As we age, we lose muscle mass. To complicate matters, we also gain body fat. If the amount of muscle loss and fat gain is profound enough, it leads to an unhealthy condition known as sarcopenia – a medical term used to describe age-related muscle loss and fat gains. Not only is this a blow to your health (by increasing the risk of type 2 diabetes and cardiovascular disease), but it also slows us down, makes us less functional, and affects our quality of life.
One of the biggest frustrations that comes with aging, besides wrinkles and varicose veins, is losing the ability to do the things you enjoy. Hanging on to more muscle mass helps preserve independence, giving us the freedom to enjoy life at a ripe, old age. But what’s driving these changes? At the cellular level, several key mechanisms contribute to the decline in muscle mass and function. Let’s take a closer look at what causes age-related muscle loss:
Anabolic Resistance
One reason you lose muscle mass, beginning in the late thirties, is a well-recognized phenomenon called anabolic resistance. This is an age-related condition where muscle cells become resistant to signals that tell them to grow. Shockingly, half of all people over eighty have sarcopenia. The problem with anabolic resistance is it makes it harder for cells to turn on the mTOR pathway, a key signaling pathway for muscle protein synthesis.
To add further insult to injury, insulin sensitivity goes down with age. This age-related decline in how cells respond to insulin is linked with an increase in low-grade inflammation, another contributor to muscle loss and sarcopenia. When cells respond less to insulin, it also boosts the risk of developing type 2 diabetes.
Finally, the energy powerhouses inside cells, called mitochondria, become damaged due to a constant onslaught of oxidative stress. This makes it harder for them to make the energy currency, ATP, to fuel muscle contractions.
Hormonal Changes
Hormonal factors also contribute to loss of muscle tissue as you go through mid-life and beyond. These hormones include:
Growth Hormone – Growth hormone begins declining during the mid portion of life due to a phenomenon known as somatopause. Growth hormone works in synergy with another protein produced by the liver called IGF-1 to build and maintain healthy muscle tissue. As IGF-1 goes down due to age, it’s harder to turn on the mTOR pathway that helps repair and build muscle tissue.
Testosterone – We think of testosterone as being a male hormone, but women produce it too, in quantities one-tenth that of men. In both men and women, testosterone production drops off with age, contributing to muscle and strength loss.
Neuromuscular Junction Changes
A neuromuscular synapse is the point at which a motor neuron, a large nerve that tells a muscle fiber to contract meets the muscle fiber it innervates. These connections change in structure with age. Plus, the muscle fibers themselves lose some of their nerve connections due to the aging process.
As you know, you have two main types of muscle fibers, slow-twitch, and fast-twitch. Fast-twitch muscle fibers are designed for strength and power while slow-twitch fibers are optimized for muscle endurance. You lose more fast-twitch fibers with age than slow-twitch. Therefore, strength and power capabilities suffer more due to aging than endurance capabilities.
So, you lose some of the ability to generate force quickly and with high force. This is particularly harmful as you need fast-twitch capabilities for balance and recovery from falls too.
Chronic Inflammation
As mentioned, inflammation contributes to age-related muscle loss. Chronic, low-grade inflammation, a smoldering fire within, increase with age. This happens as our immune systems become less precise and begin targeting normal tissue. One way it targets tissues is by producing pro-inflammatory cytokines, including interleukin-6 (IL-6) and tumor necrosis factor-alpha (TNF-α). These cytokines can be friendly, but when they become too elevated, because of aging, they sabotage muscle growth by disrupting muscle protein synthesis.
Over time, this chronic inflammation can lead to a gradual but significant loss of muscle mass and strength, a condition known as sarcopenia. It’s as if our muscles are slowly withering away, leaving us frail and vulnerable.
Reduced Regenerative Capabilities
As we journey through life, our muscle cells become less efficient at regenerating themselves. The reason? We lose a population of specialized cells known as satellite cells. These are cells that hide between the muscle fiber and its outer sheath. When you experience muscle damage, they spring into action to help your muscles repair. They do this by recruiting muscle fibers and signaling them to repair and differentiate into new, healthy muscle fibers.
Unfortunately, the number of satellite cells your muscles have goes down with age, making your muscles more susceptible to injury and chronic damage. Why do satellite cells falter? They build up genetic mutation. Plus, the environment they “live” in changes due to chronic inflammation and a drop in hormones, like growth hormone and testosterone.
Fortunately, scientists in regenerative medicine are looking for ways to boost satellite cell function. So, let’s see what the future holds. By understanding what shapes our muscle health, we can develop strategies that support satellite cells.
What Can You Do About Age-Related Muscle Loss?
Based on science, the best way to reduce the impact aging has on treasured muscle tissue is to keep strength training. Also, ensure you’re consuming enough protein to maintain and build healthy muscle tissue. By living a healthy lifestyle, strength training, and consuming enough protein, you help shift the balance of muscle protein synthesis away from breakdown to maintenance and even muscle growth.
Also, eating a healthy diet that includes antioxidant and anti-inflammatory foods may help calm the oxidative stress that damages mitochondria and reduce low-grade inflammation that tampers down muscle protein synthesis.
References:
- Montano M, Rosaly Correa-de-Araujo. Maladaptive Immune Activation in Age-Related Decline of Muscle Function. The journals of gerontology Series A, Biological sciences and medical sciences. 2023;78(Supplement_1):19-24. doi:https://doi.org/10.1093/gerona/glad036.
- Dalle S, Rossmeislova L, Koppo K. The Role of Inflammation in Age-Related Sarcopenia. Front Physiol. 2017 Dec 12;8:1045. doi: 10.3389/fphys.2017.01045. PMID: 29311975; PMCID: PMC5733049.
- Ham DJ, Rüegg MA. Causes and consequences of age-related changes at the neuromuscular junction. Current opinion in physiology. 2018;4:32-39. doi:https://doi.org/10.1016/j.cophys.2018.04.007
- Bartke A. Growth Hormone and Aging: Updated Review. World J Mens Health. 2019 Jan;37(1):19-30. doi: 10.5534/wjmh.180018. Epub 2018 May 11. PMID: 29756419; PMCID: PMC6305861.
- Bodine SC, Sinha I, Hugh Lee Sweeney. Mechanisms of Skeletal Muscle Atrophy and Molecular Circuitry of Stem Cell Fate in Skeletal Muscle Regeneration and Aging. The journals of gerontology Series A, Biological sciences and medical sciences. 2023;78(Supplement_1):14-18. doi:https://doi.org/10.1093/gerona/glad023.
- Wilkinson DJ, Piasecki M, Atherton PJ. The age-related loss of skeletal muscle mass and function: Measurement and physiology of muscle fibre atrophy and muscle fibre loss in humans. Ageing Res Rev. 2018 Nov;47:123-132. doi: 10.1016/j.arr.2018.07.005. Epub 2018 Jul 23. PMID: 30048806; PMCID: PMC6202460.
- Baumgartner RN, Koehler KM, Gallagher D, Romero L, Heymsfield SB, Ross RR, Garry PJ, Lindeman RD. Epidemiology of sarcopenia among the elderly in New Mexico. Am J Epidemiol. 1998 Apr 15;147(8):755-63. doi: 10.1093/oxfordjournals.aje.a009520. Erratum in: Am J Epidemiol 1999 Jun 15;149(12):1161. PMID: 9554417.
- Ham DJ, Rüegg MA. Causes and consequences of age-related changes at the neuromuscular junction. Current opinion in physiology. 2018;4:32-39. doi:https://doi.org/10.1016/j.cophys.2018.04.007
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