Best CM, Zelnick LR, Thummel KE, et al. - Vitamin D ₃ supplementation raises total serum vitamin D and 25(OH)D concentrations with variation based on baseline 25(OH)D, implying that 25-hydroxylation of vitamin D ₃ is more efficient when serum 25(OH)D concentration is low.

• The sample consisted of 161 patients (mean ± SD age 70 ± 6 years; 66% males) with type 2 diabetes.

• There was a positive, nonlinear relationship between total serum vitamin D and total serum 25(OH)D concentrations at baseline.

• The adjusted effects of supplementation were a 29.2 nmol/L increment in serum vitamin D and a 33.4 nmol/L rise in serum 25(OH)D.

• The serum vitamin D response to supplementation was decreased in individuals with baseline 25(OH)D < 50 compared with those with baseline 25(OH)D ≥ 50 nmol/L, although the serum 25(OH)D response was increased.