The question: Which is better if you’re trying to lose weight: low-fat foods or low-calorie ones?

The expert: Lisa Young, Ph.D., author of The Portion Teller Plan

The answer: Low-calorie foods—without question. “Ultimately, weight loss is about calorie intake. If you take in fewer calories each day, you will slim down,” says Young.

The problem with low-fat foods is that even though they may sound diet-friendly, they’re often loaded with sugar—which is super caloric. “Companies often just replace the fat with sugar to make low-fat foods taste good, which means that it can be low-fat but not low-calorie,” says Young. “A 20-ounce low-fat frozen yogurt, for instance, could have up to 500 calories, which is a huge diet bomb for something that doesn’t have any nutritional substance.”

What’s more, when it comes to weight loss, a little real fat can actually help you slim down—as long as it’s the good-for-you kind (a.k.a., monounsaturated fat).

“The good-for-you fats in foods like nuts, avocados, and salmon can help you feel more full, meaning you take in fewer calories throughout the day overall,” says Young. Just make sure to watch your portion size; since these foods do contain a healthy amount of fat, they also tend to be calorie-dense.