Just because a food is marketed as sugar-free doesn't mean it's free of calories. If you eat too many sugar-free foods, you can still gain weight if they have other ingredients that contain calories. And remember that processed foods, which often contain sugar substitutes, generally don't offer the same health benefits as do whole foods, such as fruits and vegetables.

Source: http://www.mayoclinic.org/healthy-living/nutrition-and-healthy-eating/in-depth/artificial-sweeteners/art-20046936