FOR two generations, Americans ate fewer eggs and other animal products because policy makers told them that fat and cholesterol were bad for their health. Now both dogmas have been debunked in quick succession.
First, last fall, experts on the committee that develops the country’s dietary guidelines acknowledged that they had ditched the low-fat diet. On Thursday, that committee’s report was released, with an even bigger change: It lifted the longstanding caps on dietary cholesterol, saying there was “no appreciable relationship” between dietary cholesterol and blood cholesterol. Americans, it seems, had needlessly been avoiding egg yolks, liver and shellfish for decades. The new guidelines, the first to be issued in five years, will influence everything from school lunches to doctors’ dieting advice.
How did experts get it so wrong? Certainly, the food industry has muddied the waters through its lobbying. But the primary problem is that nutrition policy has long relied on a very weak kind of science: epidemiological, or “observational,” studies in which researchers follow large groups of people over many years. But even the most rigorous epidemiological studies suffer from a fundamental limitation. At best they can show only association, not causation. Epidemiological data can be used to suggest hypotheses but not to prove them.
Instead of accepting that this evidence was inadequate to give sound advice, strong-willed scientists overstated the significance of their studies.
Much of the epidemiological data underpinning the government’s dietary advice comes from studies run by Harvard’s school of public health. In 2011, directors of the National Institute of Statistical Sciences analyzed many of Harvard’s most important findings and found that they could not be reproduced in clinical trials.
It’s no surprise that longstanding nutritional guidelines are now being challenged.
In 2013, government advice to reduce salt intake (which remains in the current report) was contradicted by an authoritative Institute of Medicine study. And several recent meta-analyses have cast serious doubt on whether saturated fats are linked to heart disease, as the dietary guidelines continue to assert.
Uncertain science should no longer guide our nutrition policy. Indeed, cutting fat and cholesterol, as Americans have conscientiously done, may have even worsened our health. In clearing our plates of meat, eggs and cheese (fat and protein), we ate more grains, pasta and starchy vegetables (carbohydrates). Over the past 50 years, we cut fat intake by 25 percent and increased carbohydrates by more than 30 percent, according to a new analysis of government data. Yet recent science has increasingly shown that a high-carb diet rich in sugar and refined grains increases the risk of obesity, diabetes and heart disease — much more so than a diet high in fat and cholesterol.
It’s not that health authorities weren’t warned. “They are not acting on the basis of scientific evidence, but on the basis of a plausible but untested idea,” Dr. Edward H. Ahrens Jr., a top specialist at Rockefeller University and prominent critic of the growing doctrine on dietary fats and cholesterol, cautioned back in the ’80s. In the face of urgent pressure to offer a solution to the rising tide of heart disease, however, he turned out to be the Cassandra of his day.
Today, we are poised to make the same mistakes. The committee’s new report also advised eliminating “lean meat” from the list of recommended healthy foods, as well as cutting back on red and processed meats. Fewer protein choices will likely encourage Americans to eat even more carbs. It will also have policy implications: Meat could be limited in school lunches and other federal food programs.
It’s possible that a mostly meatless diet could be healthy for all Americans — but then again, it might not be. We simply do not know. There are no rigorous clinical trials on such a diet, and although epidemiological data exists for adult vegetarians, there is none for children.
Since the very first nutritional guidelines to restrict saturated fat and cholesterol were released by the American Heart Association in 1961, Americans have been the subjects of a vast, uncontrolled diet experiment with disastrous consequences. We have to start looking more skeptically at epidemiological studies and rethinking nutrition policy from the ground up.
Until then, we would be wise to return to what worked better for previous generations: a diet that included fewer grains, less sugar and more animal foods like meat, full-fat dairy and eggs. That would be a decent start.
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