It may seem counterintuitive, but fatty foods aren't necessarily fattening. In fact, a diet with a moderate amount of unsaturated fats -- such as those in avocados, olives, fatty fish, nuts, and oils derived from olives, nuts, and seeds -- may prevent weight gain.
One explanation: Incorporating fat and its accompanying flavor into everyday life makes a healthful diet easier to stick with. In a recent study at Brigham and Women's Hospital, a teaching affiliate of the Harvard Medical School, scientists found that subjects who maintained a moderate-fat diet lost, on average, nine pounds over an 18-month period. By contrast, those who followed a low-fat regime actually gained six pounds.
Unsaturated fats, to their extra credit, have a favorable effect on blood cholesterol, offering powerful protection against heart disease. Fats also make it easier for the body to absorb certain nutrients, such as lycopene (in tomatoes) and lutein (from kale), which means a drizzle of olive oil is a smart addition to many vegetable dishes. As long as total calories stay at a sensible constant, good fats will maximize food and flavors, not waistlines.