Everyone asks the same question in November, usually right after the first grey morning: how much vitamin D do I actually need? I asked it myself after a blood test came back at 21 ng/mL, which the lab flagged as low-normal without explaining what that meant in practice. Turns out the answer depends on your age, your skin, where you live and whether you can even get outside before dark.
Vitamin D isn’t really a vitamin. Your skin makes it when UVB light hits cholesterol there, and the liver and kidneys then convert it into calcitriol, the hormone form your body uses. That’s why food alone rarely fixes a shortfall, and why the numbers on a supplement bottle matter more than the marketing on the front.
The Official Numbers, and Where They Stop
The NIH Office of Dietary Supplements sets the Recommended Dietary Allowance at 600 IU (15 mcg) per day for adults up to age 70, and 800 IU (20 mcg) after 70. The Tolerable Upper Intake Level, the point past which nobody should wander without a reason, is 4,000 IU a day for adults. Those two figures bracket almost every honest conversation about dosing.
| Life stage | RDA | Upper limit |
|---|---|---|
| Infants 0-12 months | 400 IU | 1,000 IU |
| Children 1-13 years | 600 IU | 2,000 IU |
| Teens 14-18 years | 600 IU | 4,000 IU |
| Adults 19-70 years | 600 IU | 4,000 IU |
| Adults 71+ years | 800 IU | 4,000 IU |
| Pregnant or breastfeeding | 600 IU | 4,000 IU |
Here’s the part the table doesn’t tell you. The RDA is the amount that covers about 97 to 98 percent of a healthy population, not the amount that corrects a measured deficiency. If your serum 25-hydroxyvitamin D comes back at 18 ng/mL, 600 IU is maintenance, not treatment. Clinicians commonly use higher short-term doses in that situation, then step back down.
And skip the megadosing reflex. More is not better past the upper limit; vitamin D is fat-stored, and the toxicity reports that exist almost all involve years of very high intake, not a single forgotten week. For related context on how to choose forms and doses across supplements generally, we compared magnesium glycinate vs citrate vs oxide using the same logic.
Reading Your Blood Test Without Panic
Serum 25-hydroxyvitamin D is the number to ask for. Four ranges do most of the talking:
- Below 12 ng/mL – deficient, by every definition, and usually symptomatic.
- 12 to 20 ng/mL – insufficient. This is where most indoor workers in northern latitudes land in winter.
- 20 to 50 ng/mL – sufficient for bone health in the general population.
- Above 100 ng/mL – the zone where toxicity symptoms start showing up, and only from intake, never from sun.
My own 21 ng/mL was technically fine on paper and still explained a lot of the tiredness I’d been blaming on sleep. That’s anecdote, not evidence, and I’m flagging it as such. The evidence is that deficiency is common, that testing is cheap, and that guessing is worse than measuring. If fatigue is your main complaint, our piece on why you wake up tired even after eight hours of sleep covers the other common causes.
D2, D3, and the Sunlight Math
Two forms sit on the shelf. Ergocalciferol (D2) comes from fungi and fortified milk. Cholecalciferol (D3) comes from lanolin, fish liver oil, and your own skin. D3 raises and holds serum levels better than D2 at the same dose, which is why most clinicians reach for it. Prescription high-dose formulations are usually D2, and that’s fine when the dose is large enough to matter.
Can you skip the bottle entirely? Sometimes. Midday summer sun on arms and legs for 10 to 15 minutes produces a few thousand IU in fair-skinned people, but that window closes fast above the 37th parallel from October to March. Sunscreen, glass windows, darker skin, and age all cut production. I work next to a south-facing window and get essentially nothing from it, which is exactly what the literature predicts.
A Sensible Way to Start
Three steps, in order:
- Get the 25-hydroxyvitamin D test. Ask your doctor, or use a validated at-home finger-prick kit.
- If you’re above 20 ng/mL, take the RDA: 600 IU under 70, 800 IU over it. Take it with a meal containing fat, since absorption roughly doubles.
- If you’re below 20 ng/mL, work with someone on a repletion plan, then retest in eight to twelve weeks instead of staying on a high dose forever.
Two things to hold onto. The dose figures above come from the NIH ODS vitamin D fact sheet, which is the reference clinicians actually use, and the sufficiency threshold of 20 ng/mL is a bone-health consensus rather than a performance target. If someone promises you more than that from a single capsule, ask which population the number came from.
Who Should Be More Careful
Some bodies handle this differently. People with darker skin synthesise less vitamin D from the same sunlight, which is why deficiency rates are several times higher in darker-skinned indoor populations living at the same latitude. Older adults lose roughly three-quarters of their skin’s production capacity compared with someone in their twenties. People with coeliac disease, Crohn’s, or who’ve had bariatric surgery absorb less of it from food and capsules alike. And body fat matters in both directions: vitamin D is stored in fat tissue, so people with higher body fat often need a higher daily dose to move the same blood number, which is the detail most dosing charts skip.
How often to test? Once a year is plenty if you’re stable and replete. Test again eight to twelve weeks after any dose change, then settle back into the yearly rhythm. Repeated monthly testing while a dose is unchanged tells you nothing new and costs money.