The Rise and Fall of Muscle
By and large, we get smaller with age – at least in terms of muscle mass and strength. Older people do not make muscle as quickly as young people, and some of the systems that help prevent muscle breakdown in youth begin to break down with age.
Simon Schenk, PhD, is an assistant professor of orthopedic surgery and a core director at the National Skeletal Muscle Research Center at the University of California, San Diego. We asked him to do the heavy lifting of explaining why.
Q: Why does muscle deteriorate rapidly in old age?
A: As with many diseases, the factors that contribute to reduced muscle mass with age are many. They include reduced physical activity (i.e., a sedentary lifestyle), reduced anabolic signaling (which could be due to reduced hormones such as growth hormone, insulin-like growth factor 1 [IGF1] and testosterone), increased adiposity and prevalence of metabolic diseases (such as insulin resistance).
It’s important to note that the news is not all bad when it comes to muscle deterioration with age. Certainly maintaining an active lifestyle and a multi-modal approach (that includes both endurance- and strength-based exercise) can thwart the loss of muscle with age, or increase it in the already aged. In fact, studies show that when you compare young muscle to muscle from an aged person who has been active their whole life, the muscles, in terms of how they function, really aren't any different.
Q: How does exercise slow this process? Why does lack of use speed up the process (atrophy)?
A: Increased adiposity commonly occurs with age, and this is due to a combination (most typically) of reduced physical activity and increased (or maintenance) food intake. The end result is weight gain and/or increased fat mass. An increase in fat mass can increase an individual's risk for developing many metabolic diseases, such as type 2 diabetes, insulin resistance, coronary artery disease and even certain cancers. Exercise can prevent or even reverse the development of such diseases. Insulin resistance in particular can affect muscle mass, as insulin is a major anabolic hormone, and when the muscle become “resistant” to its metabolic effects (which include increasing muscle/protein synthesis and preventing muscle/protein breakdown), it can directly affect muscle mass.
Exercise improves sensitivity of muscle to insulin. Exercise can also stimulate the production of anabolic hormones like GH/IGF1 and testosterone, which can positively impact muscle growth (or prevent its breakdown). For this, exercises that activate a large muscle masses (such as the legs) result in the best anabolic response.
Q: Beyond regular physical activity, has anything been shown to measurably sustain or restore muscle health in older persons?
A: Hormonal treatment with testosterone, GH or IGF1 has been shown to restore muscle health. However, GH and IGF1 studies have had mixed success unless also combined with androgens and exercise. Also, there are suggestions that these approaches may increase the risk of developing cancer and cardiovascular disease, which is concerning. Safer strategies that have shown some success have focused on nutritional changes and increasing the protein content and combined ingestion of protein and carbohydrate. This includes altering the timing of nutrition to before or during exercise rather than after. Protein/carbohydrate mixtures have been shown to increase the insulin response to a meal, and insulin has positive effects on muscle mass.
Q: What are the prospects for the future? Are there any potential treatments or pharmaceutical approaches that appear promising?
A: Supplementation with GH and IGF1 hold promise for the future, but the next frontier of research is looking into how to deliver these to the muscle only, so as to avoid potential negative effects of exposure in other tissues. In the pipeline are activators of certain receptors (known as nuclear receptors) in muscle that have been shown to increase the endurance of muscle. Interestingly, many of these treatments are most effective when given in combination with exercise. Thus, the good news is that one “cure” is readily available to most of us. That is, maintaining or initiating an active lifestyle. An active lifestyle need not mean spending hours at the gym, but can simply be gardening, walking or swimming. There are large benefits to be achieved by simply moving. In the end, an approach that considers different types of physical activity in combination with optimal nutrition holds great promise.