Aerobic exercise, such as walking, burns calories which, if combined with a sensible diet, helps create a calorie deficit, necessary for weight loss. Any aerobic exercise will burn some stored fat.
In terms of ‘where’ (on your body) or ‘what’ (fat or carbohydrate) is used as a primary energy source to fuel your exercise, misconceptions abound. It is a myth that engaging in low- to moderate-aerobic exercise, such as walking, burns more body fat than higher-intensity activities, such as jogging. For weight loss, it is all about total calories expended within a set period of time. High-intensity aerobic activities, such as running, may burn a lower percentage of body fat, but it is compensated by enabling you to burn more total calories than a lower intensity workout. Regardless of the type of aerobic exercise you choose, your body draws upon stored carbohydrate and fat for energy, more carbohydrate than fat. Exact ratios are difficult to calculate but can be estimated.
Toward the beginning of your workout, your body may draw upon stored carbohydrate and fat in a metabolic ratio of 70:30, carbohydrate to fat. Later in your workout, there is a shift in the metabolic ratio since you have limited carbohydrate stores in your body. After about 15 to 20 minutes, you gradually burn more fat than carbohydrate, or a metabolic ratio of about 60:40, fat to carbohydrate. The fittest individuals enjoy this shift earlier in their workout and draw upon more fat than their less active counterparts. During a workout, an endurance athlete may have a fat to carbohydrate ratio of up to 75:25. Again, in terms of weight loss, it is more important to create a calorie deficit than to worry about how much fat versus carbohydrate you burn.
Unfortunately, you cannot ‘spot’ reduce, or burn fat from a certain part of your body. Where and how you carry and lose body fat depends upon factors such as age, gender and genetics. The best way to get lean is to participate in regular aerobic workouts, at least 2½ hours weekly according to the Centers for Disease Control and Prevention (CDC), perform total body strength training exercises at least twice weekly and follow a healthy, fiber-rich, reduced-calorie diet.
- J Achten and AE Jeukendrup. Optimizing fat oxidation through exercise and diet. Nutrition; Jul-Aug 2004.
- MC Venables et al. Determinants of fat oxidation during exercise in healthy men and women: a cross-sectional study. J Appl Physiol; Aug 2004.
- Centers for Disease Control and Prevention (CDC). Physical Activity for Everyone.