What are the differences between muscle and fat?

For one, fat tissue is more bulky than muscle tissue, so it occupies more space under the skin. Thus, one pound of fat tissue actually has more volume (and will appear larger) than one pound of muscle tissue. For this reason, a 170-pound woman whose body is composed of 25% fat tissue will appear much leaner than a woman weighing the same but whose body fat percentage is 45%. Therefore, individuals need to assess their weight management efforts using a wide variety of body measurements and health parameters -- focusing solely on the number on the scale can conceal real and important improvements in body composition. Aim for the look and the feel -- not just a number.

Secondly, muscle tissue utilizes more calories than fat tissue. What does that mean? Let's take the two 170-pound women mentioned above. The woman with 25% body fat has more muscle tissue, so her body needs more calories to keep its systems running. Thus, she burns more calories -- even when she is just sitting around -- than does her 45% body fat counterpart. As a result, the leaner woman can actually eat more calories each day and maintain her weight as compared to the woman with more fat tissue.