Vitamin D

What is vitamin D?

Vitamin D is fat soluble. D3 is cholecalciferol; D2 is ergocalciferol. After conversion in the body, it contributes to processes including calcium absorption. Fatty fish and fortified foods can contribute to intake. [3]

A shared name does not make every form or application equivalent. A dietary reference, a measured blood concentration and a medicine dose describe different things. Meaningful comparison therefore starts with the unit and the precise question being asked.

Micrograms, IU and blood concentrations

For D2 and D3, 1 µg equals 40 international units (IU). Thus 15 µg equals 600 IU and 25 µg equals 1,000 IU. Calcidiol monohydrate requires a separate conversion and cannot simply be treated as the same number of micrograms. [2]

A blood concentration, by contrast, may be reported in nmol/L. It is not an intake amount. Drops, capsules and millilitres are not fixed vitamin-D units either: the relevant figure is the declared content of the portion actually used. A bottle may contain several hundred portions.

Three questions instead of one general promise

A useful decision separates nutritional adequacy, treatment of disease and additional prevention. Correcting established deficiency does not automatically tell us whether a well-nourished person will stay healthier by taking more. Equally, a negative prevention trial does not invalidate the importance of adequate nutrition.

For each trial, we therefore examine who participated, their initial status, the intake schedule and the outcome actually measured. A change in a laboratory marker is a different finding from fewer fractures or fewer new cases of diabetes.

What does EFSA say?

EFSA establishes an adequate intake (AI) assuming minimal production in the skin. This is a dietary reference for healthy population groups, not an extra amount that everyone should take. [1]

Who may need closer attention?

Limited outdoor exposure, older age and impaired fat absorption can make adequate supply harder. Sunshine cannot be converted into a fixed dose; intensive unprotected UV exposure is not a dosing strategy. [3]

The 2024 Endocrine Society guideline advises against routine intake beyond its reference levels in healthy adults under 75. It makes conditional recommendations for additional supply in people aged 75 and over, pregnancy and high-risk prediabetes. It does not recommend general routine testing. [4]

This clinical guideline and EFSA address different questions. Together they do not produce one capsule dose suitable for everybody. People receiving prescribed treatment should not alter it on the basis of a prevention study or a general reference table.

What do controlled trials show?

More is not automatically better

In 2023 EFSA retained an adult UL, including pregnancy and breastfeeding, of 100 µg vitamin-D equivalents daily from all sources. This is a long-term safety ceiling, not a target. Excessive intake can disturb calcium balance. [2]

Individual assessment is especially relevant with kidney problems or thiazide diuretics. [3]

An intake assessment needs to include every preparation: a multivitamin, a separate vitamin-D product and other combinations may all provide the same nutrient. A clear record of amounts and frequency is more useful than assessing each pack in isolation.

How to assess an intake statement

First establish whether the amount is per drop, capsule or daily serving. Convert different units to a common basis. Separate ordinary nutrition from prescribed treatment. When comparing a study, dose, duration, initial status and participant group must all be considered together.

A small regular intake and a large occasional intake are not scientifically interchangeable simply by division. For adults aged 50 and over with an indication, the guideline favours lower daily doses over larger intermittent doses. [4]

Start with the complete ration

Dogs and cats cannot produce an adequate vitamin-D supply through their skin. They depend on dietary intake. A sunny resting place therefore cannot replace an adequate ration. [10]

FEDIAF 2025 distinguishes nutrient concentrations by species, life stage and energy intake. Its tables concern complete rations, not an extra daily number of drops. [9]

In practice, establish which complete food is fed, the amount consumed and any additional products. Dividing a human daily intake by body weight ignores both existing food intake and differences in animal physiology.

Consider calcium, phosphorus and vitamin D together

Excess intake can damage calcium regulation and organs. [10] A low blood concentration during disease does not itself establish that additional intake improves clinical outcomes. That conclusion would require appropriate intervention studies.

For home-prepared diets, calculating the entire recipe is therefore useful. Adding one nutrient does not make an incomplete mixture balanced. Accidental ingestion of a concentrated human preparation needs prompt veterinary assessment; the human upper limit is not an appropriate benchmark.