Association between body mass index and myopia in the United States population in the National Health and Nutrition Examination Surveys 1999 to 2008: a cross-sectional study

Myopia is described as light rays entering the eye parallel to the optic axis and coming into focus in front of the retina when the ocular accommodation is relaxed [1]. The prevalence of myopia in the United States increased from 25% in 1971–1972 to 41.6% in 1999–2004 [2]. Two reports from China show that the prevalence of myopia among adolescents is 63.1% and 84.8%, and the incidence of high myopia reaches 9.4% and 19.3%, respectively [3, 4]. According to the forecast, the prevalence of myopia will reach 49.8% in 2050, and the prevalence of high myopia will reach 9.8% [5]. Not only do myopic patients suffer from decreased vision, but they are also at significant risk of developing complications, such as myopic macular degeneration, retinal detachment, open-angle glaucoma, and cataracts, which can severely reduce the quality of life [6, 7]. Globally, myopia has become a major public health concern.

Both genetic and environmental factors have an impact on the development of myopia [8‐10]. Years of education [11] and hours of outdoor activity [12‐14] have been shown to have a strong causal association with myopia and inflammation [15, 16] also correlates with myopia. Then, late sleep [17] and high glycaemic load carbohydrate diets [18] may be potentially associated with myopia. According to the carbohydrate–insulin model of obesity, a high glycaemic load carbohydrate diet leads to an increase in adipose tissue and a greater tendency to become obese [19‐21]. Obese individuals tend to have a higher body mass index (BMI) compared to non-obese individuals. However, the results of existing studies on the association between BMI and myopia are inconsistent and mostly focused on Asian populations. Some studies have associated high BMI with myopia [22, 23], but others have linked it to low BMI [24] or have not found an association between both [25, 26].

Therefore, participants in the National Health and Nutrition Examination Survey (NHANES) database from 1999 to 2008 were selected to explore the association between BMI and myopia in the United States population.

Among 8000 participants with a mean age of 16.9 years, 3149 (39.4%) were diagnosed with myopia B (spherical equivalent ≤ −0.75 D). The baseline characteristics are presented in Table 1. The p values for the mediating effect of physical activity on myopia A/B/C were 0.2189/0.184/0.1856, respectively. The results of the multivariate logistic regression analysis of BMI and myopia are presented in Table 2. The trend was the same for all three myopia diagnostic criteria. BMI was correlated with myopia, with each 1 kg/m² increase in BMI associated with a 1% increase in the risk of myopia (OR 1.01; 95% CI 1.01 1.02; p < 0.05). In myopia B (spherical equivalent ≤ −0.75 D), compared with the reference group (BMI 18.5–24.9), participants with a BMI of 25–29.9 and greater than 29.9 had a 14% and 25% increased risk of myopia, respectively (OR 1.14; 95% CI 1.01 1.29; p = 0.037, OR 1.25; 95% CI 1.08 1.44; p = 0.003), which was similar to the results for myopic A (spherical equivalent ≤ −0.5 D, OR 1.15; 95% CI 1.02 1.3; p = 0.027, OR 1.19; 95% CI 1.03 1.37; p = 0.018) and myopia C (spherical equivalent ≤ -1 D, OR 1.15; 95% CI 1.01 1.31; p = 0.035, OR 1.18; 95% CI 1.01 1.37; p = 0.032) in model 3.Smooth curve fitting showed a linear relationship between BMI and myopia B (p for nonlinearity = 0.767, Fig. 2).

In our cross-sectional study that included 8,000 individuals, there was a linear relationship between BMI and myopia (OR 1.01; 95% CI 1.01 1.02; p < 0.05). In a multifactorial analysis, participants with a BMI of 25–29.9 and greater than 29.9 had a 14% and 25% increased risk of myopia (spherical equivalent ≤ 0.75 D), respectively (OR 1.14; 95% CI 1.01 1.29; p = 0.037, OR 1.25; 95% CI 1.08 1.44; p = 0.003), and the trend remained unchanged when the diagnostic threshold was changed to −0.5D (OR 1.15; 95% CI 1.02 1.3; p = 0.027, OR 1.19; 95% CI 1.03 1.37; p = 0.018)or −1 D(OR 1.15; 95% CI 1.01 1.31; p = 0 0.035, OR 1.18; 95% CI 1.01 1.37; p = 0.032).The current focus of myopia and BMI research is on Asian children and adolescents. A cross-sectional study that included 1,359,153 Israeli adolescents aged 16–19 years who underwent medical examinations before mandatory military service showed that the BMI was a j-shaped pattern presented in the form of a bar chart for adolescent myopia and both higher and lower BMI were associated with a higher risk of myopia [22]. The results of our study are linear and presented in the form of smooth curve fitting. In the U.S. population, lower BMI did not appear to be associated with myopia, whereas higher BMI similarly had a higher OR. Similarly, a Korean cross-sectional study that investigated 24,269 participants aged 5–18 years in the KNHANES VII database from 2016 to 2018 found an association between obesity and high myopia in childhood and adolescence and an association between overweight and high myopia in girls [23]. According to the results provided in Table 2, there is not enough evidence to show that BMI and myopia are related, which is inconsistent with our results, and the different populations selected may be the source of the difference. A study using the 2003–2008 NHANES database of 6,855 participants aged 12–25 years showed that BMI was not associated with myopia (R2 = 0, P = 0.79) [26]. Three possible differences that may have contributed to the different results are as follows: first, the association between BMI and myopia was evaluated using multivariate analysis and adjustment was made with physical activity considered as one of the confounding factors; second, in addition to being analyzed as a continuous variable, BMI was also analyzed in groups. The OR of different groups reached different conclusions. Thirdly, the population from 1999 through 2008 was included, and differences in sample size (8,000) may have contributed to this difference. In a study of 19-year-old male consignors in Seoul, Korea, no association was found between myopia and BMI according to quartiles and by logistic regression [31]. In our study, a higher BMI appeared to favor a risk factor over a protective factor. Differences in population selection may account for the differences in results.

The association between BMI and myopia is affected by underlying lifestyle factors. One review found an association between decreased time spent outdoors and worsening myopia by studying children during COVID-19-induced lockdown [32]. Children who spend more time outdoors may have a lower BMI than their peers [33]. This may be a reasonable explanation for our linear results. After mediating role analysis, it was found that physical activity did not mediate the association between BMI and myopia. It may be one of the confounding factors affecting the results, so we included it in the multivariate analysis to adjust for it.

There are some limitations to this study. First, since the study design was cross-sectional, it was not possible to determine a causal relationship between BMI and myopia. In addition, measurement of refractive error in the absence of cycloplegia increases the prevalence of myopia. Therefore, three mainstream myopia diagnostic criteria were adopted to ensure the reliability of the results. Third, this study did not take into account the impact of air pollution and regional differences as confounding factors and further research is needed to explore this aspect. Fourth, due to the fact that BMI does not consider muscle mass, bone density, overall body composition, or racial and sex differences in lifespan, and also because children’s standards for obesity differ from those for adults, this study can only conclude an association between BMI and myopia, rather than a relationship between obesity and myopia. Fifth, NHANES only provided refractive data from 1999 to 2008, which may be outdated (> 15 years), and newer surveys are still needed to reflect current refractive errors, diet, exercise, or lifestyle.

In our study, myopia using all three diagnostic thresholds was positively associated with higher BMI. This suggests a potential association between myopia and higher BMI in the American population, warranting further investigations.