Muscle performance and bone density following a multi-intervention program with milk or soy milk supplementation in older adults: quasi-experimental study

Highlights

• •Immediate postexercise milk and soymilk supplementation enhanced upper bone density in older adults.
• •Milk led to greater gains in handgrip strength than soy milk.
• •Whole foods protein sources offer a practical way to support musculoskeletal health.

Abstract

Objectives

This study assessed musculoskeletal outcomes of a combined intervention comprising food-based protein supplementation, nutrition education (NE), and resistance exercise in older adults; comparing milk and soy milk as protein sources.

Design

Quasi-experimental study.

Setting and participants

Eighty-two community-dwelling adults aged ≥60 years completed the intervention.

Intervention

This 8-week cluster-based intervention assigned participants to 1 of 4 groups: exercise alone (Group 1), exercise plus NE (Group 2), exercise plus NE with milk supplementation (Group 3), and exercise plus NE with soy milk supplementation (Group 4). All groups engaged in resistance training 3 times per week, and all groups received weekly NE, except for Group 1.

Measurements

Body composition, and physical performance were measured using dual-energy X-ray absorptiometry and standardized tests at baseline and after the intervention.

Results

Within-group analyses showed that all groups experienced significant improvements in walking speed. Additionally, Group 3 exhibited improvements in handgrip strength (mean change: +4.41 kg), 5-time sit-to-stand performance (−1.94 s). Compared with other groups, Group 3 achieved greater gains in handgrip strength than Group 2 (−0.84 kg) and Group 4 (+0.52 kg), and showed a borderline significant improvement in total bone mineral density (BMD; +0.01 vs. −0.06 g/cm²) compared with Group 1. Furthermore, Groups 2 − 4 exhibited greater increases in upper-limb BMD (+0.04, +0.02, +0.01 g/cm², respectively) compared with Group 1 (−0.02 g/cm²).

Conclusions

Exercise combined with NE and protein supplementation, particularly milk, may be associated with favorable bone health in older adults.

(Clincaltrials.gov as NCT06173271)

1. Introduction

With the rapid aging of the global population, age-related declines in muscle mass and bone mineral density (BMD) have become critical health concerns [1]. These age-related physiological changes increase the risks of sarcopenia and osteoporosis, two highly prevalent conditions in older adults that are closely interrelated and often co-occur as osteosarcopenia. The concurrent decline in muscle mass and BMD accelerates functional deterioration and is associated with higher risk of falls, disability, depression, malnutrition, and mortality [[2], [3], [4], [5]]. With the continual increase in life expectancy, preserving musculoskeletal health has become an urgent public health priority.

Resistance exercise and adequate dietary protein intake are recommended as core strategies for preventing sarcopenia and osteoporosis [3]. Compared with resistance training alone, resistance training combined with dietary protein intake may provide greater benefits in preserving musculoskeletal health in older adults. However, age-related declines in appetite and digestive capacity may limit protein intake. Distributing protein intake across multiple smaller meals throughout the day is considered a more efficient approach to maximizing digestion and absorption [6]. Accordingly, to implement this approach, several studies have used commercial protein supplements because of their controlled protein content and convenience; however, older adults tend to prefer food-based protein sources [7,8]. These sources are more accessible and familiar to older adults and provide a broader range of nutrients. In addition, food-based protein options may improve long-term adherence, particularly among community-dwelling older adults.

Oral health problems, including chewing difficulties and dry mouth, are common among older adults and can limit the intake of solid protein-rich foods [9]. Accordingly, liquid protein sources such as milk and soy milk are easier to consume and generally more palatable [10]. Milk is a rich source of protein and calcium, but lactose intolerance, which is more prevalent in Asian populations, can restrict its use [11]. Soy milk, prepared from soybeans, is a traditional and widely accepted plant-based protein beverage in Asia [12]. Despite the widespread use of both beverages, few studies have directly compared the health effects of milk and soy milk as protein sources in older adults.

In addition to dietary interventions, promoting healthy eating behaviors through nutrition education (NE) has been demonstrated to improve dietary habits and protein intake. A French study that provided protein-related nutrition information to older adults reported that the intervention group had substantially higher protein intake, greater nutrition awareness, and improved eating behaviors when compared with the control group [13]. NE can enable older adults to make informed dietary choices, improve their overall nutritional status, and reduce the risk of diet-related diseases [14]. Most studies have examined the effects of either NE or protein supplementation alone. Whether combining these strategies with exercise yields additive or synergistic benefits for preserving muscle mass, muscle strength, and bone health remains unclear.

To address this gap, the present study evaluated changes in muscle mass, muscle function, and BMD following an 8-week intervention combining food-based protein supplementation, NE, and resistance exercise, compared with resistance exercise alone in community-dwelling older adults. In addition, outcomes were compared between animal-based (milk) and plant-based (soy milk) protein supplementation within the combined intervention.

2. Methods

2.1. Participants

This study recruited community-dwelling adults aged 60 years or older from 5 community centers. Recruitment was conducted through poster advertisements and announcements made with the assistance of local community leaders. Written informed consent was obtained from all participants before enrollment. Participants aged 60 years and older were eligible for inclusion. Individuals were excluded if they had medical conditions or other factors that could limit protein intake, including chronic kidney disease, diabetes, cancer, moderate to severe cognitive impairment, mobility limitations, current use of commercial protein supplements, underweight status, adherence to a strict vegan diet, or known allergies or intolerances to dairy or soy products. By referencing a previous study [15], the present study estimated the required sample size by using a 2-sided test with α = 0.05 and β = 0.10. Assuming a mean difference of 0.72 in the primary outcome and a standard deviation of 4.08, the study estimated the minimum required sample size to be 22 participants per group. A total of 86 individuals provided written informed consent and were enrolled. Two individuals withdrew during the study, and 2 did not meet the inclusion criteria. Finally, 82 participants who completed the intervention were included in the analysis. This study used a quasi-experimental design, and participants were assigned by community center into 4 groups: (1) exercise alone (Group 1), (2) exercise plus NE (Group 2), (3) exercise plus NE with milk supplementation (Group 3), and (4) exercise plus NE with soy milk supplementation (Group 4; Supplementary Figure S1).

This study protocol was reviewed and approved by the Research Ethics Committee of China Medical University Hospital (IRB No. CMUH111-REC2-164).

2.2. Exercise intervention

All participants, regardless of group assignment, received a standardized exercise intervention. The program included both resistance and balance training targeting upper- and lower-limb muscle groups. The intervention comprised 24 sessions delivered over 8 weeks, with participants attending three 60-minute sessions per week; each session included time for warming up and cooling down. Group 1 received no additional dietary or NE beyond the exercise program.

2.3. Protein food intervention

Compared with other plant protein sources such as wheat, corn, and pea protein, soy protein contains higher levels of isoleucine, leucine, and valine, meeting 157%, 136%, and 126% of the required levels of essential amino acids, respectively [16]. These 3 branched-chain amino acids promote muscle protein synthesis. Moreover, plant-based proteins, particularly soy products, are more suitable for older adults with reduced chewing ability or financial constraints and are more sustainable from an environmental perspective. Accordingly, soy products were selected as the plant-based protein source in this study. Both soy protein and milk protein have a protein efficiency ratio of 2.5 and a protein digestibility-corrected amino acid score of 1.0, and both are readily available and easy to consume [17]. Consequently, they were chosen as the respective plant- and animal-based protein sources for comparison in this intervention.

The participants in the protein supplementation groups were divided into an animal-based protein group (Group 3) and a plant-based protein group (Group 4). Group 3 received 240 mL of low-fat milk, whereas Group 4 received 230 mL of soy milk. Each participant received one serving of a food item containing approximately 7–8 g of protein per session. The protein-containing foods were provided immediately after each exercise session, and the participants were instructed to consume them within 30−60 min.

Previous studies have indicated that the coingestion of carbohydrates with protein after exercise enhances muscle protein synthesis, particularly when the carbohydrate-to-protein ratio is approximately 3:1 [18]. Thus, the participants in both groups were also provided with 60 g of steamed sweet potato. The soy milk used contained approximately 14 g of added sugar, resulting in a total carbohydrate-to-protein ratio of 3.5:1 for both intervention foods. The nutritional composition of the intervention items is presented in Supplementary Table S1.

2.4. NE intervention

The participants in Groups 2–4 attended one session of NE per week for 8 weeks, resulting in a total of 8 sessions. The primary goal of the program was to promote healthy eating behaviors, with emphasis on adequate energy and protein intake. The curriculum covered foundational topics to enhance the participants’ understanding of healthy food choices, along with education on sarcopenia, its symptoms, screening, and prevention strategies to raise awareness among older adults. Additional sessions focused on preventing exercise-related injuries to reduce risky behaviors during physical activity and daily movements. Other topics included nutrition for older adults, oral health, methods to improve food texture through cooking, and disability prevention. These lessons were designed to enhance the participants’ knowledge about oral care and appropriate ingredient selection.

2.5. Data collection

This study used structured questionnaires to collect information on sociodemographic characteristics and physical activity levels. Body composition was assessed at the Health Examination Center of China Medical University Hospital by using dual-energy X-ray absorptiometry (DEXA) using Lunar iDXA (GE Healthcare, Chicago, IL, USA). Measurements included whole body, lumbar spine, femoral neck, upper limbs, lower limbs BMD (g/cm²), and appendicular skeletal muscle mass (ASM). The T-score and appendicular skeletal muscle mass index (ASMI) was calculated for assess bone health and muscle mass, respectively, using the following formula:

$$\text{T}\text{s}\text{c}\text{o}\text{r}\text{e}=\frac{\text{M}\text{e}\text{a}\text{s}\text{u}\text{r}\text{e}\text{d}\text{B}\text{M}\text{D}–\text{y}\text{o}\text{u}\text{n}\text{g}\text{a}\text{d}\text{u}\text{l}\text{t}\text{m}\text{e}\text{a}\text{n}\text{B}\text{M}\text{D}}{\text{S}\text{t}\text{a}\text{n}\text{d}\text{a}\text{r}\text{d}\text{d}\text{e}\text{v}\text{i}\text{a}\text{t}\text{i}\text{o}\text{n}\text{o}\text{f}\text{y}\text{o}\text{u}\text{n}\text{g}\text{a}\text{d}\text{u}\text{l}\text{t}\text{B}\text{M}\text{D}}$$

$$\text{A}\text{S}\text{M}\text{I}=\frac{Appendicularskeletalmusclemass(kg)}{Height2(m)}$$

Height and weight were measured using a calibrated stadiometer and scale, and body mass index (BMI) was calculated as weight (kg) divided by height (m²). According to the criteria established by the Health Promotion Administration, Ministry of Health and Welfare in Taiwan, underweight is defined as a BMI of less than 18.5 kg/m², normal weight as 18.5–23.9 kg/m², overweight as 24.0–26.9 kg/m², and obesity as a BMI of 27.0 kg/m² or higher [19]. Handgrip strength of the dominant hand was assessed using a baseline Smedley spring-adult dynamometer (model 12−0281, capacity 220 lb).

Lower-extremity physical function was evaluated using the Short Physical Performance Battery (SPPB), which includes 3 components: a balance test, a gait speed test, and the 5-time sit-to-stand test. Each component is scored from 0 to 4, with a total score ranging from 0 to 12. Higher scores indicate better physical function [20]. The balance test included side-by-side, semitandem, and tandem stances, all performed without assistive devices. The sit-to-stand test required the participants to rise from a chair and sit back down 5 times as quickly as possible with arms crossed over the chest, and the total time to complete the task was recorded. Gait speed was assessed over a 6-m course; the participants were asked to walk at their usual pace from the starting point to the end of the path, pause briefly, and then return. The time required for each trial was recorded, and the average of the 2 walks was used for analysis.

The validated Mini Nutritional Assessment (MNA) [21] and the semi-quantitative food frequency questionnaire [22] were used to assess changes in participants’ nutritional status and intake of soy and dairy products before and after the intervention. A 20-item nutrition knowledge questionnaire (total score: 100 points) was developed based on the content of the nutrition education curriculum to assess changes in participants’ nutrition knowledge.

2.6. Statistical analysis

Continuous data are presented as means ± standard deviations or as medians with interquartile ranges (quartiles 1–3). Baseline differences in continuous variables between Group 1 and Groups 2–4 were examined using the Wilcoxon rank-sum test. Categorical data are expressed as percentages, and differences in distributions between Group 1 and Groups 2–4 were assessed using the chi-square test.

Crude changes were calculated as observed post–pre differences and are presented as mean ± SD. Estimated means ± SE and p values were obtained from linear mixed-effects models with random intercepts for participants, including fixed effects for group, time, and group × time, adjusted for age and sex. Within-group p values correspond to model-based contrasts (post vs baseline) within each group. Post hoc comparisons for interaction represent pairwise contrasts of changes between groups. Multiple-comparison adjustment was applied using the Tukey Post Hoc test. All statistical analyses were performed using SAS, version 9.4 (SAS Institute, Inc., Cary, NC).

3. Results

3.1. Baseline characteristics

Baseline demographic characteristics were compared between Group 1 and each of Groups 2–4. No significant differences in age, sex, BMI, nutritional status, nutrition knowledge score, soy and dairy products intake were observed among the groups. Furthermore, no statistically significant differences in muscle mass, muscle function, or BMD were observed between Groups 3 and 4 and Group 1; by contrast, Group 2 had a significantly higher spinal BMD than did Group 1 (median 0.97 vs. 0.88 g/cm², p = 0.04; Supplementary Table S2).

3.2. Within-group comparison of changes in muscle function and BMD before and after the intervention

Within-group analyses indicated that walking speed improved significantly in all four groups (all p < 0.05). In addition, Group 1 showed significant improvements in handgrip strength (27.6–30.3 kg, p = 0.01) and sit-to-stand performance (9.54 to 7.90 s, p = 0.02). Group 3 demonstrated significant improvements across multiple functional outcomes, including handgrip strength (25.1–29.5 kg), chair stand time (10.7–8.74 s), and SPPB scores (11.3–11.8) (all p < 0.05). No other significant within-group changes were observed in Groups 2 and 4. Furthermore, no significant within-group changes in appendicular skeletal muscle mass were noted in any of the groups (Fig. 1).

Fig. 1.

Estimated means of muscle function and muscle mass at baseline and after the 8-week intervention. Asterisks indicate statistically significant with-in group or between-group differences based on linear mixed model adjusted for age and sex (*p < 0.05; **p < 0.01; **p < 0.001). ASMI: Appendicular skeletal muscle mass; NE: nutrition education.

For the bone health, Group 1 exhibited a significant reduction in total body (1.09 to 1.03 g/cm²), spine (1.00 to 0.96 g/cm²), and upper limbs BMD (0.86 to 0.80 g/cm²) (all p < 0.05). However, Group 3 demonstrated a significant increase in T-score (-0.05 to 0.05, p = 0.02) (Fig. 2).

Fig. 2.

Estimated means of bone mineral density (BMD) at four skeletal sites and T-scores at baseline and after the 8-week intervention. Asterisks indicate statistically significant with-in group or between-group differences based on linear mixed model adjusted for age and sex (*p < 0.05; **p < 0.01; **p < 0.001). NE: nutrition education.

3.3. Comparison of changes in muscle function and bone health between groups

Compared with Group 1, Group 2 showed a significantly greater decline in handgrip strength (mean change: −0.84 vs. 2.74 kg, p for interaction = 0.02). No other significant differences in changes in muscle function over time were observed between the intervention groups and Group 1.

To further evaluate the association of protein supplementation with study outcomes, this study conducted comparisons between Group 2 and Groups 3 and 4. Group 3 demonstrated significantly greater improvements in handgrip strength than did Group 2 (mean change: 4.41 vs. −0.84 kg, p for interaction = 0.0003). However, no indicators differed between Groups 2 and 4. Milk supplementation led to greater improvements in handgrip strength (mean change: 4.41 vs. 0.52 kg, p for interaction = 0.005) compared to soy supplementation (Fig. 1, Supplementary Table S3).

Regarding bone outcomes, Group 2–4 exhibited a greater increase in upper-limb BMD (mean change: 0.04, 0.02, 0.01 vs. −0.02 g/cm², all p for interaction <0.05) than did Group 1. Group 3 exhibited a borderline significant improvement in total body BMD compared with Group 1 (mean change: 0.01 vs. −0.06 g/cm², p for interaction = 0.05) (Fig. 2, Supplementary Table S4).

For the dietary intake, Group 2 exhibited a greater increase in nutritional knowledge score (mean change: 12.6) than did Group 1 (−0.26), 3 (5.45) and 4 (3.18). Group 4 showed a significantly higher intake of soy products than Groups 1 and 2 (mean change: 4.08 vs. 0.01 and −0.90 servings/week, p for interaction = 0.005 and 0.001, respectively). No significant differences in nutritional status or dairy product intake were observed among groups (Supplementary Table S5).

4. Discussion

This study determined that among adults aged 60 years or older, immediate postexercise supplementation with protein-rich foods combined with NE improved BMD. Compared with participant who engaged in exercise only, NE and protein rich food supplementation significantly increased upper-limbs BMD. Compared with participants who received soy supplementation experienced greater gains in handgrip strength, suggesting an added benefit of postexercise milk intake. Muscle function improved in all intervention groups; however, the difference in muscle function between the protein rich food supplementation and exercise-only groups was not statistically significant. This finding highlights the central role of resistance training in enhancing muscle strength.

Our findings are consistent with those of a 6-week resistance band intervention study that reported significant improvements in bone Z-scores but not in BMD [23]. By contrast, a study involving older men at high risk for osteoporosis and sarcopenia observed significant gains in lumbar spine and hip BMD after 18 weeks of high-intensity resistance training combined with whey protein supplementation [24]. Although the present study included relatively healthy older adults and used a progressive but lower-intensity exercise protocol, the significant improvement in BMD observed in the milk and soy milk supplementation groups suggests that combining resistance training with protein-rich food supplementation is a feasible and effective strategy for maintaining bone health.

In contrast to several studies that have used commercial protein formulas, the present study employed whole foods, which provided approximately 7−8 g of protein and a fixed ratio of carbohydrates through sweet potato; this approach was more practical and appropriate for community settings. Milk provides both high-quality protein and key bone-related nutrients such as calcium and vitamin B12, both of which contribute to skeletal maintenance and help prevent bone loss [25]. In addition to being a good source of protein, soy milk contains isoflavones, which may benefit bone health in older adults. In postmenopausal women, daily supplementation with 84 mg of isoflavones for 6 months helped maintain lumbar spine and femoral neck BMD [26]. The soy milk used in the current study contained approximately 40 mg of isoflavones, and whether this dose is sufficient to produce comparable effects requires further investigation. Although no significant differences in nutritional status were observed among the groups after the intervention, the soy milk supplementation group showed a higher intake of soy products compared with the exercise-only and NE groups. The dairy products intake was higher in the milk supplementation group than in the other groups, but the difference did not reach statistical significance. Therefore, the observed changes in this study may be attributed not only to the food itself but also to the immediate post-exercise supplementation. Immediate intake of protein and carbohydrates after exercise has been suggested to promote muscle protein synthesis.

Protein may affect bone health through several mechanisms: approximately 35% of bone mass is composed of protein, and protein intake stimulates the production of insulin-like growth factor 1, which promotes bone formation and enhances calcium absorption [27]. Some studies have reported positive associations between total and animal-based protein intake and BMD, whereas associations between BMD and plant-based protein have been less consistent or negative [28]. The soy milk used in the present study, characterized by a high-quality amino acid profile and a high digestible indispensable amino acid score [29], may explain the observed benefits despite the mixed results reported in previous studies.

Although Groups 2–4 exhibited improvements in muscle function, none of them demonstrated statistically significant differences when compared with Group 1. Notably, handgrip strength in Group 2 was significantly lower than that in Group 1, suggesting that knowledge-based interventions alone are insufficient without accompanying behavioral changes. By contrast, Group 3 demonstrated significant improvements in handgrip strength, 5-time chair rise, 6-m gait speed, and SPPB scores, indicating that combining exercise with milk supplementation may be more beneficial for muscle function than soy milk supplementation.

Protein is essential for muscle protein synthesis (MPS), particularly during the 24–48 h following resistance training; however, aging-related anabolic resistance diminishes the MPS response to protein intake alone, whereas combining exercise with protein supplementation has been shown to enhance MPS and partially overcome this resistance [7,30]. Although animal-based proteins are generally regarded as more effective in stimulating muscle anabolism, when overall protein intake is sufficient, differences between animal and plant sources may be minimal [16]. The superior performance of Group 3 in the present study may be explained by the higher digestibility and amino acid availability of dairy protein compared with plant-based alternatives.

4.0.1. Strengths and limitations

This study has several strengths. First, it targeted community-dwelling older adults and used protein-rich whole foods as the intervention, enhancing the feasibility and applicability of the findings in real-world settings. Second, the inclusion of 3 intervention arms enabled comparisons of the independent contributions of protein supplementation and NE. Third, both muscle mass and BMD were assessed using DEXA, which enabled a comprehensive evaluation of musculoskeletal changes.

Despite its strengths, this study has several limitations that should be acknowledged. First, participants were recruited from community centers, and random allocation at the individual level was not feasible because of logistical constraints. Although baseline characteristics, including muscle mass, functional performance, and bone health status, did not differ significantly between the groups, potential confounding cannot be completely ruled out. Second, environmental differences across community centers may have influenced the participants’ learning outcomes and adherence to the interventions. Third, the short 8-week intervention period may limit the detection of true physiological adaptations, as observed changes could partly reflect measurement error. Moreover, due to the detection threshold of DEXA, subtle short-term changes in bone density may not be reliably captured.

5. Conclusion

This study suggests that combining resistance exercise with immediate postexercise supplementation of protein-rich whole foods, particularly milk, may improve bone health and physical function in community-dwelling older adults. Given the quasi-experimental design and limited sample size, larger randomized controlled trials are needed to confirm these findings and establish causal relationships.

CRediT authorship contribution statement

Ting Liao: Data curation; Formal analysis; Investigation; Writing – original draft.

Ting-Ying Wang: Data curation; Formal analysis; Investigation; Project administration.

Meng-Chun Lu: Conceptualization; Finding acquisition; Methodology; Writing – review & editing.

Huey-Liang Kuo: Conceptualization; Resources; Methodology; Writing – review & editing.

Yu-Lung Chen: Conceptualization; Finding acquisition; Methodology; Writing – review & editing.

Kuo-Cheng Lin: Conceptualization; Finding acquisition; Writing – review & editing.

Yi-Ling Chen: Conceptualization; Methodology; Writing – review & editing.

Ying Hsiao: Conceptualization; Writing – original draft.

Yi-Chen Huang: Conceptualization; Data curation; Funding acquisition; Resources; Supervision; Writing – review & editing.

Funding statement

This work was supported by China Medical University Hospital (grant number DMR-112-117).

Ethical standards

All participants signed informed consent forms before taking part the study. Ethical approval for the research project was granted by the Research Ethics Committee of China Medical University Hospital (IRB No. CMUH111-REC2-164).

Sponsor’s role

The sponsor of the study had no involvement in study design, data collection, analysis and interpretation, the writing of the report, and the decision to submit the paper for publication.

Data availability

The data from this study are not publicly available because the participants did not provide informed consent to share their responses publicly.

Declaration of competing interest

The authors declare no conflicts of interest.

Appendix A. Supplementary data

The following are Supplementary data to this article: