guidelines were issued.
But the report’s authors were promptly excoriated on Capitol Hill and
in the news media for denying a danger that had already been
proclaimed by the American Heart Association, the McGovern committee
and the U.S.D.A.

The scientists, despite their impressive credentials, were accused of
bias because some of them had done research financed by the food
industry. And so the informational cascade morphed into what the
economist Timur Kuran calls a reputational cascade, in which it
becomes a career risk for dissidents to question the popular wisdom.

With skeptical scientists ostracized, the public debate and research
agenda became dominated by the fat-is-bad school. Later the National
Institutes of Health would hold a “consensus conference” that
concluded there was “no doubt” that low-fat diets “will afford
significant protection against coronary heart disease” for every
American over the age of 2. The American Cancer Society and the
surgeon general recommended a low-fat diet to prevent cancer.

But when the theories were tested in clinical trials, the evidence
kept turning up negative. As Mr. Taubes notes, the most rigorous meta-
analysis of the clinical trials of low-fat diets, published in 2001
by the Cochrane Collaboration, concluded that they had no significant
effect on mortality.

Mr. Taubes argues that the low-fat recommendations, besides being
unjustified, may well have harmed Americans by encouraging them to
switch to carbohydrates, which he believes cause obesity and disease.
He acknowledges that that hypothesis is unproved, and that the low-
carb diet fad could turn out to be another mistaken cascade. The
problem, he says, is that the low-carb hypothesis hasn’t been
seriously studied because it couldn’t be reconciled with the low-fat

Mr. Taubes told me he especially admired the iconoclasm of Dr. Edward
H. Ahrens Jr., a lipids researcher who spoke out against the McGovern
committee’s report. Mr. McGovern subsequently asked him at a hearing
to reconcile his skepticism with a survey showing that the low-fat
recommendations were endorsed by 92 percent of “the world’s leading

“Senator McGovern, I recognize the disadvantage of being in the
minority,” Dr. Ahrens replied. Then he pointed out that most of the
doctors in the survey were relying on secondhand knowledge because
they didn’t work in this field themselves.

“This is a matter,” he continued, “of such enormous social, economic
and medical importance that it must be evaluated with our eyes
completely open. Thus I would hate to see this issue settled by
anything that smacks of a Gallup poll.” Or a cascade.

1 comment:

  1. Sounds like a great book. I'll read it. But consensus really has little to do with scientific 'truth.' By all means, strive for consensus, but never deeply believe in it.

    Thomas Huxley said long ago... "The great tragedy of science; - a beautiful hypothesis slain by an ugly fact." Had McGovern known that, he might have listened to Dr. Ahrens.

    When you hear some pompous twit evoking "the great weight of evidence" to defend himself against a ugly fact, be very suspicious because the great weight of evidence is irrelevant. The validity of the ugly fact is what must be pursued. It must not be dismissed because it conflicts with what we think we already know.

    Sir Karl Popper has told us that an hypothesis can never be proven correct; it can only be falsified. Heaps of data confirming an hypothesis are reassuring and interesting but the lone 'ugly fact' that falsifies it is the only path to advancement of scientific 'truth.'

    Why, then, do we suffer from informational and reputational cascades? Perhaps Upton Sinclair put his finger on it when he said "It is difficult to get a man to understand something when his salary depends upon his not understanding it."

    Brian D. McLean, D.D.S.