To the best of our knowledge, this study first investigated the association and differences between FMI and FFMI as well as LS and frailty using large-scale health checkup data in Japan. Our study provided novel insights by examining the associations simultaneously, thereby giving a clearer understanding of the effect of body composition components on LS and frailty. We hypothesized that both FMI and FFMI are associated with LS and frailty. Results revealed an association between LS and a high FMI and between frailty and a low FFMI, thereby emphasizing the importance of analyzing body composition parameters separately for FMI and FFMI in addition to BMI.
Previous reports have provided reference values for FMI and FFMI [12, 21–23], indicating differences based on age, sex, and race. The elderly population had a higher FMI than the younger population [21]. However, information on the FFMI of elderly populations is contrasting. In particular, some studies have reported minimal changes in FFMI with age [23], and others have shown that FFMI declined with age [24]. In terms of sexes, female participants had a higher FMI than male participants, and male participants had a higher FFMI than female participants [23]. Similarly, our study revealed that female participants had a significantly higher FMI than male participants, and male participants had a had significantly higher FFMI than female participants. Thus, there was no significant difference in terms of FM between female and male participants. However, after adjusting for height, the male and female participants exhibited a significant difference in FMI.
A previous study has reported racial differences, with FFMI being the lowest among Caucasians, African Americans, Hispanics, and Asians [25]. Therefore, FMI and FFMI should be analyzed separately for men and women, and Japanese data should be evaluated against references for the Japanese population. Previous reports provided reference values for FMI and FFMI in community-dwelling older Japanese men and women [23]. Our study had similar findings. The mean FMIs were 6.3 kg/m² in men and 7.5 kg/m² in women, and the mean FFMIs were 18.1 kg/m² in men and 15.5 kg/m² in women. The body composition of our study population was generally similar to that of elderly Japanese participants in previous studies [23].
FMI, an index of FM, is associated with obesity. A high FMI increases the odds of metabolic syndrome in both men and women [26]. Further, it is associated with physical dysfunction in elderly people [27] and is a predictor of worsening walking speed [28]. Hence, FMI is also associated with motor function. Meanwhile, FFMI, an index of muscle mass, is associated with motor function. A low FFMI is associated with sarcopenia [29] and poor prognosis in lung diseases such as chronic obstructive pulmonary disease [30]. Our study showed an association between LS and a high FMI and between frailty and a low FFMI. In the univariate analysis, female and male participants with LS did not significantly differ in terms of FMI. However, according to age-adjusted comparisons, female and male participants with LS had a significantly high FMI. This finding underscores the importance of preventing FM increase in LS management.
Frailty differs from LS in encompassing several concepts and biases. The univariate analysis according to sex showed that women with frailty had a significantly low FFMI. However, after adjusting for age, the FFMI did not significantly differ between female and male participants. Although frailty was associated with a lower FFMI rather than a higher FMI, further research should be performed to determine if a lower FFMI can prevent frailty.
The absence of a significant association between LS and FFMI was not expected, with consideration of the known association between muscle mass and motor function. This finding might be attributed to the specific characteristics of the GLFS-25 used to diagnose LS, which primarily assesses functional limitations rather than muscle mass directly.
Although BMC is not significantly associated with LS or frailty, it remains a valuable component of overall body composition. Future studies should still include BMC measurements to provide a comprehensive understanding of body composition in older adults. In this study, approximately 30% of participants presented with sarcopenia. The participants diagnosed with both LS and frailty had a high prevalence of sarcopenia. Further, they had a significantly high FMI and low FFMI, thereby indicating the need for combined interventions targeting both fat reduction and muscle preservation.
The clinical implications of these findings are significant. In patients with LS, interventions should focus on reducing FMI via targeted dietary and exercise programs aiming to decrease body fat. In individuals at risk of frailty, strategies should prioritize increasing FFMI by promoting muscle mass via resistance training and protein supplementation. Healthcare providers should consider incorporating BIA into routine assessments to monitor changes in FMI and FFMI, thereby facilitating early detection and individualized interventions. By addressing these specific components of body composition, patient outcomes can improve, and the incidence of LS and frailty can decrease. Ultimately, the elderly population can have a better quality-of-life.
This study had several limitations. First, the participants were from rural areas. Hence, their living and working environment differs from that of urban populations. Second, the BIA results might vary among different manufacturers. Hence, further studies using standardized protocols and cross-calibration of electrical resistance should be performed. Third, this was a cross-sectional, single-center study. Thus, our findings should be validated via longitudinal and multicenter collaborative studies.
In conclusion, this study first examined the associations and differences between FMI and FFMI as well as LS and frailty in Japan. A high FMI was significantly associated with LS, and a low FFMI was significantly associated with frailty. Measuring body fat percentage and FM is straightforward, and when assessing BMI, changes in FMI and FFMI should be considered. Preventing FMI increase and FFMI decrease may help manage and treat LS and frailty, respectively.