A Predictive Equation to Guide Vitamin D Replacement Dose in Patients

Background: Vitamin D is essential for bone health and probably the health of most nonskeletal tissues. Vitamin D deficiency is widespread, and recommended doses are usually inadequate to maintain healthy levels. We conducted a retrospective observational study to determine whether the recommended doses of vitamin D are adequate to correct deficiency and maintain normal levels in a population seeking health care. We also sought to develop a predictive equation for replacement doses of vitamin D.

Methods: We reviewed the response to vitamin D supplementation in 1327 patients and 3885 episodes of vitamin D replacement and attempted to discern factors affecting the response to vitamin D replacement by conducting multiple regression analyses.

Results: For the whole population, average daily dose resulting in any increase in serum 25-hydroxyvitamin D level was 4707 IU/day; corresponding values for ambulatory and nursing home patients were 4229 and 6103 IU/day, respectively. Significant factors affecting the change in serum concentrations of 25-hydroxyvitamin D, in addition to the dose administered, are (1) starting serum concentration of 25-hydroxyvitamin D, (2) body mass index (BMI), (3) age, and (f) serum albumin concentration. The following equation predicts the dose of vitamin D needed (in international units per day) to affect a given change in serum concentrations of 25-hydroxyvitamin D: Dose = [(8.52 − Desired change in serum 25-hydroxyvitamin D level) + (0.074 × Age) – (0.20 × BMI) + (1.74 × Albumin concentration) – (0.62 × Starting serum 25-hydroxyvitamin D concentration)]/(−0.002). Analysis of the dose responses among 3 racial groups—white, black, and others—did not reveal clinically meaningful differences between the races. The main limitation of the study is its retrospective observational nature; however, that is also its strength in that we assessed the circumstances seen in usual health care setting.

Conclusions: The recommended daily allowance for vitamin D is grossly inadequate for correcting low serum concentrations of 25-hydroxyvitamin D in many adult patients. About 5000 IU vitamin D3/day is usually needed to correct deficiency, and the maintenance dose should be ≥2000 IU/day. The required dose may be calculated from the predictive equations specific for ambulatory and nursing home patients.