There you are, putting a plan together for a healthier, leaner you. If part of that plan includes consuming "diet" or "sugar-free" branded foods, you are in for an uphill battle.
The reason: Sugar-free or "diet" foods are lower calorie options due to the restriction, or limit, to the amount of sugar they contain. One would think that since they are lower in calories, over the long-term they will help us lose weight because "hey, the box says it has only 10 calories". Unfortunately, according to the research journal, Cell Metabolism, artificial sweeteners, like sucralose or aspartame, have been shown to stimulate hunger, meaning you actually eat more as a result.
The short version as to why? Your brain and body are always attempting to balance each other out. When eating an artificially sweetened food or drink, the "sweetness" that your brain is tasting far exceeds the calories your body has absorbed. Imagine trying to balance your checkbook if your bank gave you two different account balances? This "mismatch" is what results in a stimulated appetite.