Getting told to take vitamin D in the winter sounds like something you’ve heard before. Every October, it’s said again and again: put your coat away, get your booster shot, and grab a bottle of vitamin D from the pharmacy shelf. A lot of people do it without giving it much thought. Take whatever is handy, drink a glass of water with it, and think the job is done.
Two studies from the University of Surrey, however, suggest that this assumption may need to be carefully looked at again.
The more surprising of the two results comes from a big study of UK Biobank data that included more than 36,000 people. Together with colleagues from the Universities of Reading and Oxford, researchers from Surrey discovered that people with severe vitamin D deficiency (levels below 15 nmol/L) were 33% more likely to be hospitalized for respiratory tract infections than people with adequate levels of 75 nmol/L or higher. People who have these infections don’t just have mild colds. Some of them are bronchitis and pneumonia, which are two of the leading causes of death in adults 50 and older around the world. The rate of hospitalization dropped by 4% for every 10 nmol/L rise in vitamin D levels. That is a pretty clear dose-response relationship, which is what scientists look for when they want to move from association to something more meaningful.
Take a moment to think about who is the most at risk here. Lower respiratory infections are most dangerous for older people because they can be life-changing illnesses as well as painful ones. In the UK, ethnic minority groups are already at a disadvantage because they have higher rates of deficiency, which is partly because melanin makes it harder for skin to make vitamin D from sunlight. These aren’t little groups. Based on the research, something as simple as making sure people get enough food could sometimes help them stay out of the hospital.
The second discovery changes the picture in a way that was not expected. A different study led by researchers in Surrey and published in Nutrition Reviews found that taking vitamin D2 supplements, which are one of the two main types, seems to lower the body’s levels of vitamin D3. It matters because D3 is the form that our bodies naturally make from sunlight and the form that it seems to use best. In some of the studies that were looked at, the levels of D3 in people who took D2 were lower than in the control groups that weren’t taking any supplements. It’s possible that a lot of people who think they are fixing a problem are actually messing up a different part of the equation.
Before this study, led by Professor Colin Smith at Surrey and published in Frontiers in Immunology, it was found that D3 seems to activate the type I interferon signaling pathway, which is part of the immune system’s defense against bacterial and viral threats. The idea is that D2 and D3 don’t work the same way in the immune system, but this needs to be proven by more research. They might not be doing their jobs or be doing different jobs.
This difference isn’t noticeable to most people who are just walking down the supplement aisle. It says vitamin D on the label. That usually feels like enough. But the research from Surrey brings up a good point: are people getting what they think they’re getting when they make default choices like choosing D2 because it’s in stock, vegan-friendly, or on offer?
There’s still a lot to check. The researchers are careful not to overstate the link between the two events, and more tests will have to be done before there is clear clinical guidance. But it’s getting harder to ignore the direction of the evidence. Vitamin D is no longer a niche supplement. It’s at the center of a public health debate about the NHS’s capacity, the needs of older populations, and the health needs of communities that are already poorly served by current infrastructure.
