Vitamin D: testing, intake and overdose

The blood test measures 25-hydroxyvitamin D, or 25(OH)D, which reflects intake and skin synthesis. According to the US academies cited by the NIH, deficiency risk rises below 30 nmol/L (12 ng/mL), and 50 nmol/L (20 ng/mL) or above is enough for most people. EFSA also uses 50 nmol/L as the target for setting its benchmarks.

Other professional societies use different thresholds, and a result should be read alongside age, season, intake, illnesses, treatments, and the reason for testing. Testing isn't routine screening to do every year for everyone: its relevance should be discussed with a professional.

EFSA proposes an adequate intake of 15 µg (600 IU) per day for adults, including during pregnancy and breastfeeding. The NIH recommends 15 µg up to age 70 and 20 µg (800 IU) beyond that. These are population-level benchmarks, not a prescription.

Three situations should be distinguished:

• Nutritional reference: a benchmark for planning a diet.
• Prevention: moderate supplementation can be discussed depending on age, season, and sun exposure.
• Prescription: a documented deficiency, malabsorption, or bone disease calls for a dose, duration, and sometimes monitoring set by a professional.

A useful reminder for reading labels: 1 µg of vitamin D corresponds to 40 IU.

Few foods naturally contain it: fatty fish, fish liver oil, liver, egg yolk, and cheese. Some products are fortified. Skin produces it under UVB rays, but the amount depends on season, time of day, latitude, cloud cover, skin pigmentation, age, and clothing.

Extended, unprotected sun exposure to "make" vitamin D increases other risks, notably to the skin. The sun isn't a prescription.

Vitamin D2 (ergocalciferol) and vitamin D3 (cholecalciferol) both raise 25(OH)D. According to the NIH, D3 often raises blood levels more and for longer, which isn't enough on its own to set a dose or prove a clinical benefit.

On the label, check the form, the amount per drop or capsule, the unit (µg or IU), and whether vitamin D is present in other products. A high concentration per drop makes a dosing error easy, particularly in children.

The 2014 Cochrane review pooled 53 trials and 91,791 participants, postmenopausal women and older men. Its conclusions are clear:

• Vitamin D alone: high-quality evidence shows it's unlikely to prevent hip fractures or other fractures.
• Vitamin D combined with calcium: a modest reduction in hip fracture risk, mainly seen in people living in institutions.
• Adverse effects: hypercalcemia was more common in supplemented people, and a slight increase in kidney problems was observed.

Excess vitamin D almost always comes from supplements. It causes hypercalcemia, meaning too much calcium in the blood, with nausea, vomiting, weakness, intense thirst, and frequent urination. In extreme cases, it can lead to kidney failure, calcifications, and heart rhythm disorders.

In 2023, EFSA set the upper limit at 100 µg (4,000 IU) per day for adults, including during pregnancy and breastfeeding, and 50 µg for ages 1 to 10. It identified excessive urinary calcium loss as the first sign of excess. Combining calcium and vitamin D in a supplement can increase certain adverse effects.

According to the NIH, vitamin D can interact with orlistat, statins, corticosteroids, and thiazide diuretics. Advice is especially useful with kidney disease, a calcium disorder, a granulomatous disease such as sarcoidosis, malabsorption, pregnancy, older age, or taking several supplements.

1. 01Clarify the question: symptom, test result, intake, prevention, or treatment.
2. 02Take stock of sources: diet, fortified foods, medications, supplements.
3. 03Check the form, unit, and amount per dose, without converting from memory.
4. 04Ask whether testing is useful and how to interpret it.
5. 05Reassess the dose and duration with the professional following you, without extending it on your own.