So, you want to lose body fat (or prevent it from accumulating)? Then controlling your insulin secretion is critical. Insulin is the hormone that stimulates adipose (fat) tissue to take up fatty acids and glucose and store them as triglycerides. We are most familiar with Insulin from its action on lowering high blood sugar (like in diabetics), but where do you think the blood sugar goes?
Opposing hormones, like Epinephrine, Glucagon, and Growth Hormone (to name a few) increase the muscles' uptake up sugar and fatty acids from the blood (notice I said increase, they are always taking glucose and fatty acids up at some level). These hormones, also, stimulate the increase in the breakdown (lypolysis) of triglycerides from adipose tissue and release fatty acids into the circulation.
Exercise suppresses Insulin levels and raises the levels of the opposing hormones (listed above). This should make sense because of elevated energy demands. Often overlooked, especially with the low-fat diets, is that the composition (quality) of your diet plays a significant role in Insulin secretion. Carbohydrates, especially those that are only glucose (like common starches, i.e. breads), or composed of glucose (like table sugar or high-fructose corn syrup) cause a spike in your blood glucose levels and a corresponding rise in your blood insulin levels. Carbohydrates are also necessary to 'build' triglycerides in the adipose tissue. Glucose is converted to a (two) glycerol molecule that serve as the 'backbone' of the triglyceride molecule.
So, carbohydrate restriction (especially refined, and simple carbohydrates) is key to fat loss. Also, most refined carbohydrates have little nutritional value, are concentrated, and fail to suppress your hunger. So, cut out the sugars to stimulate your body to breakdown fat, not store it.