Participants
Data for this study were obtained from the China Hainan Centenarian Cohort Study (CHCCS),[9] the centenarians survey with the largest sample size till date. It was a full-sample survey of centenarians conducted in Hainan Province, China between 2014 and 2016 that included a total of 1,002 eligible centenarians whose age had been verified. Trained investigators conducted household surveys to collect a variety of information from research subjects. Questionnaire-based interviews, anthropometric measurements (height, weight, waist circumference, hip circumference, calf circumference, blood pressure, etc.), and collection of fasting blood samples for biochemical index testing were completed by systematically trained local nurses (local language speakers that had barrier-free communication) in Hainan.
Definitions and assessment criteria
Age was calculated by subtracting the date of birth from the survey date. The date of birth was verified using information obtained from participants’ identity (ID) cards and that provided by the Civil Affairs Bureau and the Public Security Bureau. The term ‘centenarians’ refers to subjects who were 100 years of age or older at the time of the survey. Height and weight were measured by local trained nurses in Hainan using a physician weighing scale for the human body.[9]The elderly interviewees were required to take off their shoes, caps, and coats; and keep aside personal belongings such as keys and mobile phones while the measurements were being taken. Height was rounded to the nearest 0·5 cm, and an error of less than 0·5 kg was permitted between two consecutive weight measurements. Body Mass Index (BMI) was calculated by dividing the weight in kilograms by the square of the height in meters. Multimorbidity was defined as having two or more chronic diseases simultaneously.[10]
The nutritional status of the centenarians was evaluated using the Mini Nutritional Assessment – Short Form (MNA-SF) questionnaire, consisting of six components: appetite loss, weight loss, mobility, stress/acute disease, dementia/depression, and BMI/CC.[11] Out of a total of 14 possible points, a score of 12–14 points indicated good nutritional status, 8–11 points indicated being at risk of malnutrition, and ≤7 points indicated malnutrition. Higher individual and overall MNA-SF scores indicated better nutritional status.
Quality of life was measured using the EuroQol five dimensions visual analog scale (EQ-5D-VAS) instrument.[12] The EQ-5D is a widely used, validated questionnaire that includes five health dimensions (mobility, self-care, daily activities, pain/discomfort, and anxiety/depression); and each dimension contains three levels: no difficulty, some difficulties, and extreme difficulties. The Visual Analogue Scale (VAS) is a self-assessment tool where respondents self-assess their own health status on a 20 cm vertical scale. The top 100 points indicate the best health status, whereas the bottom zero points indicate the worst health status. Systematically trained local nurses in Hainan conducted interviews and collected information on EQ-5D and VAS, and used the Japanese population time trade-off (TTO) model to calculate the health indices, since previous studies had found that this model was currently the most suitable tool for use in Chinese people.[13, 14] The scores generated by this conversion table ranged from −0·11 to 1·00. An EQ-5D abnormality with a score of less than 1 in any dimension was defined as impaired health-related quality of life. If the score based on the five dimensions of mobility, self-care, daily activities, pain/discomfort, and anxiety/depression was at the no difficulty level, then the quality of life dimension was defined as normal; if not, it was defined as low.
The covariates included in this study comprise gender, ethnicity (Han, Li, and other minorities), marital status (married, widowed, and others), educational level (illiterate, elementary, and middle school and above), residential type (living with family and living alone), smoking status (non-smoker, previous smoker, and current smoker), drinking status (non-drinker, previous drinker, and current drinker) and physical activity level (low, middle, and high). Data were collected using household surveys conducted by trained Hainan local nurses who were local language speakers and had barrier-free communication.
Statistical analyses
The research data were assembled into the software database Epidata 3·1 using double entry. The variable representing nutritional status was operationalized as both total MNA-SF score and as a three-level categorical variable. Continuous variables were described in terms of mean ± standard deviation, and differences between nutritional status groups were compared using the independent sample t test. Categorical variables were described in terms of frequencies and percentages [n (%)], and the chi-square test was used to compare group differences. In addition, univariate and multivariate logistic and linear regression models were used to analyze the association between nutritional status and quality of life (measured by the EQ-5D-VAS score). The MNA-SF score (the higher the score, the better the nutritional status) and the three nutritional status groups (considering normal nutritional status as the reference group) were added as independent variables to the linear regression model that adjusted for demographic characteristics (gender, age, ethnicity, marital status, education levels, and residential type) and lifestyle variables (smoking, drinking, and physical activity). Each test of hypothesis was two-sided, with P <0·05 as the cutoff for statistical significance. All statistical analyses were performed using the SPSS 22·0 software package.
Ethics
The CHCCS was conducted in accordance with the Declaration of Helsinki and was approved by the Medical Ethics Committee of the Chinese PLA General Hospital (301hn11-206-01). All participants provided written informed consent before joining the study.