Aspartame is a low-calorie artificial sweetener used as a sugar substitute in foods and beverages. It is 200 times sweeter than sucrose and is commonly found under the trade names NutraSweet and Equal (North America), Canderel (Europe), and Pal Sweet (Asia).
Aspartame is most often used for
It's uncertain whether aspartame causes headaches due to the scarcity of blinded studies and the role of the placebo effect.
It does not inhibit fat loss at all, and may actually suppress appetite that could help fat loss (although it does not induce fat loss per se either). Diet soda, in regards to body fat, is a carbonated inert beverage.
Research suggests that diet soda is unlikely to be detrimental to metabolic health or body composition. Further controlled trial evidence is needed to explore other possible detriments.