Anytime someone successfully follows a diet-and-exercise plan for heavy weight loss, there is the possibility that the skin that held excess fat will become loose and flabby. Anaerobic weightlifting alone is unlikely to cause this, since it is more effective for gaining strength than losing fat -- but as part of a program that includes fat-burning aerobic activity, it could contribute to the creation of loose skin.
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Lose Fat, Add Muscle
This is not to say that abandoning weightlifting routines will help avoid the problem. In fact, as you continue your weightlifting program, the muscle you build will replace the lost fat, helping to keep the skin taut. By weight, muscle also burns more calories at rest than fat cells, so weightlifting will continue to help rid you of unwanted weight. But remember that muscle is denser than fat, so if you replace one pound of muscle with one pound of fat, it will not fill up the same amount of space.
Make sure you also drink plenty of water throughout the day, not just while training, because well-hydrated skin remains elastic. If you are prone to dry skin or live in an arid climate, use moisturizer. Smoking, poor nutrition and excess sun exposure can all contribute to the loosening of your skin. A good diet, no smoking and sunscreen have many benefits, one of which is keeping skin tight and healthy. Loose skin is a natural side-effect of aging, but weightlifting has been shown to mitigate the harmful effects of getting older, too.