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.