In 1984, researchers at the University of Massachusetts concluded that abdominal exercises do not decrease the amount or size of belly fat. In this study men did the equivalent of 5,000 sit-ups for 27 days. The researchers measured body fat in the abdomen, buttocks and upper back during the study. If spot reduction worked, the men should have lost fat only in the abdominal area because the buttocks and upper back are not worked during sit-ups. However there was no change in the thickness of abdominal fat or waist size. Biopsies showed there was no significant change in the diameter of abdominal fat cells either.
What ab exercises can do is tone and firm the muscles, but to reduce abdominal fat takes more than just ab exercise. You need a balanced fitness routine that includes cardiovascular exercise, resistance training, and a sensible diet.