Seasonal rhythm of serum 25-hydroxyvitamin D in a tropical monsoon climate: Evidence for behavioral modulation of a seasonal biological marker.

Serum 25-hydroxyvitamin D [25(OH)D] exhibits a seasonal pattern in temperate populations, typically peaking in late summer and declining in winter. It is still not clear whether this pattern also exists in tropical monsoon climates. To characterize the annual pattern of 25(OH)D in a tropical megacity and identify environmental synchronizers of its seasonal variation, a laboratory-based retrospective cross-sectional study was performed on 1,202 adult outpatients from Dhaka, Bangladesh. Serum 25(OH)D was measured with chemiluminescence immunoassay method. Thirty‑day moving averages of weather variables UV Index, temperature, relative humidity, cloud cover, and AQI were linked to each specimen date. Multivariable linear and logistic regression was performed. A phase shifted seasonal l rhythm was observed: 25(OH)D peaked in December (22.96 ng/mL) and declined in July (12.94 ng/mL). Deficiency prevalence doubled during monsoon months (May-August, OR 2.03-2.27 vs. January). Humidity was negatively associated with 25(OH)D (β = -0.764, p = 0.005), while cloud cover showed a positive association (β = 0.267, p = 0.015). UV index did not independently predict 25(OH)D after adjustment for calendar month. Age was protective against deficiency (OR = 0.975 per year, p < 0.001). In tropical monsoon Dhaka, the seasonal rhythm of 25(OH)D is phase-shifted relative to temperate populations, with summer nadir and winter peak. This pattern may be better explained by behavioral responses to heat and humidity than by UV availability alone. Chronobiological models of seasonal Vitamin D rhythms require climate-specific parameters.