When you lose weight through diet and exercise modification, you lose primarily fat, regardless of the source of energy burned during exercise. However, certain types of exercise expend more energy than other types, improving your capacity for burning more calories, and therefore more fat. Cardio is the bread and butter of fat loss through exercise, although strength and flexibility training are important for retaining muscle, reducing injury risk and improving your overall fitness.
Fat vs. Calories
The intensity of cardiovascular exercise determines how many calories you burn from fat and how many from carbohydrates stored as glycogen. When it comes to weight loss, the total number of calories burned is more significant than where the calories come from. This is because ultimately, if you establish a calorie deficit, your body will burn excess fat for energy even when you're not exercising. Even so, you can estimate how many grams of fat you burn through exercise by looking at the percentage of fat burned during various intensities of exercise.
A low-intensity cardio workout that raises your heart rate to 50 to 70 percent of your maximum heart rate gets approximately 60 percent of its fuel from fat, and roughly 50 percent of the fuel you burn at 70 to 75 percent of your maximum heart rate comes from fat. Higher intensity cardiovascular exercise burns a lower percentage of calories from fat, getting any where from 5 to 35 percent of its energy from fat. However, because you burn more calories overall, you may burn just as much fat with higher intensity exercise as with lower-intensity workouts; however, short burst of high-intensity exercise such as sprinting burns strictly glycogen.
To burn 1 gram of fat, you need to burn 9 calories from fat. Since moderate-intensity exercise, such as walking, hiking, dancing or moderate bicycling, burns roughly 140 to 185 calories in 30 minutes, you can estimate that 50 to 60 percent of those calories comes from fat.That translates to 70 to 111 calories burned from fat. At nine calories per gram, you burn approximately 7.8 to 12.3 g of fat in 30 minutes, or two times that amount in 60 minutes. (Ref 1,3)
At higher intensity, if you're working out at 75 to 80 percent of your maximum heart rate, for example, approximately 35 percent of burned calories come from fat. That means if you jog for 30 minutes and burn roughly 295 calories, you burn around 103 calories from fat, or the equivalent of 11.4 g of fat. (Ref 1,3)
Although you may have heard a great deal about the fat-burning zone -- that level of exercise intensity that burns the most fat -- the number of fat grams you burn or the percentage of fat you burn during exercise is not the key to losing weight or fat with exercise. The key is to burn more calories than you take in. As long as you're not severely limiting your caloric intake and causing your metabolism to rebel and slow down, you'll lose fat if you create a calorie deficit. Watch what you eat, particularly the number of total calories and the overall quality of your food and beverage choices.