Most people who go to fitness clubs or sports gyms for weight control, and many co-medicals and physicians believe that an increase in muscle mass and/or basal metabolic rate (BMR) is possible through a combination of regular exercise and optimal protein intake during weight loss. This seems a myth, and the reasons are discussed in this article. First, muscle mass is quite difficult to quantify. The limitations of body composition measurement should be well understood. Second, increasing muscle mass during weight loss is difficult. This might be attained through strict implementation of a protein-rich, low-carbohydrate diet; high-intensity resistance training; and aerobic exercise for a long duration. However, such a strict regimen is not feasible for most people. Finally, a 1-kg increase in muscle mass corresponds to an increase of only 13 kcal of BMR per day. Thus, an increase in muscle mass of 1 kg is difficult to achieve, while the gained BMR is approximately equivalent to a decrease of 13.5 kcal of BMR according to a 3-kg decrease of adipose tissue. Weight loss, unless through an extremely sophisticated weight control program, contributes to a decrease in BMR. However, it is an accomplished fact that women with significantly less muscle mass and lower BMR live longer than men with more muscle mass and higher BMR, regardless of ethnicity. Maintaining activities of daily living and daily activity function might be more essential.