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But the idea that spurning saturated fat will, by itself, make people healthier has never been fully proven, and in recent years repeated clinical trials and large-scale observational studies have produced evidence to the contrary.

After all the decades of research, it is possible that the key lesson on fats is two-fold. Cutting saturated fats from diets, and replacing them with carbohydrates, as is often done, likely will not reduce heart disease risk. But cutting saturated fats and replacing them with unsaturated fats -- the type of fats characteristic of fish, nuts and vegetable oils -- might.

Some, including representatives of the American Heart Association, disagree. In their view, the evidence for the dangers of saturated fats arises from these two ideas: Consuming saturated fats raises levels of so-called “bad” cholesterol in the blood, and higher levels of “bad” cholesterol, in turn, raise risks of heart disease.

There is a “mountain of evidence” explaining how consumption of saturated fats raises the risk of heart disease, said Penny Kris-Etherton, a nutrition professor at Penn State University and a former member of the Dietary Guidelines advisory committee.