Background
The transition into menopause is associated with a decrease in the production and circulation of estrogen, which is a naturally occurring antioxidant in the body. This reduction leads to oxidative stress in various body tissues, including skeletal muscle, resulting in negative effects on muscle protein synthesis and mitochondrial dysfunction, these factors contribute to the loss of muscle mass and strength [
2]. Sarcopenia is a chronic condition characterized by a progressive and degenerative loss of muscle mass, strength, and physical functional performance [
1]. Menopause is a natural process that occurs in all women during their final menstrual period, typically between the ages of 40 and 60, and is characterized by a decrease in estrogen production and hormonal status [
3]. On average, women experience a loss of approximately 0.6% of muscle mass in their 30s, and the rate of decline in muscle mass and muscle strength is even higher after the age of 50 [
4]. Orprayoon et al. estimated the prevalence of pre-sarcopenia to be around 12% among menopausal women under the age of 65 [
5]. Moreover, the presence of sarcopenia is associated with a negative impact on mental health, leading to a loss of self-confidence and social isolation, ultimately reducing the overall quality of life [
6,
7]. Sarcopenia indirectly contributes to an increased risk of disability, reduced independence, and being bedridden, all of which are public health concerns and place a financial burden on healthcare and social care services [
8]. Therefore, implementing health promotion and preventive interventions for sarcopenia is essential for public health.
The Asian and European Working Group for Sarcopenia supports non-pharmacological interventions such as exercise and nutrition as fundamental strategies. These interventions include exercise training, vitamin D supplementation, and protein supplementation to enhance motor neuron activity, maintain muscle elasticity, preserve muscle mass, rebuild muscle strength, and attenuate the progression of sarcopenia [
1,
9].
Exercise training can enhance muscle activation, release inflammatory and hormonal substances, and activate skeletal muscle satellite cells [
10,
11]. These mechanisms promote the muscle regeneration process, muscle protein synthesis, and help prevent muscle hypotrophy or muscle fiber hypoplasia [
10,
11]. As a result, exercise training increases the cross-sectional area of muscles, improves muscle strength, and enhances physical functional performance [
10,
11]. Vitamin D supplementation plays a crucial role in delaying skeletal muscle aging and increasing muscle protein synthesis. Vitamin D deficiency is associated with a risk of decreased muscle mass and strength, gait deficits, and an increased risk of falling [
12]. Dietary protein intake is essential for enhancing muscle strength, muscle protein synthesis, and the growth factors involved in regulating skeletal muscle function and bone remodeling [
13]. Therefore, non-pharmacological interventions are ideal goals to prevent or improve sarcopenia in menopausal women and should not be ignored.
Several systematic reviews and meta-analyses conducted between 2000 and 2023 have investigated non-pharmacological interventions to delay or prevent sarcopenia in post-menopausal women. Rubio-Arias et al. conducted a systematic review and meta-analysis based on data from five randomized controlled trials (RCTs) involving post-menopausal women aged 55 to 75 [
14]. The studies included 8 to 32 weeks of whole-body vibration training and revealed no significant effect on lean body mass. Marín-Cascales et al. conducted a systematic review and meta-analysis based on data from 15 studies [
15]. The analysis focused on post-menopausal women aged 65 and older who underwent 12 to 48 weeks of multi-component exercise training. The findings indicated beneficial effects in increasing or preventing the loss of muscle mass, although high heterogeneity among the studies was noted. Thomas et al. conducted a systematic review and meta-analysis based on data from 26 studies that included post-menopausal women aged 50 to 80 [
16]. The analysis focused on the effects of 16 weeks of resistance exercise training on lean body mass, and it showed enhancements in lean body mass. Tabrizi et al. reported a meta-analysis based on 12 studies involving post-menopausal women aged 45 to 86 [
17]. The studies investigated the effects of daily intake ranging from 10 international units (IU) to 50,000 IU of vitamin D supplementation for 12 to 96 weeks. The findings showed no significant effect on the timed up and go test, which assesses mobility. Abshirini et al. conducted a meta-analysis based on 29 RCTs involving post-menopausal women aged 65 and older. The studies examined the effects of daily or weekly intake of vitamin D2 or vitamin D3 supplementation ranging from 1,000 IU to 100,000 IU for 1 to 60 months [
18]. The analysis showed no beneficial effect on handgrip strength and the timed up and go test. Zhang et al. conducted a meta-analysis based on 13 RCTs involving post-menopausal women aged 45 to 99 [
19]. The studies evaluated the effects of daily or weekly intake of vitamin D2 or vitamin D3 supplementation ranging from 1,000 IU to 100,000 IU for 3 to 60 months. The findings showed a beneficial effect on handgrip strength but no significant effect on the timed up and go test.
These systematic reviews and meta-analyses provide insights into the effectiveness of non-pharmacological interventions and vitamin D supplementation in post-menopausal women with sarcopenia. Previous meta-analyses on exercise regimes for postmenopausal women have had methodological issues, resulting in uncertain protocols. Both vitamin D and protein seem to offer benefits for muscle health in postmenopausal older women, yet the effects of menopause on muscle changes have not been comprehensively investigated. Middle-aged women, undergoing menopausal transition, have exhibited variations in muscle morphology. Nevertheless, no prior review has thoroughly examined non-pharmacological interventions for middle-aged women. Thus, this systematic review and meta-analysis aims to analyze the influence of exercise, vitamin D, and protein on sarcopenia, specifically in menopausal women aged 40 to 60 needs to be explored and seems warranted. The study will employ a systematic evaluation of data from selected RCTs, by addressing this knowledge gap, the study aims to provide valuable information that can be applied in clinical contexts and contribute to achieving public health goals.
Discussion
This study is pioneering in its determination of sarcopenia indicators based on the guidelines from both the Asian and European working groups [
1,
9]. Additionally, the study offers new insights by conducting a comprehensive meta-analysis to evaluate the effects of non-pharmacological interventions on preventing sarcopenia among menopausal women aged 40 to 60.
The results of our meta-analysis revealed a small effect of exercise training on lean body mass, which is consistent with the findings of a previous review conducted by Marín-Cascales et al. [
15] and Benton et al.‘s study [
58]. However, it is important to note that our review and Benton et al.‘s study [
58] focused on a narrower age group (40 to 60 years old) compared to the broader age range (48 to 95 years old) examined in Marín-Cascales et al.‘s review. Despite this difference, both studies suggest that exercise training has a beneficial impact on lean body mass across different age groups. The underlying mechanisms responsible for the observed effects of exercise training on lean body mass are multifactorial. One proposed mechanism involves the reduction of inflammatory-related cytokines and the induction of growth hormone responses [
59]. These factors contribute to muscle repair and regeneration, as well as improvements in mitochondrial function within muscle cells, ultimately leading to the formation of new skeletal muscle tissue. These findings highlight the importance of exercise training in mitigating the reduction in muscle cross-sectional area and the loss of muscle mass associated with aging [
11]. Overall, our meta-analysis supports the notion that exercise training is beneficial for increasing lean body mass, particularly in the age range of 40 to 60 years old. Further research is needed to explore the specific exercise modalities, durations, and intensities that yield the most substantial gains in lean body mass in this population. Additionally, future studies should investigate the long-term effects of exercise training on lean body mass and its implications for overall health and functional outcomes.
A subgroup analysis of exercise type revealed that resistance exercise had a small positive effect on lean body mass. This finding aligns with a previous systematic review and meta-analysis conducted by Thomas et al. [
16]. Their study, which included 26 studies, demonstrated a mean increase of 0.43 kg in lean body mass after 16 weeks of resistance training in postmenopausal women aged between 50 and 80 years. Furthermore, Benton et al.‘s non-randomized controlled study revealed an increase of 1.1 ± 0.3 kg in lean body mass after completing 8 weeks of progressive resistance training among untrained middle-aged women aged 40 to 55 [
58]. Resistance training has been shown to increase the secretion of growth hormone, which stimulates skeletal muscle protein synthesis and helps prevent muscle mass loss [
60]. In contrast, our subgroup analysis indicated that aerobic exercise alone did not have a significant positive effect on lean body mass, which is consistent with a prior study by Willis et al. [
61]. In their RCTs, they observed a decrease in lean body mass (-0.10 kg) following 8 months of aerobic exercise. In contrast, resistance exercise (+ 1.09 kg) and multi-component exercise (+ 0.81 kg) resulted in positive effects on lean body mass over the same duration. The reason for the negative effect of aerobic exercise on lean body mass is attributed to the fact that different types of exercise elicit distinct physiological responses in muscle cell and tissue contractile function [
62]. While aerobic exercise does stimulate muscle protein synthesis, its primary aim is to reduce skeletal muscle fat infiltration and improve overall muscular system function [
62].
Regarding other types of exercise such as whole-body vibrations and multi-component exercises, our study results showed no significant positive effect on lean body mass (including two RCTs on whole-body vibration training and two RCTs on multi-component exercises). This is consistent with a systemic review and meta-analysis conducted by Rubio-Arias et al. which included five studies on whole-body vibration training in postmenopausal women aged 55 to 75 years and found no significant effect on lean body mass [
14]. Additionally, a literature review by Bao et al. involving 22 studies on various exercise types (resistance exercise, aerobic exercise, strengthening exercise, multi-component exercise, and vibration exercise) in older adults aged 60 to 86 years did not find a significant difference in overall skeletal muscle mass [
63]. It is important to note that the study by Bao et al. focused more specifically on aerobic exercise intervention, resulting in heterogeneity among the included studies due to the varied exercise types and regimens employed [
63]. Considering the limitations of our study, which only included 4 RCTs, further research is warranted to validate these findings. Future studies should explore the effects of different exercise modalities and regimens on lean body mass in a more diverse and larger sample population (middle-aged women). Additionally, longer-term interventions and investigations into the underlying mechanisms of exercise-induced changes in lean body mass would provide valuable insights for optimizing exercise interventions in various age groups, especially in middle-aged women.
Another subgroup analysis, conducted by Vikberg et al., focused on the duration of exercise [
64]. In their randomized controlled trial, they investigated pre-sarcopenic older adults with an average age of 70 years who underwent 10 weeks of resistance training. The study demonstrated a significant improvement in lean body mass, with an increase of 1.1 kg. Similarly, Liao et al. conducted RCTs involving sarcopenic obese women with an average age of 67.3 years [
65]. These participants underwent 12 weeks of resistance band exercise and experienced an increase of 0.79 kg in leg lean mass and 2.19 kg in handgrip strength.
In line with these findings, a meta-analysis conducted by Thomas et al. which included 26 studies involving postmenopausal women aged 50 to 80 years, revealed that 16 weeks of resistance training resulted in an increase in lean body mass [
16], similar to the results of our study.
In our study, we performed a subgroup analysis of exercise duration, categorizing it into two groups: ≤ 12 weeks (ranging from 6 to 12 weeks) and > 12 weeks (ranging from 16 to 52 weeks). We observed small to moderate significant increases in lean body mass in menopausal women aged 40 to 60 years. These findings are consistent with a review study by Nascimento et al. which suggested that the sarcopenic population should engage in exercise training for at least 4 weeks or more [
66]. Furthermore, Benton et al. recommended that untrained middle-aged women aged 40 to 55, undertaken 8 weeks of resistance training, this regimen is proposed to stimulate muscle protein synthesis, enhance muscle activation, prevent muscle loss, diminish intramuscular fat infiltration, and bolster muscle strength and morphology [
58]. Regarding the frequency of exercise training, our subgroup analysis based on two frequencies, namely 20 to 40 min/three sessions per week and 60 to 90 min/three sessions per week, revealed moderate and small effects on lean body mass, respectively. The validated training frequencies were derived from the research studies conducted by Huovinen et al. and Fisher et al. [
67,
68]. Huovinen et al. conducted a study involving older women with an average age of 71.9 years [
67]. The participants underwent 60 min of exercise training, three sessions per week, for 16 weeks. The study divided exercise frequency and volume into three groups: group 1 performed 20 to 35 min as a low dose of resistance training (RT), group 2 performed 40 min as a moderate dose of RT, and group 3 performed 60 min as a high dose of RT. Each group participated in three sessions per week. All three groups demonstrated significant improvements in muscle mass and muscle strength in response to their respective RT frequencies and volumes [
68]. Fisher et al. supported the idea of a minimal dose of exercise for muscle mass and strength improvement, with a focus on major muscle groups [
68]. The recommendation is to choose multi-joint exercises that prioritize major muscle groups. Multi-joint exercises include chest press, leg press, and seated row, primarily targeting major muscle groups. These studies emphasize the importance of choosing an appropriate frequency and volume of resistance training for improving muscle mass and strength. Both lower and higher frequencies demonstrated positive responses in terms of muscle mass improvement. However, it is worth noting that the minimal dose of resistance training with a focus on major muscle groups has shown promising results for muscle mass and strength gains.
Handgrip strength is considered the most objective and widely used measure of muscle strength in the upper limb [
69]. The meta-analysis conducted in this study has revealed a large effect of exercise training on handgrip strength. In support of our findings, a systematic review by Wang et al. analyzed 23 studies involving older adults with sarcopenia [
70]. The studies included multi-component exercise interventions ranging from 9 to 24 weeks in duration. The analysis demonstrated a significant improvement of 2.38 kg in handgrip strength. Moreover, a randomized controlled trial conducted by Toselli et al., participants with an average age of 56.2 years underwent a 24-week program with three sessions per week, has revealed a significant increase of 6.1 kg in handgrip strength [
72]. These results align with the overall findings of our study. The consistent results between our meta-analysis and Wang et al.‘s systematic review provide further support for the positive effects of exercise training on handgrip strength in various populations, including sarcopenia older adults. Handgrip strength serves as an important indicator of upper limb muscle strength and can be effectively improved through appropriate exercise interventions.
Knee extension strength is another important muscle measurement frequently used in research, and it was included in our analysis. The results of our study revealed a moderate effect of exercise training on knee extension strength, which is consistent with the findings of a previous review conducted by Lu et al. [
71]. Lu et al. conducted a meta-analysis that included 26 studies involving older adults with an average age ranging from 60.6 to 89.5 years [
71]. These studies investigated the effects of various types of training, including resistance, endurance, aerobic exercises, and whole-body vibration training, on knee extension strength. The meta-analysis showed significant improvements in knee extension strength with resistance training exercise (1.36 Nm/kg), whole-body vibration training (0.62 Nm/kg), and mixed training (0.62 Nm/kg). Furthermore, Borba-Pinheiro et al. conducted a randomized controlled trial (RCT), participants with an average age of 58.8 years, undergoing 52 weeks of resistance training; there was a significant increase of 15.28 kg in knee extension strength [
72]. The improvements in knee extension strength observed in response to exercise training can be attributed to the activation of muscle nerve fibers, increased size of muscle fibers, and enhancement of muscle regeneration. Increased muscle strength can have positive effects on static balance and reduce the risk of falls [
73].
This study revealed a low effect of vitamin D supplementation on handgrip strength. This finding is consistent with a meta-analysis conducted by Zhang et al. which included 13 studies involving postmenopausal women over 60 years old [
19]. In that meta-analysis, participants were supplemented with varying doses of vitamin D2 (ranging from 2800 IU to 210,000 IU) or vitamin D3 (ranging from 7,000 IU to 100,000 IU) for a duration of 3 to 24 months. The results showed a beneficial improvement of 0.876 kg in handgrip strength. This finding aligns with the results of our study, indicating that vitamin D plays a crucial role in increasing calcium absorption and enhancing muscle tissue repair, leading to improvements in muscle strength [
11]. Our study included 4 RCTs that examined different forms and doses of vitamin D supplementation. The interventions ranged from weekly doses of 7,000 IU to 50,000 IU of vitamin D, weekly doses of 40,000 IU of vitamin D2, to a daily dose of 0.5 µg of vitamin D3, administered over a period of 8 to 12 weeks. The heterogeneity in the interventions, including variations in the form and dose of vitamin D, highlights the need for further research to confirm the evidence regarding the effects of vitamin D supplementation on handgrip strength among middle-aged women.
This study found no significant effect of vitamin D supplementation on knee extension strength, which is consistent with the findings of a previous review conducted by Bislev et al. [
74]. The systematic review and meta-analysis by Bislev et al. examined older adults with an average age of 65 years who were supplemented with either 10,000 IU of vitamin D2 daily, 400 IU daily, or a single dose of 300,000 IU of vitamin D3 for a duration of 60 months [
74]. The results of that review indicated no significant improvements in knee extension strength. Similarly, our study included 3 eligible studies in which participants were supplemented with daily doses of 7,000 IU to 50,000 IU of vitamin D or a weekly dose of 5,000 IU of vitamin D2 for a duration of 8 to 12 weeks. Due to the variations in the forms, dosages, and duration of vitamin D supplementation, the exact beneficial effect of vitamin D on knee extension strength could not be determined. Further trials are needed to determine the optimal dose, form, and duration of vitamin D supplementation for achieving a beneficial effect on knee extension strength among middle-aged women.
This study found no significant improvements in lean body mass with protein supplementation, which is consistent with the findings of a meta-analysis conducted by Guo et al. [
75]. Guo et al.‘s meta-analysis included 17 RCTs involving individuals with an age range of 68 to 82 years [
75]. In those studies, daily oral supplementation of 0.857 to 7.5 g of leucine, a branched-chain amino acid found in various food sources, was given for durations of 4 to 48 weeks. The meta-analysis showed no beneficial effect of leucine supplementation on lean body mass, which aligns with the results of our study. Leucine is known for its ability to stimulate protein synthesis and promote muscle building and repair [
76]. Borack and Volpi have recommended a daily leucine intake of 55 mg/kg to enhance muscle protein synthesis [
77]. However, our study included 3 eligible studies that investigated isoflavones with variations in daily oral protein supplementation ranging from 40 mg to 160 mg. This differs from the previous meta-analysis in terms of the specific protein form and dosage used. Additionally, there were variations in the age of participants across studies, making it difficult to directly compare findings between studies. Further trials are needed to determine the optimal dose, forms, and duration of protein supplementation to verify the potential beneficial effects on lean body mass among middle-aged women.
Limitations
There are several limitations that should be acknowledged in this study. Firstly, the search was limited to articles published in English and Chinese, which may introduce language bias and potentially exclude relevant studies published in other languages. This could limit the comprehensiveness of the literature review. Secondly, a considerable proportion of the included studies lacked specific information about the generation of randomization sequences, blinding of participants and researchers, and registration of the clinical trial. These factors can introduce biases in the random allocation process, deviate from the intended interventions, and affect the reliability of the reported results. Thirdly, a small percentage of participants were aware of their assigned interventions, which may have influenced their performance and introduced performance bias. Additionally, the absence of pre-specific plans or clinical trial registration for analysis in the majority of the included studies may introduce selection bias. Moreover, due to insufficient data and a limited number of RCTs, it was not possible to perform a meta-analysis on the effects of exercise training, vitamin D supplementation, and protein supplementation on physical functional performance in menopausal women aged 40 to 60. Therefore, the study is unable to provide specific recommendations regarding these aspects. Finally, the variations in exercise dose (types, duration, frequency, intensity) across the included studies limited the ability to conduct subgroup analyses and provide detailed recommendations in these specific areas. The available data may not be sufficient to draw conclusive findings in this regard.
Suggestion
Based on the limitations identified, it is recommended that future research focuses on conducting larger and well-designed RCTs to examine the effects of exercise, vitamin D supplementation, and protein supplementation on muscle health in menopausal women aged 40 to 60. Additionally, systematic reviews with appropriate analytical methods should be performed to determine the optimal dosage and duration of these interventions. It is important to incorporate well-designed RCTs with rigorous randomization, allocation concealment, and blinding of interventions to improve the internal validity of the findings. Pre-specifying analysis plans and registering clinical trials will ensure transparency and minimize selective reporting. Furthermore, considering network meta-analyses can provide valuable insights into the comparative effectiveness of different interventions for preventing sarcopenia in menopausal women.