When Americans started to pack on pounds a few decades ago, fat was fingered as public enemy number one. "Low-fat" became the rallying cry for healthy eating. And so began one of the most misguided public health campaigns in history.

But even the good fat/bad fat message is turning out to be more complicated than nutritionists once thought, as researchers explore the health effects of the many different kinds of fatty acids. With evidence emerging that healthy fats not only improve cholesterol and triglyceride levels but also reduce inflammation, fats are emerging as one of the hottest new functional foods.

Polyunsaturated vs. Monounsaturated: Choosing the Healthiest Oil

"We can now say unequivocally that unsaturated fats protect against heart disease," says John Brunzell, MD, professor emeritus in the division of metabolism at the University of Washington, Seattle.