Anabolism and catabolism are the biochemical processes that build up and break down skeletal muscle, respectively. These two processes are in equilibrium during our younger years when exercise and food intake work in concert to maintain muscle mass. As we age, this equilibrium wanes, and an imbalance is created favoring breakdown, which contributes to sarcopenia – the loss of muscle mass and strength that accompanies aging.

Many factors contribute to this imbalance, including a sedentary lifestyle and dietary insufficiencies, often due to a reduced appetite in old age. A lesser-known contributor is anabolic resistance, the reduced sensitivity to a given protein dose that promotes muscle building. Dr. Phillips describes studies in his laboratory suggesting that inactivity is a driver of anabolic resistance. To forestall anabolic resistance, public health experts recommend adults engage in muscle-strengthening activities such as resistance training at least twice a week, with sessions lasting about 30 to 45 minutes each. In this clip, Dr. Stuart Phillips describes how muscle protein synthesis differs between young and older adults, and how the efficiency of protein use declines with age.