Vitamin D for Vitamin D Deficiency

This is one of the few pairings where the case is close to settled, because the supplement and the problem are the same molecule. Oral vitamin D3 reliably raises serum 25-hydroxyvitamin D, corrects deficiency, and prevents the skeletal consequences of severe deficiency: rickets in children and osteomalacia in adults. What vitamin D does not reliably do is the long list of things it has been trialled for in people who are already replete. VITAL, a trial of over 25,000 adults, found no reduction in fractures with 2,000 IU daily in a largely vitamin D sufficient population, and pooled analyses of bone mineral density show little effect from supplementation alone. So the useful framing is narrow and firm: test, treat the deficiency, retest, then maintain. Correcting a real deficiency is worth doing. Taking large doses on the assumption of broad benefit is not supported.

Cholecalciferol, whether made in skin from UVB exposure or swallowed, is hydroxylated in the liver to 25-hydroxyvitamin D, the storage form measured on lab tests, then hydroxylated again in the kidney to calcitriol, the active hormone. Calcitriol binds the vitamin D receptor and increases intestinal calcium and phosphate absorption from roughly 15% to as much as 40%. When 25-hydroxyvitamin D is low, calcium absorption falls, parathyroid hormone rises to defend serum calcium, and that PTH pulls calcium from bone. This is why a raised PTH alongside a low vitamin D is the biochemical signature of deficiency that actually matters to the skeleton, and why correcting the vitamin lowers PTH within weeks. Two practical details follow from the biochemistry. Vitamin D is fat soluble, so absorption improves substantially when taken with a meal containing fat. And magnesium is a required cofactor for both hydroxylation steps, which is why magnesium status influences how well a given dose raises your level.

Two questions have clear answers. First, form: a 2024 meta-analysis in Advances in Nutrition and an earlier 2012 American Journal of Clinical Nutrition analysis both found vitamin D3 raises serum 25-hydroxyvitamin D more effectively than D2 at equivalent doses, so D3 is the sensible default outside vegan constraints. Second, kinetics: bolus dosing studies show that free and total metabolite trajectories differ from daily dosing, supporting steady regimens. The question with a negative answer is what supplementation does beyond repletion. The 2022 VITAL fracture analysis in the New England Journal of Medicine randomised more than 25,000 adults to 2,000 IU daily and found no reduction in total, non-vertebral or hip fractures, in a cohort that was mostly not deficient. A 2014 Lancet meta-analysis similarly found no clinically useful effect on bone mineral density at most sites. A 2019 individual-participant meta-analysis did find a small protective effect against acute respiratory infection, largest in those with the lowest baseline levels. The pattern across all of it is consistent: benefit tracks baseline deficiency.

Toxicity is uncommon but real, and essentially always the result of sustained high-dose supplementation rather than sun or food. It presents as hypercalcaemia, and the doses involved are typically above 10,000 IU daily for months. Staying at or below 4,000 IU daily without blood monitoring keeps you well clear. Specific caution applies in sarcoidosis, tuberculosis and other granulomatous disease, where unregulated activation of vitamin D can cause hypercalcaemia at ordinary doses, and in primary hyperparathyroidism. In chronic kidney disease, the activation step is impaired and a clinician may prescribe an activated analogue instead. In pregnancy, standard prenatal doses of 400-1,000 IU are appropriate; high-dose regimens should be clinician led.