Introduction
Vision impairment (VI) is one of the most devastating morbidities affecting billions of individuals worldwide. As for the ageing population, VI is a widespread sensory dysfunction that could lead to multiple adverse events, both physically and mentally, such as physical dysfunctions, cognitive decline and all-cause mortality.1–4 Along with its consequent accidental injury events like falls,5 VI may also jeopardise social participation and well-being status in the ageing population.6 7 Therefore, identifying probable risk factors for VI and developing easy-to-spread screening tools to discern individuals more prone to VI have been considered pivotal public health approaches to preventing low vision and consequent adverse events in the ageing population.
Obesity has long been considered a risk factor for various adverse events in ageing life.8–10 Among numerous obesity and its comorbidities-related studies, the body mass index (BMI), a popular anthropometric and conventional adiposity index, has been widely accepted and serves as a standard obesity evaluation metric. However, there have been increasing concerns about the ‘obesity paradox’ over the past decades, which refers to when particular chronic diseases, especially in the ageing population, exhibit surprising and ‘paradoxically protective’ associations between BMI and clinical outcomes.11 One principal reason for these obesity paradoxes lies in the natural deficit of BMI. BMI is inadequate in providing information on body fat distribution, especially central obesity, which is a determining factor in predicting metabolic deterioration and obesity-related comorbidities.12
Such obesity paradox also occurs in previous studies, especially population-based epidemiological research in the ophthalmic area. Obesity could exert specific influences on many age-related and vision-threatening ocular diseases, such as age-related macular degeneration, diabetic retinopathy and glaucoma.13–16 Therefore, BMI has been investigated in its association with VI in numerous clinical and epidemiological surveys as a medical parameter or clinical characteristic of body obesity.15 However, studies focusing on BMI-VI association have yielded controversial results.17–25 These widely divergent conclusions not only bring doubts about whether obesity and adiposity indexes could be considered risk factors or screening tools of VI but also challenge whether BMI is applicable to hold its representative role of body adiposity index in VI-related surveys.
Novel adiposity indexes derived from novel anthropometry assessments exhibit excellent correlations with abdominal adipose tissue and have been considered more accurate body adiposity assessments for predicting obesity-related disorders than BMI.26 27 Recent studies also proposed that body composition and fat distribution could more accurately assess poor metabolic characteristics.28 29 Therefore, several novel adiposity indexes have been suggested as alternative indicators of obesity coexisting morbidities, including diabetes, cardiometabolic morbidity and mortality,30–33 with explicit advantages over the conventional BMI. However, the correlations between these novel anthropometric indices and VI occurrence are yet to be investigated. The application of novel adiposity indexes in VI-related population-based and epidemiological surveys still needs to be explored. From our considered perspective, a noteworthy observation in the existing body of literature is the limited attention afforded to the population with central obesity, instead primarily focusing on general obesity.
China is the most populous developing country, and it shares a considerable proportion of the obesity population and the ageing population suffering from VI.34–39 This study acquired data from the China Health and Retirement Longitudinal Study (CHARLS). CHARLS is the first nationally representative and longitudinal survey that aims to provide the most up-to-date cohort datasets for studying the health and well-being status of the middle-aged and older population in the Chinese mainland. With simple calculation using weight, height and waist circumference, six adiposity indices including the BMI, weight-adjusted-waist index (WWI), waist-to-height ratio (WHtR), body roundness index (BRI), a body shape index (ABSI) and conicity index (ConI) were enrolled and investigated in their correlations with VI among middle-aged and older Chinese population from a national level in the current study.