Sit-ups do not contribute to the loss of belly fat and have a minimal affect on burning calories. It is not possible to target specific areas of the body for weight loss, such as performing sit-ups to achieve a flat stomach or to gain six-pack abs.
While they don't help flatten the stomach, sit-ups help strengthen and build up abdominal muscles. This can have an indirect contribution to losing weight, as the body burns more calories with the increase in muscle mass.
The best exercises for losing calories and fat are full-body aerobic exercises, such as running, walking or bicycling.