MOZAFFARIAN: "So for example, dietary fats also actually in many cases improve HDL cholesterol, the good cholesterol, compared to carbohydrates, and they actually lower triglycerides, a type of blood lipid which is associated with harm."

But Mozaffarian said dietary fats may also affect blood pressure and inflammation of the blood vessels; they may even have an impact on the heart's electrical function.

Whatever the mechanism, the study does suggest some guidance for people who want to lower their risk of heart disease.

MOZAFFARIAN: "They can't just look at a product that says 'low grams of saturated fat' or 'low saturated fat' and assume that it's healthy. If you're taking out the saturated fat, what are you replacing it with?"

And it doesn't mean we have to spend a lot of time parsing the nutritional labels that are now on packaged foods in many countries.

MOZAFFARIAN: "If someone says I should eat fruits, vegetables, whole grains, fish, nuts, and avoid processed foods, sugary beverages, and foods with trans fats or high in salt, that's actually much simpler, I think, than chasing all of these numbers."

I reached Professor Dariush Mozaffarian at his office at the Harvard School of Public Health. His study was published this week in the journal PLoS Medicine.

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