Vitamin D: what the large trials showed, and what they did not

Vitamin D is produced mainly in the skin when it is exposed to sunlight, and is found in few foods, such as oily fish, egg yolks and some fortified products. It plays an important role in calcium absorption and bone health. Observational studies had found that people with low vitamin D levels were more likely to have many diseases. This led to a huge rise in the use of supplements.

The VITAL trial

To test this idea directly, Harvard researchers led by JoAnn Manson ran the VITAL trial. Over 25,000 healthy adults in the United States, men over 50 and women over 55, took 2,000 international units of vitamin D or a placebo every day, for on average more than five years.

The results, published in 2019: vitamin D did not significantly reduce the risk of invasive cancer or of major cardiovascular events, such as heart attack and stroke. Later analyses of the same trial suggested some possible effects, for example on cancer deaths after several years or on autoimmune diseases, but these need confirmation.

Why observational studies seemed to say otherwise

People with low vitamin D levels often go out in the sun less, move less, have chronic illnesses or are overweight. These factors may explain part of the link with disease. When the vitamin was given at random in a clinical trial, the expected effects were in many cases not seen.

A link in an observational study is a question, not an answer. Clinical trials are how we get the answer.

When it makes sense

• When there is proven deficiency on blood tests, as assessed by a doctor.
• In infants, according to the paediatrician's recommendations.
• In people who get very little sun, such as older people who stay mostly indoors, or who cover most of their body.
• In some conditions that affect vitamin absorption or bones, such as osteoporosis, under medical guidance.

Care with doses

Vitamin D is fat-soluble and builds up in the body. Very high doses over a long time can cause dangerous levels of calcium in the blood. Do not take high doses without medical advice. For most healthy people, a little time in the sun, a varied diet and movement remain the basis.