One of the multiple insults to you as you get older is that your muscles shrink – sarcopenia. While you can slow the speed and details of sarcopenia with exercise it is still inexorable. You might have already noticed the difference between the movements of the kids and the old people who sometime babysit them.
Women for example, on average lose approximately 6 kg of lean tissue (presumably mostly muscle) between the age of 25 and 65 years. But given that most men and women gain weight across this time period, it is apparent that they must be replacing muscle with way more fat. The point that needs to be made is that, under conditions where there is loss of muscle, body weight will give a very incomplete understanding of what is happening to body composition.
Sarcopenia not only affects the ability to lead an active lifestyle but also contributes to increased obesity, reduced quality of life, osteoporosis, and worsening metabolic health. And it goes both ways, increased obesity, decreased quality of life, and reduced metabolic health also contribute to sarcopenia. A recent review of these issues discussed some of the implications that sarcopenia has for the development of obesity and the other indignities of aging. (1)
The loss of muscles experienced by old people is thought to be caused by a combination of factors including loss of repair abilities, oxidative stress and chronic inflammation.
Besides the decrease in exercise energy expenditure, the loss of muscles leads to decreases in all the other components of the energy expenditure by our bodies. There is resting energy expenditure (REE) the energy expended by doing nothing. REE is decreased by loss of muscle mass. There is non-exercise activity thermogenesis (NEAT), the energy expended for everything we do that is not sleeping, eating or sports-like exercise. It ranges from the energy expended walking to work, typing, performing yard work, undertaking agricultural tasks and fidgeting. Even trivial physical activities increase metabolic rate substantially and it is the cumulative impact of a multitude of actions that culminate in an individual’s daily NEAT. It is, therefore, not surprising that NEAT explains a vast majority of an individual’s non-resting energy needs. The mechanism that regulates NEAT is unknown. However, brain centers have been identified that specifically and directly increase NEAT in animals. By understanding how NEAT is regulated we may come to appreciate that spontaneous physical activity is not spontaneous at all but carefully programmed. Besides sarcopenia, aching joints and loss of ease and efficiency of movements further conspires to decrease NEAT in old people and make them fat.
People have two general types of skeletal muscle fibers. Skeletal muscles are the muscles attached to bones as opposed to the heart muscle and the muscles in your intestines called smooth muscles. The 2 types of skeletal muscles are: slow-twitch (type I) and fast-twitch (type II). Slow-twitch muscles are for long-endurance feats such as distance running, while fast-twitch muscles get tired faster but are used in powerful bursts of movements like sprinting. Depending on the parts of your body and what kind of an athlete you are you generally have about 50-50% type I and type II skeletal muscle fibers. With aging we seem to lose more type II than type I muscles and this is not well explained. Aerobic exercise (running) may improve this relative shift of type II to type I and it uses 2-3 times more calories for any given time spent and it is better for maintaining metabolic health, (reduce oxidative stress and inflammation, increase insulin sensitivity, decrease blood pressure, and improve blood lipid profile) and independently strongly increases longevity. But only resistance training (weight lifting) can increase skeletal muscle mass and slow (but not stop) the loss of muscle as we age.
It is the contention of these authors that “the combination of aerobic and resistance training will slow sarcopenia development, decrease fat mass accretion (especially harmful visceral fat), and decrease the risk of developing a number of metabolic diseases throughout the lifespan.” But you’re still going to get sarcopenic and forgetful and old.
1. Hunter GR et al. Sarcopenia and Its Implications for Metabolic Health. J Obes. 2019; 2019: 8031705. Published online 2019 Mar 6. doi: 10.1155/2019/8031705
John DiTraglia M.D. is a Pediatrician in Portsmouth. He can be reached by e-mail- firstname.lastname@example.org or phone (740) 354-6605.