Frailty is a condition, in which the function of multiple physiological systems is impaired due to a decrease in homeostasis in response to stress [1
]. It is a geriatric syndrome that increases in prevalence with age [3
], and is a multifactorial syndrome influenced by physiological, nutritional, social, and cognitive factors [4
]. In older individuals, frailty is associated with adverse events, such as the risk of death [5
] and disability [5
]. Thus, to prolong the healthy lifespan of older people and reduce the burden of medical and caregiving expenses, it is necessary to reduce the number of people with frailty [8
] and to establish a sustainable, comprehensive, and effective public health program to prevent frailty.
The body mass index (BMI) is a convenient scale for evaluating thinness and fatness. In a meta-analysis including data from not only Japanese people [9
] but also those from around the world [10
], the BMI associated with the lowest risk of death increased with age. However, frailty individuals who, based on BMI, are underweight (<18.5 kg/m2
), of normal weight (18.5–24.9 kg/m2
), or obese (≥30.0 kg/m2
) have a higher risk of death than those who are not frailty [11
]. Elucidating the relationship between BMI and the prevalence of frailty is therefore important for determining the prognosis of older individuals.
A cross-sectional study on several British subjects showed that BMI has a U-shaped relationship with the prevalence of frailty; it also showed that the BMI ranges associated with the lowest prevalence of frailty are 18.5–24.9 [12
] and 25–29.9 kg/m2
]. Although the optimum BMI range in these studies varies, their findings suggest that it is important to maintain a healthy BMI to prevent frailty. However, the mean BMI differs between the Japanese and British people [15
], and it is difficult to extrapolate the results of previous studies to account for the Japanese people. To our knowledge, there is no current evidence about the relationship between BMI and the prevalence of frailty in Japanese older people. This study aimed to investigate the relationship between the prevalence of frailty and the BMI in community-dwelling Japanese older adults, with the use of two validated frailty assessment tools.
shows the characteristics of the participants in the whole cohort and subcohort. Participants from the subcohort, whose height and body weight were measured, tended to have fewer current smokers and more people with a high level of physical activity than participants from the whole cohort; however, the differences were not significant.
presents the validity and reproducibility of the self-reported height, body weight, and BMI. Differences were found between the mean values of measured height, body weight, and BMI and the mean values of the same parameters that were self-reported; however, the differences were also not significant. However, these variables showed a strong positive correlation with one another (self-reported vs. measured). Furthermore, height, body weight, and BMI, which were self-reported twice were highly reproducible. The relationships were the same even after the results were stratified according to sex and age.
shows the characteristics of participants according to BMI. A larger number of people with higher BMI tended to be men with a history of hypertension, heart disease, diabetes, or dyslipidemia and tended to be younger. These individuals also tended to consist of fewer smokers, who had a low rate of use of dentures.
shows the relationship between the prevalence of frailty and BMI on multivariate analysis. The prevalence of frailty defined using the FP model and KCL were 15.2% and 36.6%, respectively. The prevalence of frailty defined by the FP model and KCL were 25.3% and 55.5%, 19.6% and 37.7%, 14.3% and 34.2%, 12.4% and 32.6%, 12.6% and 34.3%, and 19.4% and 49.2% for each BMI category of <18.5, 18.5–19.9, 20.0–22.4, 22.5–24.9, 25.0–27.4, and ≥27.5 kg/m2
, respectively, thereby showing a U-shaped relationship. We found a positive relationship between the prevalence of frailty defined by the FP model and both, low BMI and high BMI even after adjusting for baseline confounders when using 22.5–24.9 kg/m2
as the reference (BMI of <18.5 kg/m2
: OR, 2.04; 95% CI, 1.58–2.63; BMI of 18.5–19.9 kg/m2
: OR, 1.69; 95% CI, 1.33–2.14; BMI of 20.0–22.4 kg/m2
: OR, 1.16; 95% CI, 0.96–1.41; BMI of 22.5–24.9 kg/m2
: reference OR and CI; BMI of 25.0–27.4 kg/m2
: OR, 1.00; 95% CI, 0.78–1.27; BMI of ≥27.5 kg/m2
: OR, 1.54; 95% CI, 1.15–2.07). Frailty, when evaluated using the KCL, these results were also the same. Furthermore, these relationships were also the same even after the results were stratified according to sex and age (Tables S1 and S2
depicts the relationship curve between BMI and the prevalence of frailty using the restricted cubic spline model. Even after adjusting for baseline confounders, the relationship between BMI and the prevalence of frailty defined using the FP model and KCL showed a U-shaped curve. The BMI range with the lowest prevalence rate of frailty defined using the FP model and KCL were 24.7–25.7 kg/m2
and 21.4–22.8 kg/m2
, respectively. The prevalence of IADL disability and depression evaluated using the KCL subdomains also showed a U-shaped relationship with the BMI (Figure 2
). Furthermore, a low BMI was associated with a high prevalence of oral and social frailty, while a high BMI showed a positive relationship with the prevalence of physical frailty.
In this study, we investigated the relationship between BMI and the prevalence of frailty using data from a population-based cohort study of older individuals. Even after adjusting for confounders, the BMI showed a U-shaped relationship with the prevalence of frailty as defined by both, the FP model and KCL, which are validated frailty assessment tools. The BMI range, for which the prevalence of frailty was the lowest, was 21.4–25.7 kg/m2. To our knowledge, this is the first study to show a positive relationship between the prevalence of frailty and BMI in Japanese people who are underweight (BMI < 18.5 kg/m2) and overweight (BMI ≥27.5 kg/m2). These findings suggest the need to evaluate the risk of frailty, not only in underweight people but also in overweight people.
This study showed that a low BMI was associated with a high prevalence of frailty. Several guidelines use BMI < 18.5 kg/m2
as a diagnostic marker for malnutrition [25
], which may be a risk factor for frailty, because maintaining a healthy BMI in an older person is important for maintaining a healthy skeletal muscle mass and nutrition status [25
]. Furthermore, our findings showed that a low BMI was associated with a high prevalence of oral frailty and social frailty, evaluated using the KCL subdomains. It has been reported that energy intake in older people is associated with eating together [28
] and poor oral conditions [27
]. Although the relationship between a low BMI and the prevalence of oral frailty and social frailty may be causally reversed, our results could provide important insight into risk factors for frailty, that need more careful attention in individuals with a low BMI. Furthermore, to prevent frailty, it would be necessary to establish target values for indicators, including BMI, which can evaluate energy balance. Although the Dietary Reference Intakes for Japanese (2020) establishes a personal target BMI range of 21.5–24.9 kg/m2
by considering the need to prevent both, frailty and onset of lifestyle-related diseases in older people [30
], there is no clear basis for this target to prevent frailty. Our results indicate that the BMI range corresponding to the lowest prevalence rate of frailty was 21.4–25.7 kg/m2
, which would support the BMI target value in the above guideline. Since a low BMI is a major risk factor for frailty in older people, it is therefore important to set a target BMI by considering individual characteristics and factors of lifestyle-related diseases.
Our results also showed that the prevalence of frailty is high even for individuals with a high BMI. Several previous studies have shown that the relationship between BMI and the prevalence of frailty is U-shaped [12
]. A high BMI reflects a routinely high level of energy intake, rather than the amount of energy needed [32
]. In the Wisconsin Primate Calorie Restriction study, a decrease in physical activity level was prevented to a greater extent in calorie-restricted rhesus monkeys than in rhesus monkeys fed on an ad libitum basis, and cut the metabolic cost associated with old age [33
]. In addition, caloric restriction is related to a low incidence of frailty as defined by unintentional weight loss, significant reduction in spontaneous physical activity, and decreased kinetic energy efficiency [34
]. Our results indicate that the prevalence of physical frailty defined using the KCL subdomain was high in overweight individuals, and these previous studies support our results. Therefore, to prevent frailty, it would be important to control body weight and to evaluate the risk of frailty in overweight people.
The strength of this study is that we were able to show the relationship between BMI and the prevalence of frailty defined using two validated assessment tools. Furthermore, as we have confirmed the validity of self-reported BMI by comparing it with measured BMI, a misclassification of self-reported BMI would be unlikely. Therefore, our results showed a strong relationship between variables and are therefore highly generalizable. However, our study has some methodological limitations. First, it has a cross-sectional design. Therefore, we cannot account for temporal and direct causal relationships between BMI and the prevalence of frailty. Second, our study included participants with a history of diabetes, dyslipidemia, heart disease, and stroke. These limitations may prevent generalization of the results. However, our results were similar after excluding participants with these diseases. Finally, we were unable to completely eliminate systematic errors from self-reporting. Thus, self-reported data such as education, income, smoking habit, and medical history may be affected by recall bias. However, as we used multivariate analysis to adjust for factors such as social and economic status, which are known to be associated with the results of frailty and BMI, we believe we were able to minimize the effect of confounding factors.
Hanlon et al. reported that the prevalence of frailty defined using the FP model (weight loss, exhaustion, grip strength, low physical activity, and slow walking pace) in 493,737 British individuals aged 37–73 years showed a difference of only around 2% between individuals aged 37–64 years and individuals aged ≥65 years [13
]. Furthermore, the relationship between BMI and the prevalence of frailty in this population was U-shaped [13
]. Since April 2008, Japan has been implementing the Specific Health Checkups and Specific Health Guidance and has since been evaluating the risk of metabolic syndrome in individuals aged 40–74 years [35
]. Middle-aged and older individuals with obesity and metabolic syndrome may already be experiencing frailty; hence, adding items to current tools used for evaluating frailty among middle-aged and older individuals and evaluating metabolic syndrome and frailty at the same time are important. However, the concept of frailty and the use of screening methods in middle-aged people need to be applied with care to ensure that the results are similar to those in older people [13
]. Furthermore, although appropriate diet [4
] and physical activity [31
] are crucial for preventing frailty, it is highly unlikely that a single method of intervention will apply to all individuals with frailty; thus, the method of intervention needs to be tailored according to individual characteristics [1
]. Early intervention for people with frailty will provide many benefits for both, the individuals and the health care system. The population mean of BMI is rising not only in East Asia including Japan but also all over the world [15
]. Therefore, frailty is an issue to be addressed in countries that are experiencing the double burden of malnutrition, and the control of BMI may be given high priority to decrease the prevalence of frailty.