The Effect of Oral Loading Doses of Cholecalciferol on the Serum Concentration of 25-OH-Vitamin-D

Background/Objectives: Severe vitamin D deficiency can be treated with oral loading doses of cholecalciferol. Our objective was to develop an algorithm to accurately calculate the amount of cholecalciferol needed for a loading dose, and what factors should be taken into account. Methods: Two studies were conducted on subjects with Vitamin D deficiency. Study 1 was observational, retrospective and included 88 subjects treated with a daily supplementation of cholecalciferol. 60 of these furthermore received a loading dose, calculated by an algorithm. Study 2 was prospective and included 29 subjects treated with a cholecalciferol loading dose, calculated by an algorithm developed based on data from study 1, which included BMI. Results: Baseline 25OH-vit.D was below 25 nmol/L (study 1) and 23 nmol/L (study 2). Subjects were given a single loading dose of cholecalciferol, averaging 172,000 IU (study 1) and 212,000 IU (study 2), based on their baseline 25OH-vit.D level. 25OH-vit.D increased by 35 nmol/L (study 1) and 56 nmol/L (study 2)(range 113.0, SD 29.79) respectively. In study 2 the increase lead to an end 25OH-vit.D of 79 nmol/L - not significantly different from the target value of 80 nmol/L (P = 0.46). The increase in 25OH-vit.D in study 1 was significantly lower than in study 2 (P<0.001). Conclusion: When calculating loading doses of cholecalciferol, taking subject BMI into account gives a better estimate of the loading dose of vitamin D3 needed to treat vitamin D deficiency. It does not, however, remove the large interindividual variation in dose-response.