Sit-ups alone cannot target belly fat - only overall exercise combined with caloric deficit reduces abdominal fat.
The evidence shows that aerobic exercise and resistance training can reduce abdominal fat, but there's no support for spot reduction through targeted exercises like sit-ups. Multiple systematic reviews demonstrate that general exercise programs reduce waist circumference and visceral fat, but this occurs through overall fat loss, not localized burning. The most effective approaches combine aerobic exercise, resistance training, and dietary changes.
Caveats
While exercise effectively reduces abdominal fat, it works through creating an overall caloric deficit and general fat loss, not through targeting specific body areas. Spot reduction remains unsupported by scientific evidence.