Vitamin D is unusual among vitamins because the body can synthesise it from sunlight exposure on skin, which means the right supplementation advice depends heavily on where someone lives, what season it is, and how much of their skin is typically exposed.
What UK guidance says
The NHS recommends that adults and children over four consider a daily supplement containing 10 micrograms of vitamin D during autumn and winter, when sunlight in the UK is too weak for the skin to produce enough. Certain groups — including people with limited outdoor exposure, those who cover most of their skin for cultural or religious reasons, and people with darker skin — are advised to consider a supplement year-round.
What US guidance says
The National Institutes of Health's Office of Dietary Supplements sets a recommended dietary allowance of 15 micrograms (600 IU) per day for most adults, rising to 20 micrograms (800 IU) for adults over 70. These figures are population-level reference intakes rather than treatment doses for diagnosed deficiency, which a clinician determines separately based on blood testing.
Not medical advice
This article summarises published guidance from health agencies. It is general information, not a personal recommendation, and vitamin D supplementation — particularly at higher doses — should be discussed with a healthcare professional, especially for people with existing kidney conditions.
Groups agencies flag as higher risk of deficiency
- People who get little or no sun exposure, including those who are housebound or covered for most of the day.
- People with darker skin pigmentation living at higher latitudes, where UVB intensity is lower for more of the year.
- Older adults, whose skin synthesises vitamin D less efficiently with age.
- Pregnant and breastfeeding women, and infants, who have distinct guidance from the same agencies.
- People with conditions affecting fat absorption, since vitamin D is fat-soluble.
Why more is not automatically better
Vitamin D is fat-soluble and stored in the body, so unlike water-soluble vitamins, excess intake is not simply excreted. Agencies including the NIH publish tolerable upper intake levels precisely because very high, sustained supplementation has been associated with harm, including elevated blood calcium. High-strength over-the-counter products sold at multiples of the recommended allowance are not automatically appropriate for someone without a diagnosed deficiency.
Testing versus routine supplementation
Public agencies generally do not recommend routine population-wide blood testing for vitamin D; testing is typically reserved for people with symptoms, risk factors, or specific clinical indications, with treatment doses set by a clinician based on the result.
- Check whether you fall into a group your national health agency flags for year-round supplementation rather than assuming universal advice applies.
- Favour the standard recommended daily amount unless a clinician has recommended a higher, monitored dose for a diagnosed deficiency.
- Reassess in season: many countries' guidance is explicitly split between summer and winter recommendations.
- Discuss supplementation with a healthcare professional if you have kidney disease or are taking medication that affects calcium metabolism.
The guidance is targeted for a reason: sunlight, skin, latitude and life stage all change the calculation.
