Muscle Function and Protein

Original Editor - Lucinda hampton

Top Contributors - Lucinda hampton, Alexandra Stead, Kim Jackson and Vidya Acharya  

This article provides evidence-based information on protein intake recommendations for older adults. Understanding the relationship between protein consumption, muscle health, and functional capacity is essential for optimising patient outcomes in geriatric populations.

Protein Intake in Older Adults

Age-related loss of muscle mass and function increases the morbidity and mortality risk of age-related chronic diseases and disability. These concerns are becoming increasingly critical given the global rise in the ageing population. The aetiology of sarcopenia is complex and multifaceted, and despite advances in understanding, there remains no widely available pharmacological intervention with an appropriately low side-effect profile to directly combat this condition.

Recent evidence continues to demonstrate that the recommended amounts of protein may be insufficient for elderly people to maintain optimal muscle health and function. Current research suggests seniors may require 1.0-1.3 grams of protein per kilogram of body weight daily. For example, an individual weighing 82 kg may need to consume 82-107 grams of protein daily, regardless of calorie intake. The current protein Recommended Dietary Allowance (RDA) does not appear to meet the optimal protein requirements for older adults to maintain skeletal muscle protein mass. Given that sarcopenia is, in part, underpinned by the reduced ability of dietary protein to stimulate muscle protein synthesis (MPS), increasing protein intake has been demonstrated to result in better preservation of muscle mass. As part of a multimodal intervention, increasing dietary protein intake may support not only muscle mass maintenance but also bone health when calcium and vitamin D intakes are adequate.

Protein Requirements in Older Adults

The original RDA recommendations were established based on studies conducted in young healthy adults and do not adequately account for the numerous physiological changes that occur with ageing. These recommendations were based on the concept of preventing deficiency rather than promoting optimal health, which may be of greater importance in the geriatric population.

Accumulating evidence demonstrates that older adults require more dietary protein than their younger counterparts to support optimal health, promote recovery from illness, and maintain functional capacity. The most common age-related causes of protein shortfall include inadequate dietary protein intake (loss of appetite, gastrointestinal dysfunction, reduced energy requirements, alterations in food preferences), a reduction in the utilisation of available protein (anabolic resistance, insulin resistance, higher splanchnic extraction), and elevated basal requirements (acute and chronic diseases, inflammatory conditions, increased oxidation of protein).

Anabolic Resistance

Anabolic resistance is responsible, in part, for skeletal muscle atrophy associated with ageing, muscle disuse, and during disease states. Anabolic resistance describes the blunted stimulation of muscle protein synthesis in response to a given dose of protein or amino acids and contributes to progressive declines in skeletal muscle mass. Physical inactivity induces anabolic resistance (which is likely exacerbated with ageing), insulin resistance, systemic inflammation, decreased satellite cell content, and reduced capillary density, creating a cascade of detrimental effects on muscle health.

Protein Overview

Proteins are essential nutrients for the human body. They serve as primary building blocks of body tissue and can function as a fuel source. Protein is the only macronutrient that lacks an inactive reservoir compound, meaning dietary amino acids must be incorporated into functional proteins to maintain physiological homeostasis.

Protein is one of three macronutrients essential for human nutrition, alongside lipids and carbohydrates. It is predominantly found in animal products (meat, fish, dairy, eggs), though also present in plant-based sources such as legumes, nuts, seeds, and whole grains.

Essential Amino Acids

The nine amino acids that humans cannot synthesise endogenously are termed essential amino acids (EAAs): histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine. Protein sources containing all essential amino acids are classified as complete proteins. Complete protein sources include fish, meat, dairy products, eggs, quinoa, hemp seeds, chia seeds, and soy products. Many plant-based proteins are incomplete proteins, lacking one or more essential amino acids. These include beans, grains, and legumes. However, incomplete proteins can be strategically combined to create complete amino acid profiles through complementary protein pairing (e.g., beans with rice, peanut butter with wholegrain bread).

Skeletal muscle contractile proteins represent the largest protein reservoir in the body and can be rapidly mobilised to supply amino acids during periods of fasting or physiological stress. Insufficient protein intake leads to negative protein balance, resulting in skeletal muscle atrophy, impaired muscle growth, and functional decline. Therefore, adequate protein consumption is essential to prevent muscle wasting and maintain skeletal muscle mass and function in older adults.

Optimising Muscle Preservation

Optimising nutritional intake through the introduction of protein-rich whole foods, isolated proteins, and nutrient compounds with purported pro-anabolic and anti-catabolic properties can offset impairments in muscle protein turnover and, ultimately, attenuate the degree of muscle atrophy. The greatest effects are observed when resistance training and higher protein diets are combined, appearing to act synergistically.

Exercise and Protein Recommendations

To optimise outcomes, exercise is essential. Older individuals should perform progressive resistance exercise using varied loading protocols two to three times per week, ingest at least 1.2 g of protein per kilogram body weight daily, distribute protein intake evenly across meals in boluses of approximately 0.40 g/kg, and consume protein within 2 hours of retiring for sleep. Rapidly digested proteins that contain high proportions of essential amino acids (EAAs) and adequate leucine content are most effective in stimulating MPS. Protein intake should prioritise whole food sources that contain all EAAs required to stimulate MPS.

Dietary Strategies

At every meal and snack, adults over 65 should consider implementing several dietary strategies. Substituting higher protein foods for starches or carbohydrates is recommended, including smoked salmon, cheese, nuts (almonds, hazelnuts, mixed nuts, peanuts, peanut butter, sunflower seeds, walnuts), seeds, and minimally processed meat products. Protein supplementation through protein shakes provides a convenient method to increase protein intake. Both plant-based and animal-sourced protein powders are effective options. Some protein powders can be mixed directly into beverages such as coffee or tea. Clinicians should advise patients to select high-quality protein powders that are free from added sugar, excessive sodium, and unnecessary additives. Current evidence suggests whole food sources are preferable, though supplementation may be beneficial when dietary intake is insufficient.

Modifying meal composition is another effective strategy, replacing traditional sandwich formats with protein-rich alternatives such as turkey or chicken wraps with salad vegetables. Incorporating diverse protein sources should include legumes, eggs, lean meats, fish, poultry, soy-based products (tofu, tempeh), Greek yoghurt, cottage cheese, and seafood options. Increasing leucine intake is particularly important, as leucine performs numerous physiological functions, including blood glucose regulation, wound healing facilitation, and growth hormone production. Leucine is particularly important for muscle health due to its role in stimulating muscle protein synthesis. Whilst leucine supplementation shows promise, it appears most effective when combined with resistance training and adequate total protein intake.

Clinical Implications for Physiotherapists

Physiotherapists play a crucial role in addressing sarcopenia and age-related muscle loss through comprehensive assessment and intervention strategies. When working with older adults, consideration should be given to several key areas. Nutritional screening should incorporate questions about dietary protein intake during initial assessments and reassessments, particularly for patients presenting with sarcopenia, frailty, or functional decline. Interprofessional collaboration is essential, requiring physiotherapists to work collaboratively with dietitians and nutritionists to optimise protein intake alongside exercise prescription for patients at risk of or experiencing muscle loss. Patient education should provide evidence-based guidance on the importance of adequate protein intake in conjunction with resistance exercise for maintaining muscle mass and function. Exercise prescription must be designed to incorporate progressive resistance training programmes that complement increased protein intake, recognising the synergistic effects of combined interventions. Monitoring and progression require physiotherapists to regularly assess muscle mass, strength, and functional capacity to evaluate the effectiveness of combined nutritional and exercise interventions.

Conclusions

Elderly adults demonstrate reduced responsiveness to the anabolic stimulus of lower doses of amino acid intake compared with younger adults. However, this blunted response in older adults can be overcome with higher levels of protein consumption combined with appropriate exercise prescription. The attenuated muscle responsiveness to lower protein doses in older adults can be effectively addressed through increased protein intake and progressive resistance training. The most substantial effects are observed when resistance training and higher protein diets are combined, demonstrating synergistic benefits for muscle mass and function. The requirement for larger protein doses to generate responses in older adults comparable to younger individuals provides robust support for recommending increased protein intake in geriatric populations. The consumption of high-quality proteins that are easily digestible and contain high proportions of EAAs, particularly leucine, appears most beneficial for stimulating muscle protein synthesis and preserving muscle mass in older adults.

Contemporary evidence strongly supports the implementation of higher protein intake (1.0-1.3 g/kg/day) combined with progressive resistance exercise as a foundational intervention for maintaining muscle health in older adults. Physiotherapists should integrate nutritional considerations into comprehensive treatment plans for optimal patient outcomes.