Why don't I lose weight when I gain muscle from exercising?

To gain weight whether it be muscle or fat the body needs to consume more calories than it burns. If the body is building lean muscle mass you are eating slightly more calories than you burn to give you enough energy and raw material to build new muscle tissue. Pursuant to this net calorie intake there is no need for your body to tap into its calorie storage tissue fat to get calories for basic functions. Once you have gained the amount of lean muscle you wish to gain you can then go into a mild calorie deficit and continue to weight train and slowly lose the fat stores you wish to lose. 
Dr. David L. Katz, MD, MPH
Preventive Medicine
Consider a pound of feathers and a pound of iron: Both weigh the same, but the feathers take up more space. The same is true of fat. Muscle is much denser than fat, so as you increase muscle mass, you become heavier because your body is denser.

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Important: This content reflects information from various individuals and organizations and may offer alternative or opposing points of view. It should not be used for medical advice, diagnosis or treatment. As always, you should consult with your healthcare provider about your specific health needs.