Effects of High‐Protein Food Intake on Appendicular Skeletal Muscle Mass Index in Community‐Dwelling Older Adults: A 3‐Month Pre–Post Study

Dear Editor,

Sarcopenia is a progressive skeletal muscle disorder characterized by loss of muscle mass, strength, and physical performance, threatening functional independence in older adults [1]. A prospective cohort applying the AWGS 2019 criteria reported a sarcopenia prevalence of 11.5% in men and 16.7% in women among community‐dwelling Japanese older adults, with independent associations with all‐cause mortality and incident disability [2]. Guidelines recommend dietary protein intake of ≥ 1.0–1.2 g/kg/day for healthy older adults [3, 4], yet many Japanese older adults fail to meet this target [5], and higher protein intake has been associated with greater appendicular skeletal muscle mass [6]. We therefore examined the effect of a 3‐month high‐protein food intervention on the appendicular skeletal muscle mass index (ASMI) in community‐dwelling older adults.

This is a secondary analysis of a single‐arm pre‐post trial conducted in 2023 in Itabashi, Tokyo. The parent trial (UMIN000054344) evaluated albumin redox balance as its primary endpoint; the present analysis used data from the same trial to examine ASMI. Community‐dwelling older adults (≥ 65 years) were recruited with a target enrollment of 50 participants. One participant withdrew after developing a back skin rash, which medical evaluation deemed unrelated to the intervention; other participants continued after a brief precautionary suspension. After excluding withdrawals and participants with adherence < 80%, 47 participants were included in the analysis (mean age 71.6 ± 4.2 years; 70.2% women). Participants consumed 18 g/day of protein from a high‐protein powder (Enjoy Protein; Morinaga Milk Industry Clinico Co. Ltd., Tokyo, Japan; 4.5 g whey protein per 5 g serving) mixed into foods or beverages for 3 months. According to the manufacturer's product specifications, the intervention product contained approximately 22 g of branched‐chain amino acids (BCAAs) per 100 g of product, with leucine accounting for approximately 13% of the total amino acids. Thus, the 18 g/day protein dose provided approximately 4.4 g of BCAAs and 2.3 g of leucine per day.

ASMI and BMI were assessed at baseline, 1.5 months, and 3 months; ASMI was measured by bioelectrical impedance analysis (InBody S10), and BMI was calculated as body weight (kg) divided by height squared (m²). Habitual energy intake and protein intake (excluding the supplement) were assessed at baseline and 3 months using the Brief‐type Self‐administered Diet History Questionnaire (BDHQ) [7]. To examine changes across the time points, a linear mixed model (LMM) was used, with post hoc comparisons corrected by the Bonferroni method. The ASMI model was adjusted for baseline age, sex, and baseline BMI, whereas the BMI and dietary intake models were adjusted for baseline age and sex only. All analyses were performed using IBM SPSS Statistics version 29.0, with a two‐sided significance level of 5%. This study was approved by the ethics committee of the Tokyo Metropolitan Institute for Geriatrics and Gerontology (R23‐100).

Measurements at each time point are shown in Table 1. LMM adjusted for sex, baseline age, and baseline BMI demonstrated a significant main effect of time on ASMI (F(2, 45.7) = 7.20, p = 0.002). Bonferroni‐corrected post hoc comparisons revealed that ASMI significantly increased from baseline to 1.5 months (adjusted mean difference = 0.181, 95% CI: 0.062–0.300, p = 0.001), with the increase sustained at 3 months (adjusted mean difference = 0.128, 95% CI: 0.013–0.242, p = 0.024). No significant change was observed between 1.5 and 3 months (adjusted mean difference = −0.053, 95% CI: −0.151 to 0.044, p = 0.545). In contrast, LMMs adjusted for sex and baseline age showed no significant main effect of time on BMI (F(2, 46.2) = 0.171, p = 0.843), energy intake (F(1, 46.0) = 0.393, p = 0.534), or protein intake (F(1, 46.0) = 0.180, p = 0.674).

TABLE 1.

Participant characteristics and outcome measures at baseline, 1.5 months, and 3 months.

Variable	Baseline	1.5 months	3 months
n	47	47	47
Appendicular skeletal muscle mass index (kg/m2)	6.36 ± 0.99	6.54 ± 0.99	6.49 ± 0.93
Body mass index (kg/m2)	23.1 ± 3.30	23.1 ± 3.30	23.1 ± 3.37
Habitual energy intake (kcal/day)	1934 ± 604	—	1969 ± 719
Habitual protein intake (g/day)	81.5 ± 30.7	—	80.4 ± 32.4

Note: Data are presented as means ± standard deviations.

A 3‐month high‐protein food intervention significantly increased ASMI in community‐dwelling older adults without changes in BMI or habitual protein intake. These findings are consistent with a meta‐analysis of 38 RCTs by Hettiarachchi et al., which reported a positive effect of protein supplementation on muscle mass in community‐dwelling older adults (SMD = 0.116; 95% CI: 0.032–0.200; p = 0.007) [8].

Concerns have been raised that protein supplementation may suppress appetite and reduce habitual intake, offsetting the intended benefit. However, Ben‐Harchache et al. reported that supplementation did not significantly reduce total energy intake in healthy older adults [9]. Consistent with this, habitual protein intake was maintained throughout our intervention, suggesting that the ASMI gain was driven, at least in part, by the supplemental protein itself rather than by changes in overall dietary behavior.

Several limitations should be acknowledged. The single‐arm pre‐post design without a control group precludes firm causal inference. The small sample (n = 47) of relatively healthy older adults limits generalizability to frail or sarcopenic populations, and the 3‐month follow‐up precludes evaluation of long‐term effects. In addition, physical activity, which is a major determinant of skeletal muscle mass and may act synergistically with dietary protein intake, was not assessed during the intervention period. Consequently, we cannot exclude the possibility that changes in participants' physical activity levels contributed to the observed increase in ASMI, and future studies should incorporate concurrent assessment of physical activity to disentangle the independent and combined effects of protein intake and physical activity on muscle mass.

As ASMI was not the parent trial's primary endpoint, findings should be regarded as exploratory and hypothesis‐generating. Prospectively designed RCTs with ASMI as a primary endpoint are warranted.

Funding

This work was supported by the Morinaga Milk Industry Co. Ltd.

Conflicts of Interest

This study was conducted as a collaborative research project with Morinaga Milk Industry Co. Ltd., which provided research funding and supplied the high‐protein powder used in this study free of charge as part of the collaborative research agreement. T. Shibasaki is an employee of Morinaga Milk Industry Co. Ltd. but was not involved in the interpretation of the study results. The other authors declare no conflicts of interest.

Data Availability Statement

Research data are not shared.