Unraveling Anabolic Resistance in Sarcopenia: A Narrative Synthesis of Age-Related Impairments in Muscle Protein Synthesis

Molecular mechanisms

The molecular basis of anabolic resistance involves multiple interconnected pathways affecting muscle protein turnover. Age-related changes in the mechanistic target of rapamycin (mTOR) signaling pathway represent a primary mechanism [6]. In younger adults, protein ingestion rapidly activates mTOR through amino acid sensing, particularly leucine-mediated signaling [7]. However, older adults demonstrate impaired mTOR activation, requiring higher amino acid concentrations to achieve comparable responses [6].

Additionally, age-related alterations in muscle blood flow and amino acid transport contribute to anabolic resistance [8,9]. Capillary density decreases with aging, reducing nutrient delivery to muscle fibers [10]. Simultaneously, amino acid transporter expression and function decline, limiting the muscle’s capacity to extract circulating amino acids even when adequate protein is consumed [11].

Figure 1.

Mechanistic Pathways Underlying Anabolic Resistance in Aging Skeletal Muscle.

Clinical manifestations

Clinically, anabolic resistance reflects a diminished MPS response to typical protein doses, indicating a shift in the dose-response relationship [4,12]. While young adults achieve maximal MPS stimulation with approximately 0.24 g/kg of highquality protein per meal, older adults require nearly double this amount (0.40 g/kg) to achieve equivalent responses [13].

This elevated threshold has profound implications for clinical practice. Standard dietary recommendations based on younger adult populations become inadequate for older adults, potentially contributing to progressive muscle loss despite seemingly adequate protein intake.

The inactivity-resistance cycle

Physical inactivity represents a potent amplifier of anabolic resistance, creating a pathological feed-forward cycle [9]. Even short periods of reduced activity (14 days) can induce measurable anabolic resistance in previously healthy older adults [14]. This inactivity-induced resistance persists beyond the period of reduced activity, suggesting deeper metabolic changes rather than simple reversible deconditioning.

This creates a vicious cycle through several interconnected problems. When people become less active, the reduced mechanical stress on muscles makes mTOR signaling even less sensitive [6], while decreased muscle blood flow limits amino acid delivery and uptake [8,9]. Prolonged inactivity also triggers inflammatory responses that further compromise anabolic signaling pathways, creating a pattern where muscle becomes increasingly resistant to anabolic stimuli.

This cycle poses particular risks for older adults who may face periods of enforced inactivity due to illness, hospitalization, or injury. Breaking this pattern requires early intervention with structured resistance training and targeted nutritional support to restore anabolic sensitivity before irreversible changes occur. This mechanism highlights why maintaining regular physical activity is critically important throughout aging.

Diagnostic challenges

Unlike other age-related conditions, anabolic resistance lacks standardized diagnostic criteria or easily accessible biomarkers. Current research relies primarily on stable isotope methodology to measure MPS rates, which is limited to specialized research facilities [15]. This creates a significant gap between research understanding and clinical application.

Surrogate markers

Several potential surrogate markers have been proposed for clinical assessment of anabolic resistance. Functional Measures such as grip strength, gait speed, and sit-to-stand performance may reflect the cumulative effects of anabolic resistance on muscle mass and quality [16]. However, these measures lack specificity and may be influenced by multiple factors beyond muscle anabolism.

Circulating amino acid profiles, particularly branchedchain amino acid concentrations, may provide insights into muscle anabolic capacity [17]. However, validation studies are lacking, and normal variation limits clinical utility. Dual-energy X-ray absorptiometry (DXA) and bioelectrical impedance analysis (BIA) can assess muscle mass but fail to capture muscle quality or anabolic capacity [18]. More sophisticated imaging techniques, such as magnetic resonance imaging (MRI) or computed tomography (CT), can assess muscle quality but remain expensive and inaccessible for routine clinical use.

Resistance training: the primary intervention

Among all non-pharmacological strategies, resistance training remains the cornerstone in reversing anabolic resistance. Notably, even a single session can re-sensitize aging muscle to anabolic signals—particularly amino acids—for up to 48 hours. This transient “window of opportunity” underscores the importance of consistent and strategically timed interventions [19].

Resistance training exerts its anabolic effects through multiple integrated mechanisms that collectively restore muscle sensitivity to anabolic stimuli [20]. The primary mechanism involves direct mechanical stimulation of the mechanistic target of rapamycin (mTOR) signaling pathway, where mechanical tension and stretch activate mTOR complex 1 (mTORC1) through a cascade involving focal adhesion kinase (FAK) and phosphatidylinositol 3-kinase (PI3K)/protein kinase B (Akt) signaling [5,21]. This mechanical activation bypasses the agerelated impairments in amino acid-induced mTOR stimulation, providing an alternative pathway for initiating muscle protein synthesis [22].

Simultaneously, resistance exercise enhances muscle perfusion and nutrient delivery through acute vasodilation and chronic adaptations in capillary density. The exerciseinduced increase in muscle blood flow facilitates the delivery of circulating amino acids to the muscle tissue, while the mechanical stress of contraction enhances microvascular recruitment and endothelial function [23]. These vascular adaptations are particularly important in aging muscle, where baseline perfusion is often compromised.

Furthermore, resistance training upregulates the expression and function of amino acid transporters, including system L amino acid transporter 1 (LAT1) and sodium-coupled neutral amino acid transporter 2 (SNAT2), which are essential for leucine and glutamine uptake respectively [24]. This enhanced transport capacity allows the muscle to more effectively extract amino acids from the circulation, partially compensating for age-related reductions in transporter efficiency.

Finally, resistance training induces neuroplasticity adaptations that increase motor unit recruitment and firing frequency, leading to enhanced muscle fiber activation. These neural adaptations occur within days to weeks of training initiation and contribute significantly to early strength gains, even before measurable hypertrophy occurs [25]. Meta-analyses consistently demonstrate that resistance training improves muscle mass, strength, and functional capacity in older adults [26]. Importantly, strength gains often exceed mass gains, suggesting improvements in muscle quality and neural control beyond simple hypertrophy.

Optimized protein nutrition

Protein nutrition must be tailored to overcome the elevated anabolic threshold characteristic of aging muscle. This requires both adequate total intake and optimal distribution throughout the day. Clinical evidence supports protein intake of 1.2-1.6 g/kg/day for older adults, substantially higher than the current RDA of 0.8 g/kg/day [27]. Since muscle protein synthesis has a ceiling effect, it's important to spread protein intake evenly throughout the day rather than loading up at one meal. Each meal should contain approximately 0.4 g/kg of high-quality protein to achieve maximal anabolic stimulation [13]. This typically translates to 25-30 grams of protein per meal for most older adults.

High-quality proteins containing adequate leucine content (2.5-3.0 grams per meal) are essential for optimal MPS stimulation [28,29]. Animal-based proteins generally provide superior amino acid profiles compared to plant-based alternatives, although properly combined plant proteins can achieve similar effects [30].

Synergistic effects of combined interventions

The combination of resistance training and optimized protein nutrition produces synergistic effects that exceed the sum of individual interventions. Resistance training sensitizes the muscle to amino acid stimulation, while adequate protein provides the necessary substrate for enhanced MPS responses. This interaction works through both immediate and longterm mechanisms that reinforce each other. Meta-analyses demonstrate that protein supplementation augments resistance training-induced muscle mass gains by approximately 38% beyond training alone, with particularly strong effects in older adults [31]. The timing of this synergy is critical, occurring within the 24-48 hour post-exercise period when exerciseinduced mTOR priming and upregulated amino acid transporter expression maximize protein utilization [24].

Combined interventions also show superior functional benefits, with improvements in walking speed, chair rise time, and daily activities exceeding single interventions by over 20% [26]. The synergistic response depends on adequate dosing, with peak benefits seen at protein intakes of 1.4-1.6 g/kg/day paired with resistance training 2-3 times weekly [13].

Long-term application of combined interventions leads to adaptations beyond immediate anabolic responses. These include metabolic changes in muscle tissue, increased mitochondrial activity, and better insulin sensitivity, all of which support ongoing anabolic processes [6,20]. These findings highlight why comprehensive approaches are essential for addressing anabolic resistance. Clinicians who apply these synergistic principles can achieve better outcomes in preventing age-related muscle loss and maintaining functional capacity in older adults.

Creatine monohydrate

Creatine monohydrate represents the most evidence-based nutritional supplement for augmenting resistance training effects in older adults [32]. By increasing intramuscular phosphocreatine stores, creatine enhances high-intensity exercise capacity, allowing for greater training volumes and improved anabolic stimulation.

Vitamin D

Vitamin D supplementation provides benefits primarily in deficient individuals, correcting impaired muscle function rather than enhancing normal physiology [33]. Given the high prevalence of vitamin D deficiency in older adults, screening and supplementation represent important clinical considerations.

Adherence and compliance

Implementing evidence-based interventions for anabolic resistance faces significant practical challenges. Resistance training requires supervised instruction, progressive overload, and long-term commitment. Protein distribution requires substantial dietary modification and ongoing monitoring.

Individual variability

Substantial individual variation exists in anabolic resistance severity and intervention responsiveness. Factors including genetics, comorbidities, medication use, and baseline fitness levels all influence treatment outcomes [4]. Personalized approaches may be necessary for optimal results.

Healthcare system integration

Current healthcare systems often lack infrastructure for implementing comprehensive anabolic resistance interventions. Integration of exercise professionals, dietitians, and physicians is essential for successful outcomes but remains challenging in many settings.