Novel anthropometric indices are superior adiposity indexes to portend visual impairment in middle-aged and older Chinese population

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.

Methods

Results

In total, 7750 participants aged 45 years or older were deemed eligible for the cross-sectional analysis in the current study from CHARLS 2011, among which 4133 accomplished the following two interviews in 2013, and 2015 were adapted in the longitudinal analyses (figure 1). The baseline demographic and socioeconomic characteristics, medical conditions, lifestyle-related factors and six adiposity indexes of the study sample were grouped by visual status and shown in table 1. Participants free from VI appeared to have a lower status of all six adiposity indexes.

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We first ran a univariate logistic regression to indicate potential confounders before performing a multivariate regression analysis. We discovered that gender, age, marital status, educational level, living area, drinking habit and smoking status would probably confound the correlation between obesity index and visual status (online supplemental table 1). To our surprise, according to univariate logistic regression, we first noticed that the conventional adiposity index, BMI, was not positively associated with VI (OR: 0.994, 95% CI 0.979 to 1.009, table 2). The other five novel adiposity indexes showed profound and positive associations with VI (all p values <0.0005, table 2). To further clarify these cross-sectional associations between adiposity indexes and VI, we analysed their relevance by controlling for various covariables (model 1 to model 5, table 3, complete version with 95% CI shown in online supplemental table 2). By this point, only WWI showed significant and detrimental impacts on visual status after being adjusted for multiple confounders (table 3, model 1–4, all p values <0.05). Even after adjustment of BMI, such correlation remained robust (table 3, model 5, p=0.0538).

We first tested the associations between time-varying WWI and VI over time in longitudinal observation. Consistent with cross-sectional analyses, we found that WWI was still significantly associated with VI, even after being adjusted for various confounders, including the BMI (table 4, all p values <0.0001). Moreover, WWI exhibited a robust dose–response association with the prevalence of VI since the OR values increased linearly with the increasing quintile of WWI, even after adjustment (p value for linear trend <0.0001) (online supplemental figure 1). In the fully adjusted model (model 5), including the BMI as one of the confounders, the OR (with 95% CI) of VI associated with the second, third, fourth and fifth (vs first) WWI quantile was 1.228 (1.000 to 1.508), 1.330 (1.070 to 1.653), 1.578 (1.275 to 1.954) and 1.900 (1.407 to 2.565), respectively.

Discussion

In the current population-based cross-sectional and longitudinal study of a nationally representative sample of middle-aged and older Chinese adults, we found that novel anthropometric indices, including WHtR, WWI, ABSI, BRI and ConI, were associated with increased prevalence of VI. Among these, WWI exhibited a robust correlation with the prevalence of VI, which indicated that WWI could be considered an independent risk factor of VI. In contrast, other adiposity indexes were not associated with VI after adjustment of demographics, socioeconomic, lifestyle-related factors, chronic health conditions and even the conventional adiposity index, BMI. Notably, WWI also exhibited a robust dose–response association with the prevalence of VI according to longitudinal observation across 5 years, which indicated WWI as a superior adiposity index to portend VI in the ageing Chinese population than other adiposity indices. To the best of our knowledge, the current study is the first population-based study to perform horizontal comparisons of associations between various adiposity indices and the prevalence of VI among the ageing population.

There are two main subtypes of body obesity: general and central obesity. Although BMI is the most widely used assessment and classification of general obesity in epidemiological and clinical studies,41 it cannot capture the distribution features of body fat, especially in senior adults.12 Since ageing contributes to distasteful changes in body fat distribution, researchers have long considered that BMI may not be a reliable and representative indicator for body adiposity in the ageing population. On the other hand, BMI has also long been criticised for the limitation of distinguishing the amounts of fat-free mass (lean mass) and fat mass, which leads to severe misclassification of individuals with excess body fat or with some chronic conditions such as diabetes.42 Consequently, the BMI-based obesity paradox could probably bias the actual relationship of adiposity with VI in the ageing population. Although such a paradox could be found according to previous studies, limited concern has been attached to further investigation.22–24 Based on this nationally representative survey and our results of higher BMI associated with decreased prevalence of VI in ageing Chinese adults, we propose that the conventional adiposity index, BMI, may not be suitable to denote body adiposity in VI-related surveys.

WWI was recently developed to eliminate the obesity paradox of BMI for mortality by adjusting waist circumstances for body weight.33 Previous studies have indicated its better correlations to cardiometabolic morbidity and related mortality.33 43 According to the present study, we also discovered the linear association of WWI with the prevalence of VI in the ageing Chinese population. Our results proved that WWI was superior to the general adiposity index (eg, BMI) when examining their associations with the prevalence of VI. On the other hand, WWI also exhibits more robust correlations to VI compared with other abdominal obesity indexes (eg, ABSI, WHtR). This may be explained by the fact that, along with its ability to assess fat and muscle mass reciprocally,44 WWI also correlates to bone mass in older adults,45 so WWI has been considered an excellent anthropometric index of a comprehensive evaluation of body composition, which could echo the changes in body composition by ageing. Abdominal obesity has been proven to correlate to vision-threatening ocular disorders, including diabetic retinopathy, age-related macular degeneration and the risk of glaucoma, which are prevalent in the ageing population. Greater body fat deposition is associated with narrower retinal arterioles and broader retinal venules,46 47 and changes in retinal vascular calibre have been proven to correlate with retinal senescence, diabetic retinopathy and age-related macular degeneration.48–50 On the other hand, higher WWI could also indicate osteopenic obesity and sarcopenic obesity. However, these concepts have been barely investigated or noticed in the ophthalmic area. Therefore, our findings may highlight the essential roles of abdominal obesity, excessive fat mass and changes in body composition in visual dysfunction among the ageing population, which will light future exploration of such detrimental associations.

Compared with WWI, ABSI is further adjusted by height in the denominator. Similarly, other adiposity indexes, including WHtR/BRI/ConI, were also derived from waist and height. According to our statistical work, although these indexes showed significance with VI in univariate regression, such associations were less robust than WWI-VI associations after the adjustment of confounders. Compared with body weight and BMI, body height may have a stronger correlation to axial length,50 and longer axial length is apparently associated with moderate to high myopia and ocular complications, which may lead to VI among the older population.51–54 Previous studies also reported a direct correlation between body height and VI among younger adults.55 From a statistical view, owing to the fact that other adiposity indexes were further adjusted by height in the denominator and had positive correlations with VI, the statistical power may consequently be attenuated by the body height in the denominator. Accordingly, we failed to observe significant correlations between these adiposity indexes and VI in the current study. Such a hypothesis warrants investigation in future exploration.

Conclusion

In conclusion, we propose a novel adiposity index, WWI, as a valuable and reliable alternative marker of obesity, which exhibits a robust dose–response association with VI in the ageing Chinese population. Such a finding may eliminate the BMI-based obesity paradox from epidemiological studies in the ophthalmic area. The WWI-VI correlation indicates the potential roles of abdominal obesity, excessive fat mass and changes in body composition in VI among the ageing population, which shed light on future exploration of such detrimental associations.