Like few other issues, dietary cholesterol - that is, the kind you consume in eggs and other foods - and the fifty-plus years of debate over its role in heart disease shows the profound difficulties in reaching definitive conclusions about nutrition and the reluctance to withdraw public health warnings.
The issue of dietary cholesterol, of course, is distinct from the level of cholesterol in the blood, which should continue to be monitored, experts say, because it has been linked to heart disease. According to research cited by the American Heart Association, blood cholesterol levels are determined by heredity or the consumption of foods high in saturated fats.
The idea that dietary cholesterol represents a danger goes back at least as far back as 1961, when the American Heart Association warned Americans of the perils of consuming cholesterol-rich foods such as eggs, and liver.
The warning about cholesterol was embraced by the first version of the government's "Dietary Goals" in 1977, and it has remained a fixture in them ever since. Indeed, when new nutritional labels were introduced in 1994, cholesterol was one of the nutrients that was called out.
And as late as last year, the Food and Drug Administration asserted that “Current dietary recommendations continue to recognize the well-established relationship between consumption of cholesterol and its effect on blood cholesterol level.”
The cholesterol warnings have been based on the idea that eating cholesterol-rich foods would significantly raise the levels of "bad" cholesterol in the blood, and that phenomenon would in turn lead to heart troubles.
Whatever its merits, the theory had remarkable staying power, hanging on despite years of doubts from prominent scientists.
The flaw in the logic behind the warning is that eating cholesterol doesn't much affect the levels of cholesterol in the blood, at least for humans, scientists have found.
As the Dietary Guidelines Advisory Committee concluded in its report today: "available evidence shows no appreciable relationship between consumption of dietary
cholesterol and [blood] cholesterol."
What is remarkable is how uncertain the science was behind the longstanding warning.
As far back as 1955, Ancel Keys, one of the key researchers in this field, could conclude in the Journal of Nutrition that repeated studies had shown that "cholesterol level [in the blood] is essentially independent of the cholesterol intake over the whole range of natural human diets."
Keys had done repeated studies of the issue. For example, in one experiment, 23 men doubled their cholesterol consumption, another 41 men cut theirs in half. Over four to 12 months, their blood cholesterol levels of both groups failed to show any response to the change , according to Keys.
In 1977, as a Senate committee was published the first set of dietary guidelines - which called for limiting cholesterol- three on the committee objected, noting that the Canadian government had recently concluded that "a diet restricted in cholesterol would not be necessary."
"Because of these divergent viewpoints, it is clear that science has not progressed to the point where we can recommend to the general public that cholesterol intake be limited to a specific amount," according to the letter from Senators Charles Percy(R-Illinois), Richard Schweiker(R-Pennsylvania) and Edward Zorinsky(D-Nebraska).
The recommendation nevertheless made it into the final publication. "I can't tell you why previous committees did not deal with this," said Marian Neuhouser, a member of the current panel.