vitamin D deficiency

Laura is a 40-year-old engineer of eastern European ancestry who presents to your office to request testing of her vitamin D levels. She generally wears sunscreen in the summer and wears sun-protective clothing year round. She is fairly healthy, eating a balanced diet high in fresh fruits and vegetables, but experiences migraines for which she takes triptans for rescue about 4 times per year. She and her partner own a house in a middle-class neighbourhood where they live with their son. Laura informs you that her mother has been taking vitamin D supplements for many years based on her doctor’s recommendations to improve her bone health. Additionally, Laura tells you that her neighbour, a nurse about her age, recently had her vitamin D levels checked and they were “very low,” so she has considered starting vitamin D supplementation.

Laura is hoping that by testing her vitamin D levels she will know how much supplementation she needs to take to protect her from COVID-19, cancer, and fractures as she gets older. Laura is worried that her levels might be low because she avoids direct sunlight and is concerned that she may be missing an easy opportunity to improve her health with supplementation she can afford.

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.