Everything you need to know about Vitamin D

For adults with no documented deficiency, 1,000-2,000 IU/day of vitamin D3 is a defensible maintenance range, especially in winter or at higher latitudes. Adults with documented deficiency (25-OH-D below 20 ng/mL) usually need 4,000-5,000 IU/day for 8-12 weeks under monitoring, then a maintenance dose. Avoid bolus dosing schedules; the trial record there is unambiguously negative.

D3 (cholecalciferol) raises and sustains 25-OH-D more reliably than D2 (ergocalciferol). D2 is the vegan/vegetarian option and is still effective, but typical doses must be roughly 2-3 times higher to match D3 outcomes. For non-vegans, D3 is the standard choice.

For most people taking 1,000-2,000 IU/day of D3, a co-administered K2 is not clinically necessary. K2 may matter more at higher D3 doses (5,000+ IU/day chronically) or in people with calcification concerns. The strongest K2 trials use MK-7 at 90-180 mcg/day. If in doubt, take K2 from food (natto, hard cheeses, egg yolks) rather than escalating doses on the supplement side.

Yes. The American Academy of Pediatrics recommends 400 IU/day from the first few days of life through the first year, because human breast milk does not contain enough vitamin D to prevent rickets in exclusively or predominantly breastfed infants. Drops should be precisely measured — overdose cases from concentrated formulations are documented.

When combined with calcium in older adults with low baseline 25-OH-D, yes — vitamin D reduces fracture risk modestly. In replete adults the trial record is much weaker, and high-dose bolus schedules have actually increased falls and fractures. The story is: replete the deficient, do not over-treat the rest.

Routine testing in healthy adults is not recommended by most professional bodies and is rarely covered by insurance. Testing makes the most sense if you have a known risk factor: dark skin, limited sun exposure, malabsorption, glucocorticoid use, post-bariatric surgery, chronic kidney disease, osteoporosis, or unexplained musculoskeletal pain. The target is generally a 25-OH-D level of 30-50 ng/mL (75-125 nmol/L); pushing above 50 ng/mL with supplements has no documented benefit and crosses into the toxicity gradient.

Yes, but it takes either chronic very-high dosing (typically 50,000+ IU/day for months) or accidental concentrated-drop overdoses in infants. Vitamin D toxicity raises calcium, causes nausea, kidney stones, and in extreme cases cardiac arrhythmia. The IOM upper limit for adults is 4,000 IU/day. Stay below that without a clinician-supervised reason to exceed it, and avoid the concentrated infant-drop formulations where a small measurement error becomes a hospital visit.