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Vitamin D
What Vitamin D is, what the NIH ODS fact sheet says the evidence supports, typical doses, timing, and the cautions that come with it.
Supplements are not medicines and this page is not medical advice. Doses that help one person can harm another — with medicines, kidney or liver conditions, pregnancy, or before surgery, talk to a clinician before starting anything. More is not better.
What it is
A fat-soluble vitamin the skin makes in sunlight; needed for calcium absorption and bone.
What the evidence supports
ODS: prevents and treats deficiency; with calcium it reduces fracture risk in older adults. Beyond bones, evidence is mixed.
Dose and timing
RDA: 600 IU (15 mcg) to age 70, then 800 IU (20 mcg). UL 4000 IU. With a meal containing fat; it is fat-soluble.
Cautions
High doses without a deficiency have no proven benefit. Blood level testing is the way to know.
Interactions are the hidden risk: supplements can change how medicines work and vice versa. Give your clinician the full list before anything else.
Source: NIH Office of Dietary Supplements — Vitamin D fact sheet · checked 2026-09-03
Frequently asked questions
What does Vitamin D do?
A fat-soluble vitamin the skin makes in sunlight; needed for calcium absorption and bone.
Does Vitamin D work?
ODS: prevents and treats deficiency; with calcium it reduces fracture risk in older adults. Beyond bones, evidence is mixed.
How much vitamin d should I take?
RDA: 600 IU (15 mcg) to age 70, then 800 IU (20 mcg). UL 4000 IU. With a meal containing fat; it is fat-soluble.
Is Vitamin D safe?
High doses without a deficiency have no proven benefit. Blood level testing is the way to know. And the standing rule: tell your clinician everything you take — supplements interact with medicines.
Creatine
Studied doses: 3-5 g per day; loading protocols use about 20 g/day in 4 doses for 5-7 days.
Magnesium
RDA: men 400-420 mg, women 310-320 mg per day.
Omega-3s (EPA/DHA)
No RDA for EPA/DHA; ALA: men 1.6 g, women 1.1 g per day. Many products supply 250-500 mg combined EPA+DHA per day.
Melatonin
Studied doses commonly 0.5-5 mg, taken 30-60 minutes before bed.
Iron
RDA: men 8 mg, women 19-50 18 mg, pregnancy 27 mg per day.
Vitamin B12
RDA: 2.4 mcg per day.
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