How Exercise Affects Your Body's Composition

Weight-training increases muscle and bone mass, altering your body composition.
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Instead of using weight as a measurement of your health progress, use body composition to determine the amount of change in fat and muscle mass when you exercise. Body composition addresses fat mass and fat-free mass in which the latter consists of protein, water and minerals. Since exercise stimulates various changes in body composition, it's a cheap method to ensure long-term changes in getting a healthier body composition.

Reduced Body Fat

Fat is the preferred fuel source for your body, especially during low-intensity exercise, such as walking and yoga. During long-duration aerobic exercise, your body uses primarily carbohydrates as the main fuel source for the first two to five minutes. As the duration increases, the percentage of fat used also increases and the percentage of carbohydrates decreases. In a 2011 eight-month study published in the "Journal of Applied Physiology," two groups of overweight and obese adults who performed aerobic training and a combination of aerobic and resistance training significantly lost more fat mass than the group who performed only resistance training. However, both groups who did resistance training had an increase in lean muscle mass, while the aerobics-only group had no increase in muscle. Therefore, doing a combination of aerobics and resistance training will decrease fat mass while gaining some muscle mass.

Increase Metabolism and Muscle Tone

Strength training greatly stimulates muscle growth and bone density, whether you're lifting heavy weights or doing calisthenics. Although strength training uses carbohydrates as the main fuel source and burns almost no fat, it raises your metabolic rate as much as doing high-intensity cardio. A study published in the "International Journal of Sports Nutrition and Exercise Metabolism," young women who performed a 100-minute, strenuous strength-training session increased and maintained their metabolic rate for more than three hours. This is known as EPOC -- excess post-exercise oxygen consumption -- in which your body uses oxygen and energy derived from carbohydrates and fats to repair damaged tissues, cool body temperature and deliver nutrients to your cells. Thus, strenuous resistance training provides an indirect change in fat loss.


One major problem with changes in body composition, particularly in fat loss, is that your body compensates when you have a diet-induced negative energy balance, which is the state when the number of calories you expend is greater than the number consumed. As you get better in exercise, your body burns fewer calories and craves more food. This protective behavior is often triggered by aerobic exercise. In a 13-week study published in the "American Journal of Physiology," subjects who performed a 60-minute aerobic exercise session lost almost the same amount of fat mass as another group that performed a 30-minute aerobic exercise session. Thus, body composition between both groups didn't have any significant differences, even when groups ate the same number of calories per day and had similar lifestyles. In the end of the study, the 30-minute group had an 83 percent more-than-expected increase in negative energy balance, while the 60-minute group had a 20 percent less-than-expected decrease in negative energy balance. This shows that long-duration aerobics can lead to greater compensation in how much energy you expend during exercise.

Healthy Body Composition Range

The range of body fat percent is different between men and women. The American Council on Exercise recommends that men maintain between 18 to 24 percent body fat for optimal health and between 14 to 17 percent for optimal fitness. Women should maintain between 25 to 31 percent for optimal health and 21 to 24 percent for optimal fitness. Check your body composition once a month for at least six months to see if the weight changes are from fat or fat-free mass.

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