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Age-related muscle loss, also known as sarcopenia, is a natural part of aging. It is characterised by a gradual decline in muscle mass, strength, and function, which can lead to significant health risks and impact an individual's quality of life. The rate of muscle loss varies, but it typically accelerates with each passing decade, starting as early as age 30. This process can be influenced by various factors, including dietary protein intake, physical activity levels, hormone levels, inflammation, and genetics. Understanding the mechanisms behind age-related muscle loss is crucial for developing strategies to mitigate its effects and promote healthy aging.
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What You'll Learn
Loss of muscle fibres
Age-related muscle loss, known as sarcopenia, is a natural part of ageing. The loss of muscle fibres is a principal cause of sarcopenia. This loss of muscle fibres is associated with denervation and the remodelling of motor units.
The mechanisms of muscle fibre atrophy in humans relate to imbalances in muscle protein synthesis (MPS) and breakdown (MPB). Anabolic resistance, which lowers the body's ability to break down and synthesise protein, is fundamental to age-related fibre atrophy. This anabolic resistance to fundamental environmental cues regulating diurnal muscle homeostasis (physical activity and nutrition) underlies age-related catabolic perturbations in muscle proteostasis.
The importance of physical activity in regulating muscle mass homeostasis is exhibited in relation to the impact of complete immobilisation, such as bed rest, or partial immobilisation, such as reduced movement due to a sedentary lifestyle. Both can induce rapid muscle atrophy. Exercise, in particular progressive resistance training (PRT), is therefore key to building muscle mass.
Nutrition also plays a role in building muscle mass. Protein is key to muscle food. The anabolic responses to dietary protein are both dose-dependent and transient in nature. Maximal MPS responses are achieved with ≥10 g of EAA in younger individuals. Omega-3 fatty acids, consumed through seafood or supplements, may also increase muscle growth at any age.
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Reduced protein synthesis
Age-related muscle loss, or sarcopenia, is a natural part of ageing. It is characterised by a gradual loss of skeletal muscle mass, strength, and function, which can begin as early as age 25-30 and accelerate after age 60. This loss of muscle mass is associated with impaired physical function and an increased risk of developing chronic metabolic diseases.
One of the underlying factors contributing to sarcopenia is an age-related alteration in the ability for stimuli such as protein/amino acids, insulin, and exercise to increase skeletal muscle protein synthesis in older adults. This phenomenon is known as anabolic resistance, where the body becomes less responsive to anabolic stimuli, leading to reduced protein synthesis.
Studies have shown that the muscle protein synthetic response to food intake and physical activity is blunted in older adults. This means that the body's ability to effectively use these stimuli for muscle protein synthesis decreases with age. Specifically, the sensitivity of skeletal muscle to amino acid availability appears to be reduced with age, requiring higher quantities of protein to stimulate muscle protein synthesis.
Additionally, age-related hormonal changes, such as a decrease in testosterone levels, may also contribute to reduced protein synthesis. Insulin resistance, which is more prevalent in older individuals, can negatively impact muscle protein synthesis by impairing the muscle cells' ability to respond to circulating insulin.
Furthermore, age-related inflammation may also play a role in reduced protein synthesis. Acute and chronic inflammation can interfere with protein turnover and affect the anabolic sensitivity of older muscles. However, the precise mechanisms of age-related muscle loss are still not fully understood, and further research is needed to determine the exact causes.
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Lack of physical activity
Physical inactivity can lead to muscle atrophy, or disuse atrophy, which is the thinning or wasting away of muscle tissue due to insufficient use. When muscles remain inactive, the body interprets this as a sign that they are no longer needed and begins to break them down, resulting in a decrease in both muscle size and strength. This process can begin within two to three weeks of muscle disuse, and the loss of strength during such a period has been attributed to the loss of contractile proteins.
The impact of physical inactivity on muscle loss is particularly evident in individuals who are bedridden or immobilised due to injury or illness. In these cases, antigravity muscles, such as the gastrocnemius and soleus, are especially susceptible to atrophy. Additionally, athletes and physically active individuals who experience short-term immobilisation may also be at risk of muscle atrophy.
The good news is that muscle loss due to physical inactivity can often be reversed through regular exercise and improved nutrition. Progressive resistance training (PRT), which involves gradually increasing workout volume, weight, reps, and sets, has been shown to effectively build muscle mass in older men. Additionally, a higher-protein diet can support muscle growth, as protein is essential for providing the body with the amino acids needed to build muscle.
It is important to note that age-related muscle loss is likely influenced by multiple factors, including hormonal changes, nutrition, and individual responses to exercise and nutritional stimuli. However, addressing physical inactivity through appropriate exercise programmes and nutritional interventions can help mitigate the effects of muscle loss and improve overall health in older individuals.
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Poor nutrition
Nutrition plays a vital role in preventing and managing sarcopenia. A poor-quality diet, particularly one lacking in protein, is a suspected contributor to muscle loss. As people age, their bodies have increasing difficulty converting protein into energy. This is known as anabolic resistance, which reduces the body's ability to break down and synthesise protein. A diet lacking sufficient protein can lead to a decline in muscle mass. Experts recommend consuming 20-35 grams of protein per meal, which can be achieved through protein-rich foods such as meat, fish, cottage cheese, or lentils.
In addition to protein deficiency, a diet low in fruits and vegetables has been linked to age-related muscle loss. Fruits and vegetables provide essential vitamins and antioxidants that contribute to overall health and muscle function. For example, vitamin C and beta-carotene have been associated with improved physical performance in older women. Additionally, vitamin E and carotenoid deficiencies have been linked to a decline in physical function and frailty.
Ultra-processed foods, which are high in sugar, salt, additives, and unhealthy fats, have also been implicated in low muscle mass. These foods can contribute to obesity, which further exacerbates sarcopenia. Obesity increases inflammation and alters the body's response to insulin, accelerating muscle loss and creating a cycle of muscle loss and fat accumulation.
Finally, vitamin D deficiency has been associated with age-related muscle loss. Vitamin D is believed to have protective effects against muscle loss, and adequate intake may help prevent sarcopenia.
While the role of nutrition in age-related muscle loss is evident, further research is needed to fully understand the complex interplay between various nutritional factors and their impact on muscle health.
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Insulin resistance
Insulin plays a significant role in regulating muscle protein metabolism, increasing the net gain of muscle protein in both animals and humans. Insulin resistance, which develops with aging, is a condition in which the body's cells become less responsive to insulin, impairing the body's ability to properly utilise insulin to increase muscle protein synthesis. This results in a decrease in muscle mass and strength, a condition known as sarcopenia.
Several factors contribute to insulin resistance and its impact on muscle loss. Firstly, structural and functional modifications occur in skeletal muscle during aging, leading to impairments in muscle strength, contractile capacity, and performance. Secondly, there is a dysregulation of protein synthesis and breakdown, which is frequently reported in age-related muscle loss. This dysregulation further contributes to the decline in muscle mass.
Additionally, specific actions of insulin on various muscle proteins, particularly mitochondrial proteins, have been observed. These actions suggest that insulin also plays a significant role in regulating mitochondrial oxidative phosphorylation in human skeletal muscle. Impairments in this process due to insulin resistance could contribute to age-related muscle protein loss.
While the exact mechanisms are not yet fully understood, studies in rodent models of diabetes mellitus, specifically type 2 diabetes (T2DM), have provided valuable insights. For example, in streptozotocin-induced type 1 diabetic (T1DM) mice, insulin deficiency leads to increased muscle proteasomal protein degradation and rapid muscle atrophy. Similarly, in TallyHo mouse models of T2DM, the impact of insulin resistance and hyperglycemia on mature muscle has been studied, as these mice develop diabetes at a mature age. These models have helped researchers investigate the effects of exercise and diabetes on muscle size, glucose tolerance, and protein synthesis and degradation.
Furthermore, limited studies in young men have indicated a positive correlation between insulin sensitivity and lean body mass. However, more research is needed to establish a direct link between early insulin resistance and muscle loss in prediabetic humans. Nevertheless, it is clear that insulin resistance is a contributing factor to age-related muscle mass loss, and its impact on muscle wasting and sarcopenia cannot be overlooked.
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Frequently asked questions
Age-related muscle loss is called sarcopenia.
Age-related muscle loss can start as early as age 30, with a decline of 3-5% per decade.
Age-related muscle loss can lead to a decrease in mobility and an increased risk of falls and fractures. It can also result in functional dependence and disability.
Age-related muscle loss can be prevented by progressive resistance training and a higher-protein diet.
