Does vitamin D help your immune system, or is that overrated?

Vitamin D has a direct line to your immune system. Immune cells such as T cells, B cells and macrophages all carry vitamin D receptors and have the means to activate vitamin D themselves. Once activated, it prompts these cells to produce substances that directly attack bacteria and viruses. It also helps cells clear pathogens, such as the tuberculosis bacterium, through an internal disposal process. The biological foundation is therefore solid.

The broader picture is more nuanced. Multiple meta-analyses of human studies show that low vitamin D levels are associated with a greater risk of acute respiratory infections. Supplementation helps, but the effect is modest. It is most pronounced in people with a demonstrable deficiency and in children. Those who already have an adequate level have little reason to expect benefits from higher doses.^1,2

In COVID-19, clinical studies show that correcting a vitamin D deficiency reduces the risk of hospitalisation, intensive care admission and death. The researchers emphasise: the goal is to correct a deficiency, not to take megadoses on top of an already adequate level.³

For autoimmune diseases such as type 1 diabetes, multiple sclerosis and lupus, meta-analyses show an association with low vitamin D levels. Laboratory and animal studies illustrate how vitamin D can dampen excessive immune responses. Modest benefits have been found in type 1 diabetes and inflammatory bowel disease, but a fully proven causal relationship has not yet been established.⁴

A deficiency is not uncommon. US population research found that 61% of white Americans and 91% of Black Americans had insufficient levels.⁵ Practical advice: have your level tested if you have symptoms or are at higher risk. If a deficiency is confirmed, supplementing is worthwhile. Taking high doses on a hunch makes little sense if your level is already in order.