Targeted fat loss, also known as spot reduction, has long been an intense area of focus by infomercials, fitness and supplement companies and sports magazines. The idea of target fat loss where you lose fat more readily in areas you exercise more seems to be reasonable and intuitive. It also appeals to the general public, as most of us tend to gain fat in certain areas while not others, causing the prevalence of "love handles" and "beer bellies" among us. The reality though, it's that the basic physiology of the human body prevents targeted fat loss from happening no matter what form of exercise you do, and this is why: Fat is stored in the human body as triglycerides, they have to be broken down into glycerol and free fatty acids and then enter the blood stream before can be utilized by muscle cells. This means that the fat burnt from whatever you do can come from anywhere in the body, not just the area you've worked on. Scientific studies conducted on athletes have shown that prolonged, intense work-outs targeted at one part of the body does not reduced the amount of subcutaneous fat in the trained area compared to the rest of the body. Fat loss is universal.
The most effective way of losing fat is through good calorie burning exercises, for instance, high intensity interval training. Sit-ups alone don't burn enough calories, and while they can indeed improving your core strength, they do almost nothing to get rid of the fat on your belly. Fat loss cannot be targeted, if you want to lose weight, do exercise that burn the most calories.