Vitamin D: what the large trials actually found

Vitamin D is the best-selling supplement in the pharmacy and one of the most argued-about in research. Here is where the confirmed benefit ends and the expectations begin.

What is not in dispute

Vitamin D is needed for calcium and phosphate absorption. Severe deficiency causes rickets in children and osteomalacia in adults. Deficiency is common in groups with little sun exposure, darker skin, older age, obesity or intestinal disease, and correcting it there makes sense.

Where expectations parted ways with the data

VITAL was a randomised, placebo-controlled trial of nearly 26,000 people taking 2,000 IU of vitamin D3 daily, followed for a median of about five years. It found no significant difference from placebo in cardiovascular events or cancer incidence. Participants were not selected for deficiency, so this is a question about prevention in generally healthy adults.

The picture repeats elsewhere: D-Health in Australia, using monthly high doses in older adults, and the D2d trial, which did not meaningfully slow the progression from prediabetes to type 2 diabetes.

No effect in a population without deficiency does not mean deficiency should go untreated. Those are two different questions.

What to do with this

• Testing 25(OH)D is reasonable if you are in a risk group or already taking a supplement blindly.
• Dose follows the blood level, not the marketing on the bottle.
• Very large intermittent doses have been linked to more falls in older adults in some trials — extremes do not help.
• Vitamin D is fat-soluble and accumulates. Overdose is possible and causes hypercalcaemia.

Read the originals

The sources below are the papers themselves rather than summaries. It is a good habit: check who was studied and what was measured.