Association of Urbanization and Lower Light Exposure With Increased Body Mass Index

Abstract

Introduction: Light/dark cycles are the main synchronizing signal (zeitgeber) that entrain human’s internal clock to the 24h-days. Some aspects of urban environments, including irregular light exposure and weak zeitgebers, influence the circadian organization and thereby may have an impact on metabolism. Comparing communities at different levels of urbanization and with different histories of access to electricity might provide evidence to support associations previously found between disrupted patterns of light exposure and increased populational rates of overweight and obesity. The present study aimed to investigate whether living at a higher level of urbanization would be associated with higher body mass index (BMI). It was hypothesized that BMI is higher in urbanized communities, since their inhabitants have weaker zeitgebers, often associated with disrupted circadian rhythms. Methods: We conducted a cross-sectional study in Quilombolas communities, located in the south of Brazil. Subjects were categorized into 5 groups based on their communities’ stage of urbanization and history of access to electricity: from rural with no access to electricity to highly urbanized communities that have access to the grid. We used data from 134 participants aged 16 - 92 years old (63% women), who had 7 days of light exposure recordings collected using wrist-worn actimeters. We also collected anthropometric data to calculate BMI, which was then categorized as follows: ≥18.5 kg/m² to <25 kg/m² = normal weight; ≥25 kg/m² to < 30 kg-m² = overweight; ≥ 30 kg/m² = obesity. We used Shapiro-Wilk to test for normality, Kruskal-Wallis followed by Dunn to compare BMI between groups and Spearman to assess whether there was an association between patterns of light exposure and BMI. Results: Kruskal-Wallis/Dunn test showed a significant difference in BMI between the urban group and the rural ones (KW: X² = 11.987, p < 0.001). Lower average light exposure between 7 am and 5 pm was significantly correlated with higher BMI (Spearman, r = - 0.296, p < 0.001). Also, higher average light exposure at night (from 1 am to 6 am) was significantly correlated with higher BMI (Spearman, r = 0.256, p = 0.002). Conclusions: Our results support the hypothesis that low amplitudes of light exposure may be a risk factor contributing to the high prevalence of obesity worldwide. Studies have previously shown associations between BMI and social jetlag, suggesting the correlations found in our study may be related to higher levels of circadian misalignment, more often present where zeitgeber strength is lower, as in urban environments. Future research is needed to address causal relationships between light exposure and excessive body mass in humans. Provided light exposure is a risk factor for obesity, these results point to potential new targets for intervention and prevention strategies.