1. Research Design, Participants, and Methods
This was a cross-sectional study. The survey population was 900 stratified randomly selected residents of Nagatoro town, Saitama Prefecture, Japan, aged ≥ 65 years. Nagatoro has a population of 6,807 and an aging rate of 39.6%, compared with 28.6% for Japan as a whole [14]. Nagatoro is located in a basin surrounded by mountains. The survey participants were mailed a self-administered questionnaire. The survey period was from August to September 2021. To examine individual attributes associated with prefrailty, individuals judged as prefrail or healthy according to the frailty criteria described below were included in the analysis, and those judged as frail were excluded from the analysis.
2. Survey Items
1) Basic Attributes
The basic attributes of age and gender were determined.
2) Frailty
The Kihon Checklist (KCL) was used to assess frailty. The KCL was developed in Japan in 2006 [15] and has been validated to assess frailty [16]. The checklist has been translated into English [17], Portuguese [18], Spanish [19, 20], Chinese [21], Thai [22], Turkish [23], and other languages.
The KCL is a comprehensive assessment of frailty that comprises 25 questions on physical (motor function, nutritional status, oral function), mental (cognitive function, depressed mood), and social (being housebound) aspects and activities related to daily living. Possible responses on each item are 0 or 1; higher total scores (0–25) indicate more problems with daily functioning. The total score was used to determine frailty, with scores of ≥ 8 indicating frailty, 7–4 prefrailty, and ≤ 3 robustness [24]. As frailty includes physical, mental, and social factors, the KCL was selected because it is a multifactorial index.
3) Physical Aspects
Medical history, height, weight, regular exercise habits, and food intake status were ascertained. Body mass index (BMI kg/m2 ) was calculated from height and weight, and a BMI of < 18.5 was defined as “thin.” Food intake status was determined using the Dietary variety score (DVS) [25] for 10 foods or food groups (seafood, meat, eggs, milk and dairy products, legumes, green and yellow vegetables, seaweed, potatoes, fruit, and oils and fats). Responses are a 4-point scale ranging from “eat every day” to “rarely eat,” with 1 point given for “eat every day” and 0 for all other responses. The total score ranges from 0 to 10.
4) Mental Aspects
Subjective health and depressive status [26] were assessed. Subjective health was measured on a 4-point scale ranging from “fairly healthy” to “not healthy.” The presence of depressive status was defined as a “yes” response to either the question “I feel down or depressed” or the question “I’m just not interested in things” during the last month.
5) Social Aspects
Household composition, employment status, economic status, frequency of going out, presence of intimate friends, community participation in middle age, social activities, solitary eating, and presence of friends who listen to one’s concerns were assessed.
3. Method of Analysis
Participants with KCL total scores of 4 to 7 were categorized as the prefrailty group and those with scores of ≤ 3 as the healthy group. Significant differences in basic attributes, and physical, mental, and social dimensions in the two groups by gender were confirmed using the chi-square test or Fisher’s exact test (for nominal variables) and the t-test (for continuous variables).
Next, odds ratios and 95% confidence intervals (CI) were calculated using logistic regression analysis with the presence or absence of prefrailty as the dependent variable. A univariable analysis was conducted in which each survey item was individually entered as an independent variable, and a multivariable analysis was conducted in which all independent variables were entered. The statistical package SAS ver9.4 (SAS Institute Inc., Cary, NC, USA) was used for statistical analysis, and the significance level was 5%.
4. Ethical considerations
The research was explained to the participants and their written informed consent was obtained. The study was approved by the ethics review committee of Teikyo University School of Medicine (Teirin 21–129). The study was carried out in accordance with relevant guidelines and regulations, including the Declaration of Helsinki and the ethical guidelines for epidemiological research, published in collaboration with the Ministry of Education, Culture, Sports, Science and Technology and the Ministry of Health, Labour and Welfare, Japan.