The Combined Effect of Resistance Training and Vitamin D Supplement on Skeletal Muscle Hypertrophy: A Review

Muscle hypertrophy is probably one of the most important goals of resistance training (RT). Most people who do resistance exercises seek to increase muscle mass and then muscle performance, especially muscle strength (1). Resistance training is the most common training program for developing strength and stimulating muscle hypertrophy. This type of training improves muscle strength by increasing the cross-sectional area of the muscle caused by hypertrophy (2). Note that although creating hypertrophy is one of the main goals of bodybuilders and competitive athletes, muscle hypertrophy or maintaining muscle mass has therapeutic applications for non-athletes, too, because the reduction of muscle mass is closely associated with the occurrence of some underlying diseases (3). That is why RT, with the approach of muscle hypertrophy, is highly recommended to everyone, including athletes and non-athletes. To obtain the desired outcome in muscle adaptations caused by RT, the correct design of RT components, including intensity, duration, number of repetitions and sets, and duration of rest periods between sets, is of great importance (4). This is because these components can increase protein synthesis in skeletal muscles and cause hypertrophy by activating signaling pathways (5). Spiering et al. (2008) believe that muscle hypertrophy caused by RT results from a series of interrelated events, including muscle activation, signaling events caused by mechanical deformation of muscle fibers, hormones, and immune/inflammatory responses; another reason is protein synthesis that occurs because of increased transcription and translation; the last reason is fiber hypertrophy, which occurs consecutively (6). Lim et al. (2022) pointed out that skeletal muscle hypertrophy caused by RT depends on external variables such as the characteristics of the RT program, the designated diet, some supplements, and internal variables such as mechanical transport, ribosomes, gene expression, and satellite cell activity (7).

The available evidence indicates that the use of natural substances, nutritional diets, supplements, and vitamins can alter genes, proteins, and the overall physiological homeostasis of the body (8-10). There is substantiated evidence that vitamin D plays an important role in muscle hypertrophy. Bass et al. (2020) showed that the overexpression of the vitamin D receptor in skeletal muscles increased protein synthesis in myofibrils by raising anabolic signaling, especially the mammalian target of rapamycin (mTOR), which can be observed in the form of muscle hypertrophy (11). Closely consistent with this study, Bass et al. (2021) reported that the knockdown of the vitamin D receptor in vitro and in vivo increases skeletal muscle atrophy. Knockdown of the vitamin D receptor of the muscle caused atrophy by decreasing the total protein content, which reduced the myofiber level. Finally, vitamin D receptor (VDR) knockdown disrupted myogenesis (cell cycle, differentiation, and myotube formation) in vitro. These results show the importance of VDRs and signaling pathways in maintaining skeletal muscle protein (12). Nonetheless, evidence suggests that skeletal muscle is a direct target for vitamin D, and it is widely reported that vitamin D plays a determining role in regenerating human skeletal muscles. It is also a contributing factor in the maintenance of serum 25(OH)D, which might be necessary for increasing the repair processes and potentially facilitates the hypertrophy process (13). Since both RT and vitamin D affect muscle hypertrophy, this review aimed to summarize the existing knowledge about the simultaneous effect of RT and vitamin D on muscle hypertrophy in more depth and detail.

The process of selecting the most relevant articles involved meticulous cross-checking and elimination of cases that did not meet the criteria. The search results from different databases provided valuable insights. For instance, PubMed yielded 99 results, and after a thorough screening, 4 articles (13-16) were found to be eligible. ScienceDirect produced a substantial number of results, totaling 14,978. After a meticulous evaluation, 2 articles (17, 18) were deemed suitable. Google Scholar, with its extensive database, generated a significant number of results, amounting to 54,900. Following a rigorous assessment, 13 articles (19-31) were selected for inclusion.

The selected articles underwent an extensive review process to ensure their reliability and accuracy. Each article was read and scrutinized multiple times to thoroughly understand its content and implications. However, during this rigorous evaluation, it became evident that certain articles needed to be excluded for specific reasons. For example, 1 article focused primarily on the human heart rather than skeletal muscle (15), which did not align with the objectives of the present study. Additionally, several articles did not directly address the topic of hypertrophy (18, 19, 29). Furthermore, 1 article specifically investigated cardiac fibrosis (20), and another article solely discussed muscle morphology (21), topics that were not directly relevant to the research question.

After careful consideration of these factors and the elimination of articles that did not meet the criteria, the remaining studies were categorized into two distinct sections: Cross-sectional area and muscle performance. This division allowed for a comprehensive analysis of the selected studies and facilitated a deeper understanding of the impact of RT interventions and vitamin D on muscle hypertrophy.

Skeletal muscle is a dynamic tissue, and its size and cross-sectional area are affected by various factors. One of the most important external factors playing a role in skeletal muscle hypertrophy is mechanical load, the most common type of which is RT. Evidence shows that RT leads to hypertrophy by increasing muscle protein synthesis (33). The increase in protein synthesis caused by RT is the result of a series of cascade reactions activated by growth factors and anabolic hormones, especially androgens and IGF-1. In other words, RT activates the process of intracellular signaling for protein synthesis by increasing the release of anabolic hormones (34). Protein synthesis in skeletal muscles is regulated by a complex biological network of intracellular signaling mechanisms. The IGF-1/PI3K/Akt signaling pathway is one of the most effective molecular signaling systems for muscle growth and hypertrophy. This pathway plays a key role in the hypertrophic process because it carefully regulates the molecular basis of protein degradation and synthesis (35). In this pathway, Akt (protein kinase B) plays a central role in the signaling pathway of muscle hypertrophy and atrophy (36). Various stimuli, such as growth factors, cytokines, and hormones, activate AKT. The activation of AKT plays a critical role in muscle protein synthesis, followed by muscle hypertrophy so that, in skeletal muscle, the expression of the active isoform of Akt1 leads to myotube hypertrophy in vitro and in vivo (37). The activation of AKT activates the mTOR (mammalian target of rapamycin) pathway and the GSK3β (glycogen synthase kinase-3 beta) pathway, which play an important role in skeletal muscle hypertrophy (38). The activation of mTOR by AKT causes the phosphorylation and activation of p70S6K, as well as the phosphorylation and release of the inhibitory effect of PHAS-1/4E-BP1, while the activation of GSK-3β by AKT inhibits the inhibitory effect of eIF2B on protein synthesis (39). Baar and Esser (1999) found that RT activates p70S6K; moreover, the close relationship between p70S6K and the increase in muscle mass indicates that mTOR-induced p70 (S6k) phosphorylation may be a good marker for muscle hypertrophy and could be involved in muscle growth. The skeleton caused by the biomechanics of resistance exercises may also play a role (40). It has also been reported that the induction of rapamycin as an mTOR inhibitor inhibits muscle hypertrophy caused by RT (41). Besides, RT inhibits eIF2B immediately and 3 hours after exercise by activating GSK-3β AKT and stimulates the process of protein synthesis in skeletal muscles (42). Overall, RT leads to hypertrophy by increasing muscle protein synthesis, which is regulated by a complex network of intracellular signaling mechanisms mentioned above. These complex interplay of signaling pathways and protein synthesis mechanisms contribute to the hypertrophic impacts of RT on skeletal muscle.

Another mechanism by which exercise can affect skeletal muscle hypertrophy is increasing insulin sensitivity and improving the insulin signaling pathway in skeletal muscle. It has been reported that RT in prediabetic obese people increases the expression of phosphorylated Akt2, mTOR, and muscle hypertrophy (43). Consitt et al. (2013) also reported a rise in Akt2 protein expression in young and old people after RT (44). Holton et al. (2004) showed that 6 weeks of RT with one leg increased the expression of insulin receptor protein and Akt in the active leg compared to the inactive leg, indicating an adaptation in the insulin signaling pathway in skeletal muscles following RT (45). In general, RT improves the insulin signaling pathway in skeletal muscles, and in this way, it prevents muscle protein destruction and muscle atrophy. Therefore, resistance exercises can exert their effects on hypertrophy or muscle mass maintenance.

Skeletal muscles are the direct target of vitamin D. Vitamin D has been implicated in human skeletal muscle regeneration. Evidence suggests that maintaining serum 25(OH) D may be beneficial for enhancing repair processes and potentially facilitating hypertrophy (13). The presence of vitamin D receptors in skeletal muscles shows that this vitamin is essential for muscle function and metabolism (46). Low levels of vitamin D (VDR) are associated with skeletal muscle fiber atrophy, muscle pain, weakness, and increased risk of sarcopenia (47). Vitamin D affects muscle strength, muscle function, and muscle metabolism due to changes in protein synthesis, myogenesis, mitochondrial activity, muscle regeneration, and glucose metabolism in muscles (13). Vitamin D, like RT, can stimulate IGF-1 and thereby exert its anabolic effect on skeletal muscle tissues (48). Tanaka et al. demonstrated that myoblasts rely on signals transmitted through VDR in order to undergo differentiation into myocytes. They also highlighted the significance of VDR expression in skeletal muscles for the preservation of muscle volume in older individuals (49). In addition, the serum 25(OH) D3 level and VDR expression in muscle cells decrease with age, which is consistent with the decline in muscle mass and the incidence of sarcopenia caused by aging (50). Significant muscle atrophy, decreased muscle strength, reduced muscle fiber size, lower bone density, and disturbance in the regulation of myogenic regulatory factors were observed in mice in which VDR has been knocked out and in mice that received a diet with a low vitamin D content, compared to the control group (51, 52). It has been reported that vitamin D increases the phosphorylation of Akt and GSK3β and thus improves insulin signaling by regulating the insulin receptor, which itself can activate the protein synthesis process in skeletal muscle (53). Since vitamin D has an antioxidant effect, evidence shows that in conditions of vitamin D deficiency, the enzyme antioxidant defense is reduced and increases proteolysis in skeletal muscle tissue (54). The results of numerous studies show that vitamin D improves protein synthesis and hypertrophy in C2C12 muscle cells (55). Furthermore, vitamin D influences the production and secretion of insulin, affects the survival of β-cells of the pancreas, and increases the secretion of insulin from β-cells of the pancreas (56, 57). Vitamin D not only affects the production and secretion of insulin from pancreatic beta cells but also influences the response of peripheral tissues to insulin or, in other words, the insulin signaling pathway in skeletal muscles (53). It has been reported that vitamin D raises insulin sensitivity by binding 1,25(OH) 2D3 to VDR, inducing insulin-receptor substrate expression in target tissues and activating PPARδ (58). In confirmation of the effect of vitamin D on the insulin signaling pathway in peripheral tissues, evidence suggests that in conditions of vitamin D deficiency, insulin resistance is observed due to the down-expression of the insulin receptor (59). Besides, supplementation with 1,25(OH)2D3 improves glucose metabolism by regulating the SIRT1/IRS1/GLUT4 signaling cascade and glucose uptake in C2C12 myotubes (60). Another mechanism for the effect of vitamin D on improving insulin function and reducing insulin resistance in skeletal muscle is its impact on the regulation of intracellular calcium ions, which causes the transfer of GLUT4 from the depth of muscle cells to the cell membrane and glucose absorption; as a result, the rate of glucose uptake by skeletal muscle increases (60). Since insulin signaling through the activation of the PI3k/AKT/mTOR signaling pathway causes the maintenance and development of muscle hypertrophy, evidence shows that the lack of insulin signaling in skeletal muscles is associated with a decline in muscle mass and muscle function, which can justify the reduction of intramuscular protein synthesis (61). Therefore, the reduction of vitamin D can harm the process of muscle hypertrophy through the disruption of the insulin signaling pathway. Both RT and vitamin D can activate the signaling pathways in muscle cells that increase protein synthesis, develop hypertrophy, and promote muscle function, particularly in areas such as muscle strength; thus, the simultaneous use of these two interventions can be a suitable solution for strengthening the impact of each on muscle hypertrophy, preventing muscle atrophy, and enhancing sports performance in athletes. Since both of these interventions can inhibit oxidative stress, inflammation, and apoptosis, in addition to the stimulating signaling pathway of muscle hypertrophy, they can be used as an efficient method for people suffering from muscle-wasting diseases such as diabetes and cancer and in the elderly. They can also be used to mitigate the complications of the aforementioned diseases. In conclusion, vitamin D plays a crucial role in skeletal muscle function, regeneration, and metabolism. Low levels of vitamin D are associated with muscle atrophy, weakness, and a higher risk of sarcopenia. Vitamin D affects muscle strength, function, and metabolism by influencing protein synthesis, myogenesis, mitochondrial activity, muscle regeneration, and glucose metabolism. It can stimulate IGF-1 and exert an anabolic effect on skeletal muscle tissue. The presence of vitamin D receptors in skeletal muscle highlights its importance for muscle volume maintenance.

Generally, Studies have shown that vitamin D deficiency leads to muscle atrophy, decreased muscle strength, and disturbances in myogenic regulatory factors. Vitamin D improves insulin signaling, increases insulin sensitivity, and affects glucose metabolism. It also regulates intracellular calcium ions, which enhance glucose uptake by skeletal muscle. The activation of the insulin signaling pathway is crucial for muscle hypertrophy and protein synthesis. Both RT and vitamin D can activate signaling pathways that increase protein synthesis and promote muscle hypertrophy. Combining these interventions may enhance their effects on muscle hypertrophy, prevent muscle atrophy, and enhance sports performance. Additionally, RT and vitamin D can reduce oxidative stress, inflammation, and apoptosis, which makes them potential therapeutic strategies for muscle-wasting conditions such as diabetes, cancer, and aging-related muscle loss.