Obesity may extend the time required to reach a steady-state 25(OH)D level after initiating vitamin D supplementation

Obesity is known to influence the circulating 25(OH)D level but less is known about whether it influences the time required to reach a stable 25(OH)D level after initiating vitamin D supplementation. This observational study was done to investigate whether BMI modified the time required to reach a steady-state 25(OH)D level in response to vitamin D supplementation. Participants in the Boston STOP IT study who were treated for 12 mo with 700 IU of vitamin D₃ and 500 mg of calcium daily and had 25(OH)D measures at 0, 6, and 12 mo, were included. We assessed the trajectory of 25(OH)D levels by baseline BMI category (normal weight, BMI 18.5-24.9 kg/m², n = 62; overweight and obese, BMI ≥ 25 kg/m², n = 105). Baseline 25(OH)D levels were 78 ± 31.3 nmol/L (normal weight) and 74.7 ± 36.5 nmol/L (overweight and obese). In a linear mixed model examining the influence of time and baseline BMI category on change in the mean 25(OH)D level, there was a significant time x BMI group interaction (p = .024. The normal weight participants had reached steady-state 25(OH)D levels by 6 mo whereas 25(OH)D levels continued to rise between 6 and 12 mo in the overweight and obese participants. This analysis suggests that the time required to reach a steady-state 25(OH)D level after initiating vitamin D supplementation in overweight and obese adults is greater than the usual 3-mo time point commonly used in clinical practice. A more refined definition of the time course of circulating 25(OH)D response to supplementation is needed in overweight and obese individuals in order to optimize clinical monitoring of vitamin D status.