The purpose of this study was to quantify body composition changes during amyotrophic lateral sclerosis (ALS) progression and to determine whether these subjects were losing or maintaining the energy stored in their bodies. The body composition of 12 males in the early stages of ALS and 6 age-matched controls was measured twice over a 6-month period using dual X-ray absorptiometry. During the study period the control group did not change. The ALS group lost an average of 2 kg of lean mass while gaining 0.55 kg of fat mass, resulting in a 1.45 kg loss in total body mass. When the changes in mass were converted to their energy equivalents, the ALS subjects lost an average of 1800 kcal of energy stored in lean mass but gained 4900 kcal in fat mass, resulting in a net increase of 3100 kcal stored. In conclusion, a small increase in fat mass can successfully compensate for the energy lost in lean mass from disease progression. Therefore, it is possible to preserve the amount of energy stored in the body of ALS patients, even when there are significant losses in lean and overall body mass. Consequently, a moderate loss of body mass should be expected and even encouraged among this patient population.