While most people know that exercising will cause you to lose weight and decrease body fat, what you may not be aware of is that exercising regularly causes your body to decrease fat easier and more efficiently than people who are sedentary or less active. In fact, those who have low activity levels tend to use more of the body’s stored sugar (from muscle and liver called “glycogen” or carbohydrate) than stored fat. Ideally, you want to store some glycogen in your muscles and liver to support your daily activities and keep your blood sugar levels stable (this helps you maintain your energy levels throughout the day). If you are looking to lose weight, you don’t want to lose your stored carbohydrate, you want to lose fat tissue.
Exercising regularly is the best way to use fat tissue for energy, and save you stored carbohydrates. Studies show that sedentary, obese individuals who begin and maintain regular exercise programs, are able to shift from using stored carbohydrate to stored fat (in fancy terms we call this a ”shift in substrate oxidation”). One particular study carried out by Dr. J. F. Brun and associates  looked at sedentary obese individuals with insulin resistance syndrome and observed changes in triglycerides, insulin, glucose levels, blood pressure, and substrate use (fat or carbohydrate) before and after incorporating an exercise program.
After these individuals maintained a regular exercise program, their insulin resistence was corrected and they demonstrated a shift to using more fat for fuel over carbohydrate (a good thing!).
One reason for this is that trained individuals develop what is called “Insulin Sensitivity” (not to be confused with “insulin resistance”, which is the opposite scenario). Insulin sensitivity is when the body becomes very efficient at utilizing sugar and requires less insulin to do so. Those with insulin resistance need extra insulin to process sugar and are therefore inefficient at properly maintaining blood sugar levels.
I’ll stop myself from getting too carried away on the scientific explanation of this topic, but the take-home message is that if you want to lose weight (fat tissue), you must become well-trained and exercise consistently.
 Dumortier, M., Perez-Martin, A., Pierrisnard, E., Mercier, J., & Brun, J. (2002). Regular exercise (3×45 min/wk) decreases plasma viscosity in sedentary obese, insulin resistant patients parallel to an improvement in fitness and a shift in substrate oxidation balance. Clinical Hemorheology And Microcirculation, 26(4), 219-229.