The removal of trans fats from all kinds of foods is certainly good for our health. But how good? That depends on what kind of fat has replaced them.

Three doctors from Gentofte University Hospital in Denmark decided to find out. They picked 19 popular kinds of fast foods, cookies, cakes and snacks that were known to be heavy on the trans fats, then compared them with 19 similar foods with low trans fats.

Before we get to the results, a quick primer on the relative merits of different kinds of fat. Trans fats are vegetable oils with added hydrogen to make them more solid. That makes them useful to food manufacturers, but bad for people – they raise bad cholesterol and reduce good cholesterol. The more you eat, the more you increase your risk of coronary heart disease.

The same is true of saturated fats, which occur naturally in foods like cheese, ground beef and chocolate. But monounsaturated and polyunsaturated fats (from fish, avocados and vegetable oils, among other items) don’t raise bad cholesterol. In fact, when used as a substitute for trans fats or saturated fats, unsaturated fats can help reduce your cholesterol.

So if food manufacturers were simply replacing trans fats with saturated fats, they really wouldn’t be making their products that much better.

And now, drum roll please ….

French fries were a natural candidate for this analysis. Fries that were low in trans fats had a corresponding increase in polyunsaturated fats, the best kind. For microwave popcorn, about half the trans fats were replaced with monounsaturated and polyunsaturated fats, with saturated fat making up the difference. Cakes and cookies swapped nearly one-quarter of their trans fats for the better unsaturated fats, but most of the substitution was for saturated fat. The results were published today in the New England Journal of Medicine.

The authors concluded that when trans fats were removed from foods, they really did get healthier.

Here are a few useful backgrounders on dietary fat from the National Institutes of Health and the American Heart Assn.

-- Karen Kaplan

Photo: Ringo H.W. Chiu / For The Times