Maintaining muscle mass is essential for health, but muscle mass decreases markedly with aging, compromising overall fitness and contributing to frailty, disability, and falls. Animal models demonstrate that intermittent fasting is generally beneficial for maintaining muscle mass, but in some disease states, such as ALS, or Lou Gehrig's disease, it can be detrimental. Dr. Mattson describes how, in the early 1990s, Dr. Roy Walford was a physician and crewmember for the Biosphere 2, a science research facility located in the Arizona desert. Dr. Walford was a long-time proponent of caloric restriction, a dietary practice characterized by marked reduction in food intake. Later in life, Dr. Walford developed ALS, which may have been exacerbated by his caloric restriction and low body mass. Dr. Mattson also shares a personal anecdote in which he lost a tremendous amount of muscle mass following an injury. He cautions that having a low bodyweight is generally beneficial, as long as muscle mass is maintained. In this clip, Dr. Mark Mattson discusses the importance of maintaining muscle mass during aging.