The results of this study showed that compared with the nonfrail group, the age of the prefrail group was higher, suggesting that the risk of frailty increases with age. Second, BMI and VFA were higher in the prefrail group than in the nonfrail group, and BMR, protein content and SMI were significantly lower in the prefrail group than in the nonfrail group, suggesting that prefrail older people may have more fat, less muscle, poor metabolism and inadequate nutrition. In particular, a higher VFA will release more inflammatory signals and thus damage the function of the immune system. Clinical attention should be given to the adverse effects of sarcopenic obesity in prefrail elderly individuals. Second, ECW, ICW and TBW were decreased in the prefrail group. With increasing age, along with the decline in muscle mass and muscle strength, ICW and TBW also decreased gradually [7]. In the prefrail group, ECW decreases but to a lesser extent, and the ECW/TBW is increased. Body water composition can be used as a health parameter, and a previous study showed that body water composition was associated with muscle strength and physical function [18]. Whether a high ECW/TBW is associated with decreased physical function and whether it can be used as a potential screening marker for prefrailty warrants further investigation.
Our present results suggest that the ECW/TBW is negatively correlated with grip strength and gait speed, and positively correlated with 5-repetition sit-to-stand time in older men living in the community. After adjusting for confounding factors (age and BMI), the ECW/TBW remained independently associated with grip strength, gait speed, and 5-repetition sit-to-stand time. Several studies have shown that the degree of cellular dehydration is negatively correlated with nitrogen balance and can affect the anabolism of protein, which can lead to muscle atrophy, impair muscle contraction function and affect physical health [19], which may also be one of the reasons why the increase in the ECW/TBW is closely related to the decline in physical function. Park et al. [15] reported that individuals with low muscle strength and muscle mass had a higher ECW/TBW (0.391 or higher) than that of healthy individuals.
The ECW/TBW may be an effective research tool to evaluate the muscle strength and physical function performance of elderly individuals. Yamaguchi K. et al. [20] found that the grip strength (r=-0.39, P < 0.001) of those aged 70 and above was closely related to the ECW/TBW, which was similar to the results of our study. At the same time, our study also found that the most frequently detected indicator of the Fried phenotype in prefrail older adults was reduced grip strength, accounting for 70.18%, with reduced grip strength reflecting reduced muscle strength. Muscle dehydration is caused by reduced intracellular water content, which is thought to be partly due to cell membrane damage caused by inflammation and reactive oxygen species. Changes in body water composition may lead to cell damage and affect muscle function [4]. Therefore, the ECW/TBW as a potential parameter for the early detection of muscle weakness or decreased physical activity may be a significant early indicator of impending frailty for prefrail elderly individuals.
Logistics regression analysis showed that after adjusting for age and other confounding factors, an increased ECW/TBW and longer 5-repetition sit-to-stand time were found to be independent influences in prefrail older adults. Similar to our findings, a cross-sectional study that included 550 community-dwelling older adults found that after adjusting for age, sex and BMI, multiple regression analysis showed that an increased ECW/TBW was significantly associated with locomotive syndrome risk (P < 0.001) and frailty (P = 0.001). A higher ECW/TBW may reflect a higher risk of frailty [21]. It has been suggested that excess extracellular fluid can increase inflammation by transferring bacteria and endotoxins to the circulatory system through the oedematous intestinal wall [22], which may also contribute to the development of frailty. Kehayias J.J. et al. suggested that the ECW/TBW could be used as a frailty factor (FF), which mimics the metabolic effects of body cell quality, as a measure of frailty [14]. It has also been suggested that the ECW/TBW may be a potential objective biomarker for screening lung cancer patients with frailty [23]. In this study, the ECW/TBW measured by the BIA method was used as a more objective indicator that may become a marker of prefrailty in community-dwelling elderly men. At the same time, it is closely related to the decline in physical function, which has certain reference value for the early identification of prefrailty. Second, the 2019 Asian Working Group for Sarcopenia (AWGS) suggested that the 5-repetition sit-to-stand time test could replace the gait speed test [17]. The gait speed test is influenced by subjective factors, is not timed correctly when subjects are crossing the line and has a risk of falling, whereas the 5-repetition sit-to-stand test is simple and easy to perform and can reflect lower limb muscle function more objectively. If the 5-repetition sit-to-stand test is included in the diagnostic criteria of frailty, it may better reflect the lower limb muscle function and joint movement of frail elderly individuals, which may have certain significance for the early diagnosis of frailty.
There are some limitations in this study. First, the study was designed to be cross-sectional, and future follow-up cohort studies are still needed to clarify the causal relationship between them. Second, the study did not delineate the normal range of the ECW/TBW; therefore, the boundary value of the ECW/TBW should be studied in the future. Third, the main population of this study was elderly men in the community, and it is necessary to recruit elderly women for inclusion in future analyses. Fourth, the recruited population mainly included elderly individuals in the prefrailty stage; therefore, more patients in the frailty stage need to be included for further research in the future.