Sarcopenia, a geriatric syndrome characterized by progressive loss of skeletal muscle mass, diminished muscle strength, or impaired physical performance, has emerged as a significant public health concern in aging populations (1). This condition is strongly associated with functional decline and increased disability risk, substantially impacting healthcare systems worldwide. Epidemiological studies indicate that the prevalence of sarcopenia among older Asian populations ranges from 6.8 to 25.7%, underscoring its clinical significance (2). The pathogenesis of sarcopenia is multifactorial, involving complex interactions between age-related physiological changes and molecular mechanisms (3, 4). Key contributing factors include the downregulation of anabolic hormones (insulin, sex steroids, and growth hormone), increased muscle fiber apoptosis, and elevated circulating levels of pro-inflammatory cytokines (5–8). Given the pivotal role of chronic inflammation and anabolic-catabolic imbalance in sarcopenia progression, therapeutic strategies targeting these pathways, particularly through immunonutrition interventions (e.g., arginine and omega-3 fatty acids from fish oil), have shown promising potential in clinical research.

Immunonutrition supplementation (particularly with L-arginine (L-Arg), a conditionally essential amino acid with significant anabolic properties in muscle metabolism) has demonstrated efficacy in improving muscle mass and function. The therapeutic mechanisms of L-Arg are primarily mediated through two pathways: (1) its metabolite, creatine, activates the PI3K-Akt/PKB-mTOR signaling pathway to enhance muscle protein synthesis (9), and (2) its conversion to nitric oxide (NO) stimulates satellite cell proliferation and increases satellite cell numbers in muscle fibers, thereby restoring skeletal muscle function (10). Similarly, fish oil, a rich source of n-3 polyunsaturated fatty acids (PUFAs), has shown significant potential as a nutritional intervention for sarcopenia. The n-3 PUFAs exert their effects through multiple mechanisms: reducing plasma levels of pro-inflammatory cytokines (e.g., IL-6 and TNF-α), increasing anti-inflammatory factors (e.g., IL-10 and TGF-β) (11), and stimulating the mTOR signaling pathway to enhance muscle protein synthesis while inhibiting protein degradation (12–14).

Emerging evidence suggests a synergistic relationship between fish oil and arginine, with studies demonstrating that fish oil can enhance arginine bioavailability and activity (15, 16). Extensive clinical research has established the beneficial effects of arginine and fish oil supplementation in various pathological conditions, including cancer, gastrointestinal disorders, renal diseases, critical illness, and cerebrovascular accidents (17). Furthermore, numerous clinical trials and meta-analyses have consistently demonstrated the efficacy of combined L-arginine and fish oil formulations used in surgical populations (18–21).

Notably, plasma levels of arginine and n-3 PUFAs decrease with age (22, 23), which provides a strong rationale for supplementation in older adults. While many studies have explored the effects of arginine or fish oil supplements individually in patients with sarcopenia, there is a significant gap in research on their combined therapeutic potential.

This study aims to assess the effectiveness of combining arginine and fish oil supplementation in improving muscle strength, physical function, and body composition in older adults with sarcopenia, and examine the impact of this intervention on inflammatory markers, nutritional status, and other relevant health factors. We hypothesize that the combined supplementation will significantly improve muscle function and quality of life in patients with sarcopenia through two key mechanisms: (1) modulating inflammatory pathways and (2) boosting muscle protein synthesis. To our knowledge, this is the first to comprehensively explore the combined effects of arginine and fish oil supplementation in sarcopenia patients, which could help develop new multi-target strategies for managing this debilitating condition.

Supplementary Figure 1 shows the trial enrollment based on the CONSORT flow diagram. About 464 patients were screened in the present study; 113 were diagnosed with sarcopenia; 71 refused to participate because of remote residence, inconvenient transportation, refusal of venous blood collection, and private time schedule conflict with trial. Finally, a total of 42 eligible participants were recruited and included in the study. The recruitment of participants commenced on December 2023, and extended with follow-ups until the end of October 2024. Throughout the study, 13 participants dropped out due to several reasons such as non-compliance with supplements or three-quarters of the supplements were not consumed, refusal to provide blood sample at the end of the study, change of place of residence. Consequently, the final number included in the analyses was 29 participants.

In this randomized controlled clinical trial in older adults with sarcopenia, we investigated for the first time the effects of arginine and fish oil supplementation on sarcopenia. Our findings demonstrate that arginine and fish oil significantly improved handgrip strength and gait speed in older adults with sarcopenia. Additionally, the Arg + fish oil group showed significant improvements in PASE and frailty status compared to the placebo group. Notably, at the end of the trial, TNF-α and IL-6 levels were significantly lower in the Arg + fish oil group than in the placebo group. These findings highlight the promising effects of arginine and fish oil supplementation in managing sarcopenia in older adults.

After 12 weeks of intervention, significant improvements in handgrip strength and gait speed were observed in the Arg + fish oil group compared to the placebo group. These findings suggest that the Arg and fish oil intervention may exert beneficial effects on muscle-related outcomes in older adults with sarcopenia. Our results align with previous studies demonstrating the positive impact of arginine on muscle health (31). Patricia et al. (32) reported that arginine-containing nutritional supplements enhance muscle strength and mass. Similarly, Børsheim et al. (33). found that supplementation with essential amino acids (EAAs) plus arginine improved lean body mass, muscle strength, and physical performance in elderly individuals with glucose intolerance. The underlying mechanisms may involve arginine’s metabolite, creatine, which activates the PI3K-Akt/PKB-mTOR pathway to promote muscle protein synthesis (34). Additionally, arginine enhances nitric oxide (NO) production, stimulates satellite cell proliferation, and restores skeletal muscle function (10).

Fish oil, rich in n-3 polyunsaturated fatty acids (PUFAs), particularly eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), has also been linked to musculoskeletal health. Randomized controlled trials have demonstrated that n-3 PUFA supplementation can enhance muscle protein synthesis in older adults (35–37). Gordon et al. (38). supplemented older adults with 1.86 g EPA and 1.5 g DHA daily for 6 months and observed significant improvements in thigh muscle volume, handgrip strength, and one-repetition maximum (1-RM) muscle strength. Previous studies have also found that fish oil and curcumin can improve muscle mass, gait speed, and prevent age-related declines in muscle health in rats (39). These benefits may be attributed to n-3 PUFAs’ ability to activate the mTORC-1 signaling pathway, enhance protein translation, and inhibit muscle protein degradation, thereby improving muscle protein synthesis efficiency (40). Da Pan et al. found that the fish oil-derived ω-3 PUFA and wheat oligopeptide may enhance skeletal muscle strength by targeting the up-regulation of Tnnt1, Myosin, Fbln5 and Itgav (41).

However, this study found that there was no statistically significant change in SMI, suggesting that the improvement in sarcopenia may not be achieved through a direct increase in muscle mass. This seemingly contradictory result may stem from the fact that the 12-week intervention period may have focused more on inducing neuromuscular adaptations and metabolic improvements (42–44). This can enhance muscle strength and physical function, whereas structural muscle hypertrophy generally requires longer-term cumulative stimulation (45). Therefore, the synergistic effect of arginine and fish oil may have optimized the intrinsic functional properties of muscle without substantially altering its physical cross-sectional area in the short term.

In addition to muscle-related outcomes, our study also assessed the effects of arginine + fish oil supplementation on physical activity (PASE) and frailty in older adults. Frailty, a complex syndrome marked by decreased physiological reserve and multisystem dysfunction, is a strong predictor of negative outcomes like hospitalization and mortality (46). The improvements observed in PASE scores and frailty may be linked to the recovery of gait speed, as shown in previous studies (47). For example, Zhu et al. (48). found that nutritional supplements containing protein, β-hydroxy β-methylbutyrate, vitamin D, and omega-3 fatty acids improved physical activity scores in older adults. Similarly, Hsieh et al. (49). reported that combined home exercise and nutritional interventions had a positive effect on frailty and physical function in pre-frailty or frailty older adults.

However, there is currently less research on the effects of arginine and fish oil supplementation on skeletal muscle. The exact mechanisms behind the effects of arginine + fish oil supplementation on skeletal muscle remain unclear. Both arginine and fish oil are considered immunonutrients, with clinical studies showing their effectiveness in reducing inflammation and improving health outcomes in various populations. A prospective randomized study by Braga et al. found that (50) combinations of arginine and omega-3 fatty acids significantly reduced postoperative infections and hospital stays in surgical patients. Alexander further highlighted the anti-inflammatory and infection-preventing effects of combined arginine and fish oil supplementation (51). These findings suggest that the synergistic effects of arginine + fish oil may help reduce the harmful impact of inflammation on sarcopenia.

To elucidate the mechanisms, it is important to highlight the critical role of inflammation in the pathophysiology of sarcopenia. TNF-α and IL-6, as the main pro-inflammatory cytokines, play a key role in the process of muscle atrophy (52). TNF-α activates the NF-κB pathway, which not only mediates pro-inflammatory responses but also perpetuates a vicious cycle of cytokine overexpression (53). The TNF-α/NF-κB pro-inflammatory signaling pathway plays a dominant role in the mechanism of sarcopenia. This TNF-α also inhibits the Akt/mTOR signaling axis, leading to impaired protein synthesis and increased proteolysis in skeletal muscle (54, 55). Elevated levels of TNF-α or IL-6 can inhibit the expression of hormones such as growth hormone and IGF-1 (56, 57). This leads to impaired muscle production and increased proteolysis, which together promote structural destruction and functional decline of muscles (58). Studies have shown that arginine can increase the production of NO and inhibit the activation of the NF-κB signaling pathway, thereby effectively reducing the expression of TNF-α and IL-6 (59). In this study, we also demonstrated that arginine and fish oil supplementation significantly reduced TNF-α and IL-6 levels in patients with sarcopenia. This is consistent with previous studies in which Irandoust (60) et al. found a significant decrease in plasma IL-6 following arginine supplementation in trained men. Similarly, n-3 PUFAs in fish oil have been shown to reduce pro-inflammatory cytokines while increasing anti-inflammatory factors like IL-10 and TGF-β (11). Multiple studies have also demonstrated the effects of arginine and fish oil in reducing IL-6 and TNF-α (61–63). In addition, both arginine and fish oil can stimulate the mTOR signaling pathway and increase the rate of muscle protein synthesis, thereby counteracting the inhibitory effect of inflammation on mTOR. Therefore, we speculate that arginine and fish oil can improve the symptoms of sarcopenia and delay the progression by reducing IL-6 and TNF-α.

However, our study did not observe significant improvements in muscle mass, possibly due to the complex pathogenesis of sarcopenia (64–66). Factors such as malnutrition, physical inactivity, aging, chronic comorbidities, and hormonal changes contribute to inflammation and malabsorption, all of which can exacerbate muscle loss and dysfunction. Aging is known to reduce the sensitivity of skeletal muscle protein synthesis to dietary amino acids, and older adults need more dietary proteins and essential amino acids to stimulate the same rate of synthesis as younger adults (67). Therefore, optimizing protein intake or supplementing with amino acids for specific functions has been used as a basis for the prevention and management of muscle loss in patients with sarcopenia. In our study, completers exhibited stable muscle function and strength parameters during the continuation of the trial. This suggests that while supplementing with function-specific amino acids does not improve muscle mass, it may reduce the natural progression of muscle dysfunction and muscle loss.

The advantages of our research are as follows. First, our results were obtained in older adults with sarcopenia, as few studies have focused specifically on the effects of arginine and fish oil in this population. Second, we observed the additive effects of RET-based arginine and fish oil on sarcopenia. However, our study has some limitations. First, the control group did not receive placebo capsules that were visually identical to the fish oil capsules. Although all assessors performing measurements for the objective outcome measures were blinded to group allocation, and all primary outcomes were objective indicators, the trial design did not achieve perfect double-blinding at the participant level, theoretically introducing the potential for participant expectation bias. Second, the mechanistic discussion above is primarily based on inferences drawn from existing literature, as this study did not directly measure relevant molecular biomarkers. Therefore, the conclusions presented should be regarded as hypotheses regarding potential mechanisms underlying the clinical outcomes. Third, the small sample size may have limited statistical power, making it difficult to detect significant differences in certain parameters. Future studies should involve larger cohorts to enhance the robustness and generalizability of the findings. Fourth, the 12-week intervention period cannot answer the potential effect of interventions on a long-term perspective. Fifth, the absence of separate component intervention groups limits our ability to dissect the individual contributions of each constituent to the combined effect. Future research should utilize placebos that are perfectly matched in dosage form, quantity, and method of administration, implement a multi-group design, extend the intervention period, and incorporate related mechanistic studies to validate and expand upon the findings of this study.

In conclusion, combined supplementation with arginine and fish oil improves muscle strength, gait speed, and inflammatory status in older adults with sarcopenia. These findings suggest that Arg + fish oil may serve as a promising therapeutic strategy for sarcopenia management. However, further large-scale, multicenter studies are needed to confirm these results and explore the long-term benefits of this intervention.