Losing muscle is not generally healthy, slows our metabolism and can be debilitating in the long run when it comes to daily functioning. Therefore the goal is to lose only body fat during weight loss unless you are extremely overweight. In the latter situation it's normal to lose a small amount of muscle since as you gained weight, even if not exercising, about 25% of the weight gained would be muscle because you needed it to carry the extra body fat. Below are other facts on why losing fat only is the best course and why we recommend using changes in body fat to gauge your results whenever possible. By measuring body fat you can see exactly how much fat and/or muscle is lost or gained and it's also shows you why your weight might not change but your body fat has gone down, meaning you added valuable muscle.
•Calories are burned in muscle tissue. One pound of lean body mass burns approximately ~6cals/LB/day at rest and much more when moving. Conversely, body fat is a storehouse for calories. One pound of fat only burns ~2.1cals/LB/day and stores 3500 calories of energy. Adding or maintaining muscle keeps the metabolism revved up.
• When losing weight quickly (more than 1.5LBS/week), ~25 percent will be lost from lean muscle tissue unless you are supplementing the diet. If 25 percent or more of the weight you lose is from lean muscle tissue, weight regain and more is likely.
• Muscle tissue is denser and more attractive than fat tissue because it is 70 percent water and firm, while fat is approximately 20 percent water.
• Rapid weight loss and under-eating cause muscle tissue to be used for energy, which decreases metabolism.
• Providing your body with the food and nutrients it needs will sufficiently fuel working muscles, initiate fat loss and develop a healthier metabolism.