You probably know the advice to step outside and catch some midday sun for your vitamin D.
Most of us think of it as a simple summer habit. Where you live and the time of year can decide whether it actually works.
According to the NIH Office of Dietary Supplements, around 5 to 30 minutes of sun is enough for most people.
Hit the face, arms, hands and legs, roughly between 10 a.m. and 4 p.m., either daily or at least twice a week without sunscreen.
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That guidance only holds when the sun is strong enough.
A classic study in the Journal of Clinical Endocrinology & Metabolism found that in Boston, at 42.2 degrees north, winter sunlight produced no vitamin D at all from November through February.
In Edmonton, Canada, further north, the vitamin D winter stretched from October through March.
Five things that change your daily sun dose
Your personal need depends on five variables that the NIH highlights. The first is latitude and season.
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The further you live from the equator, and the deeper into winter you go, the less UV-B reaches your skin.
Above roughly 40 degrees north, winter sunlight does not drive meaningful vitamin D production, according to the Linus Pauling Institute at Oregon State University.
That band includes Boston, New York and much of the northern United States.
The second is skin tone. Melanin, the pigment that darkens skin, competes with the vitamin D precursor for UV light and acts as a natural sunscreen.
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People with darker skin can need up to ten times as long in the sun to make the same previtamin D as someone with fair skin.
The third is age. Older adults have less of the vitamin D precursor in their skin, so the same ten minutes outside produces less vitamin D at 70 than at 25.
The NIH names older adults as one of the main groups at risk of low vitamin D levels.
The fourth is sunscreen. The NIH notes that sunscreens with SPF 8 or more block most of the UV rays that make vitamin D.
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In practice, people rarely apply enough to cover every patch of skin, so some vitamin D is still made.
The fifth is simply time outdoors. If your work, your clothing or your health keep you indoors, no amount of midday guidance will fix that.
When sun alone may not be enough
The current recommended intake from the NIH is 15 micrograms (600 IU) a day for adults aged 19 to 70, rising to 20 micrograms (800 IU) above 70.
The tolerable upper limit for adults is 100 micrograms (4,000 IU) a day from all sources combined.
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In June 2024, the Endocrine Society issued a new guideline. It suggests healthy adults under 75 do not need more than the recommended intake.
The panel named four groups likely to benefit from higher doses. These are children and adolescents, pregnant people, adults aged 75 and older, and people with prediabetes.
It also recommended against routine blood testing for vitamin D in healthy populations.
For anyone who lives above 40 degrees north, has dark skin, is over 70, or spends most of the day indoors, a daily vitamin D supplement is often the simpler route from fall through early spring.
The right dose is a conversation to have with your doctor, who can weigh any medications and other conditions you have.
This article is made and published by Ida-Marie Palm Varbæk, who may have used AI in the preparation.