It likely is not news to you that as we age, our body tends to change in a variety of ways; one of those ways is in body composition. While body composition changes, this article will focus largely on muscle mass and strength during the aging process. In this article, we will discuss what sarcopenia is, the rate at which it occurs, a few key reasons why it occurs, and any possible preventative measures to be taken against sarcopenia.
What is sarcopenia?
Sarcopenia is the loss of muscle mass with aging, and consequently, strength loss .
How does it impact the body?
Sarcopenia impacts the body in several ways that are predictably akin to any general muscle atrophy (aka, loss); those three ways are a decrease in strength, metabolic rate, and maximal oxygen consumption . Strength decreases are a majority due to muscle cross sectional area (aka, size of muscle) atrophy (especially Type II), but also affected by neural changes . Secondly, metabolic rate declines due to muscle tissue being highly metabolically active (aka, uses many calories) and the loss of that tissue with age linearly decreases daily caloric expenditure . Finally, as a large portion of oxygen consumption is attributed to muscle, the decline in muscle also decreases overall oxygen consumption .
What is the rate of sarcopenia?
On average, the rate at which we lose muscle is about 1% per year up to age 50; after 50, the rate increases to about 2% per year . Meanwhile, strength seems to decrease more substantially (which is in accordance with cross sectional area not being the only factor on strength) at an estimated rate of 0.8 – 2.0+ %, per year, with recent evidence pointing more toward the higher end of that range, especially from 50-60+ years of age (1.5 – 5.0% per year) .
Why does sarcopenia occur?
There are several reasons for sarcopenia becoming more and more of an issue and even speeding up in the latter stages of life. The first, and the most obvious, is a lack of activity; if a person does not train or stress their musculature, then there is no reason for the muscle to be stimulated and grow or maintain itself (especially true for Type II fibers) . This concept also fits in nicely with a change in protein metabolism as at younger ages, the body has a similar protein synthesis as it does breakdown, leading to a relatively neutral outcome; however, as we age, the body becomes more “anabolic resistant” and decreases in muscle protein synthesis are seen . This eventual change in muscle protein synthesis leads to a decrease in muscle mass as degradation then, even slightly, outpaces synthesis leading to a net loss per year . Not only that, it may also be true that the degradation process is rendered less functional and mistakes occur in which proteins are not correctly degraded to amino acids for resynthesis, but rather, are left as useless contractile components in the muscle – which, then, could account for some strength loss even in the face of no change in muscle mass .
Other factors spawn from the endocrine system with a change in the amount of various hormones. Insulin, to begin with, normally decreases muscle protein breakdown and as we age, we become more insulin resistant, leading to insulin having less of an effect . Factors like testosterone, which is known to decrease over time, and growth hormone, as well as insulin growth factor, have an impact on protein synthesis – these may be the main drivers of why protein synthesis decreases considering their presence also decreases with age . Finally, on the other side, the hormone cortisol is also slightly higher, chronically, as we age and this causes issues as it further stimulates protein breakdown.
There are, however, changes in neuromuscular connection are seen on the motor endplate (aka, signaling site between muscle cells and neuron cells) as the axons of the neuron degenerate and withdraw, as well as fragment . So, a decrease in motor unit functionality certainly has some influence on the loss of muscle; however, if this neural remodeling or the muscle loss occurs first is still a topic of debate . Either way, this does lead to a slight, although noticeable decrease in strength and force production .
As the last point discussed in this article, it is also possible for muscle specific mitochondria mutations to appear in older individuals that are not formerly present . These mutations, specifically called DNA deletion mutations, lead to degradation of the mitochondria to create energy effectively through the electron transport chain; these mutations likely occur due to free radical damage . However, not all who age have these mutations, although a majority has one or more of these mutations later in life .
While sarcopenia can be argued to be a progressive issue spanning the later part of life, there is some evidence showing that for most individuals this may not be the driving problem. Rather, that this functional degradation can lead to increased risk for catastrophic injury as age increases . When these injuries occur and a person is limited to bed rest for weeks or even as little as a few days, there is a rapid decrease in muscle due to a lack of stress being applied to the musculature (even things as simple as standing can help tremendously); so, having surgery or going through a non-weight bearing period of time leads to rapid decreases in functionality that need to be actively rehabilitated immediately after the period of immobility – otherwise the process of sarcopenia has just accelerated years in a matter of days or weeks .
Is sarcopenia preventable?
Oh, you wanted to know how? Fine!
There are two major ways to mitigate sarcopenia. The first is to consume adequate calories and protein . We do this by simply eating at maintenance (use a calorie calculator or self-experiment for a few weeks) in terms of calories and consuming 25-30g of protein a few times a day to reach 1.0 – 1.3 g per kg of bodyweight per day (0.45 – 0.59 g per kg of BW). Secondly, resistance training has an incredible impact on muscle growth and the rebirth of function from decreasing anabolic resistance, increasing neural efficiency, increasing muscle protein synthesis, among other factors . Resistance training seems to lead to a reversing of many of the issues that come as a result (or lead to) sarcopenia with the noticeable results seen in the increase in strength and muscle mass; even in 90 year olds first starting resistance training there are increases as high as 174% in strength and nearly 12% in muscle size in a matter of 8 weeks . However, the benefit might come before as those who have been resistance training for some time have comparable muscle mass and strength, at age 70, to 28 year old sedentary individuals . So, clearly, resistance training and adequate nutrition both lead to a mind blowing-ly powerful reversal and prevention of many of the issues that would normally plague an individual that is sarcopenic; however, no matter what intervention is used, the muscle does become somewhat weaker over time.
All in all, what do we now know? We know that sarcopenia is the condition in which we lose muscle and strength over the aging process. We know that sarcopenia decreases our metabolic rate, our muscle fibers get smaller, our neurons are less efficient, and our overall fitness decreases. We also know that sarcopenia starts out mild with only a 1% per year drop in muscle after age 30 (0.8 – 2.0+% drop in strength), and speeds up after the age of 50 to 2% per year muscle loss (closer to 5.0% in strength loss). Sarcopenia occurs for a variety of reasons from habitual to hormonal to neural, but luckily sarcopenia can be largely (not entirely) reversed and fought off with strength training and adequate nutrition intake (calories and protein).
Writer: Nicolas Verhoeven
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