Seasonal variation in natural abundance of ²H and¹⁸O in urine samples from rural Nigeria

The doubly labeled water (DLW) method is used to measure free-living energy expenditure in humans. Inherent to this technique is the assumption that natural abundances of stable isotopes ²H and ¹⁸O in body water remain constant over the course of the measurement period and after elimination of the loading dose of DLW will return to the same predose level. To determine variability in the natural abundances of ²H and ¹⁸O in humans living in a region with seasonal shifts in rain patterns and sources of drinking water, over the course of 12 mo we collected weekly urine samples from four individuals living in southwest Nigeria as well as samples of their drinking water. From ongoing regional studies of hypertension, obesity, and energy expenditure, we estimated average water turnover rate, urine volumes, and sodium and potassium excretion. Results suggest that ²H and ¹⁸O in urine, mean concentrations of urinary sodium and potassium, urine volume, and total body turnover differed significantly from dry to rainy season. Additionally, seasonal weather variables (mean monthly maximum temperatures, total monthly rainfall, and minimum relative humidity) were all significantly associated with natural abundances in urine. No seasonal difference was observed in drinking water samples. Findings suggest that natural abundances in urine may not remain constant as assumed, and studies incorporating DLW measurements across the transition of seasons should interpret results with caution unless appropriate doses of the tracers are used.