Vitamin D and chronotype: is there any relationship in individuals with obesity?

Purpose: Chronotype is the attitude to perform most of the daily activities in the morning ("morning chronotype", MC) or in the evening ("evening chronotype", EC). The latter, as well as vitamin D deficiency, has been associated with an increased risk of obesity-related complications, likely through the promotion of insulin resistance. Therefore, we aimed to investigate whether there is any relationship between vitamin D (25-OH-D) and chronotype in individuals with obesity. Methods: In this cross-sectional study, we enrolled 59 individuals (43.1 ± 13 years; 17M/42F) with obesity. Anthropometric parameters, lifestyle habits, personal medical history, chronotype, insulin resistance, and 25-OH-D were assessed. Results: Individuals with EC presented significantly higher BMI than MC (p < 0.001), greater waist (p = 0.012), and hip circumferences (p = 0.001). Individuals with EC showed significantly lower insulin sensitivity (p = 0.017) and 25-OH-D than MC. In addition, the prevalence of vitamin D deficiency and impaired fasting glucose was significantly higher in EC than in MC. 25-OH-D directly correlated with chronotype score (r = 0.351; p = 0.019) whereas inversely with BMI (r = - 0.363; p = 0.016). The regression analysis showed that BMI was most tightly associated with 25-OH-D concentrations (β = - 0.323, p = 0.032), followed by chronotype score (β = 0.340, p = 0.042). Using chronotype score as the dependent variable, BMI significantly predicted a lower chronotype score (β = - 0.586, p < 0.001). Conclusion: The present study showed that 25-OH-D, as well as chronotype, correlate independently with obesity.