Does muscle really weigh more than fat? And if so, how much more?

— Bob Jeffries, Omaha, Nebraska

Yes, a volume of lean, or fat free, tissue does indeed weigh more than the same volume of fat. The density of fat is about 0.9g/cm³, whereas the density of lean tissue is 1.1g/cm³. In other, more unscientific words, lean tissue is about 22% more dense than fat tissue. Or, if your thigh stays the same size but, through an exercise program, is more muscle and less fat (yeah!) then your thigh would be heavier (oh no!).

The hidden issue behind the question is weight loss vs. body composition. Many people begin exercising with the intention of shedding unwanted pounds, and panic as they discover that instead of losing weight, they are gaining weight. Much of this can be attributed to a redistribution of body composition: youíre gaining muscle tissue in response to your exercise program and losing body fat as a result of the energy demands of the exercise.

There are a couple of ways to determine if youíre on the right track. One of the easiest is the most obvious one-- how your clothes fit. If your clothing is getting looser, you're losing size, even though the scale says your not losing weight. That's a good thing. You can also have your body fat percentage tested, either through skinfold measurements (they pinch you at all the wrong places, add it up and calculate your body fat) or hydrostatic weighing (you're dunked in a pool of water and weighed while the amount of water volume you displace is measured). Both are accurate and will give you an objective sense of whether you're heading in the right direction.