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Glossary

Vitamin D

Defined in topic 19

The vitamin the skin makes when ultraviolet B light strikes a cholesterol-like molecule, a process melanin slows without stopping. It prevents rickets; in large trials, supplements did not prevent cancer, fractures or heart disease.

Published Updated

Graded claims that mention Vitamin D

  • Is sunscreen bad for you?Against: the one trial runs the other way.Evidence grade: Probable. Good observational evidence with a plausible mechanism, or small or few trials.

Chapters of the book that mention Vitamin D

Where this comes from

In the reference topic that owns this term

Reference topic 19, The Largest Organ, states the mismatch as a card: what the system evolved to expect, what it gets instead, and what happens in the gap.

Mismatch card

What it expects

Sun matched to its pigment, changing gradually with the seasons, mostly outdoors and often in shade.

What it gets

Pale skin under subtropical sun, dark skin at high latitudes, indoor lives and sudden, burning doses.

What happens in the gap

Skin cancer where sun outruns pigment; low vitamin D, and at the extreme rickets, where pigment outruns sun.

The full card, and the topic behind it

What that chapter recommends

3 of the 7 graded recommendations in Reference topic 19, strongest evidence first. How the grades work.

  1. For long exposure, use shade, clothing and a hat, then sunscreen

    StrongEvidence grade: Strong. Multiple randomized controlled trials, or consistent large cohort studies with a well-understood mechanism.

    When the UV index is 3 or above, Cancer Council Australia advises SPF 50 or 50+, broad-spectrum and water resistant, as "the last line of defence." Use it to prevent burning, not to stay out longer.

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  2. Wash as often as suits you, and wash your hands always

    StrongEvidence grade: Strong. Multiple randomized controlled trials, or consistent large cohort studies with a well-understood mechanism.

    Plain soap is enough. With eczema, seal each bath with moisturizer straight away: in a small trial in children, twice-daily baths sealed this way beat twice-weekly ones (●●○).

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  3. Get short, regular, unburnt sun

    ProbableEvidence grade: Probable. Good observational evidence with a plausible mechanism, or small or few trials.

    When the UV index is 3 or above, a few minutes outdoors on most days keeps most people's vitamin D adequate; when it falls below 3, as in a southern Australian winter, spend time outside at midday with some skin...

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Graded recommendations elsewhere that mention Vitamin D

Other reference topics that mention Vitamin D

Corrections that mention Vitamin D

The argument behind this term, its studies and its notes are in Reference topic 19 of the reference library. The book gives the practical version. Get the book.