In October, practically every British home has a small bottle on the kitchen counter. D vitamin. The winter supplement. It’s something that medical professionals have been suggesting for years—almost as casually as telling people to drink more water. The majority of people take whatever is on the shelf; D2 and D3 sound alike. It isn’t.
Researchers have long suspected that vitamin D deficiency causes more than just aches and pains, and a recent study from the University of Surrey is giving this theory significant weight. People are sent to the hospital by it. Additionally, the type of supplement you’re taking may be exacerbating your condition.
On its own, the headline finding is striking enough. Researchers found that individuals with severe vitamin D deficiency, defined as levels below 15 nmol/L, were 33 percent more likely to be hospitalized for respiratory tract infections than those with healthy levels of 75 nmol/L or higher, based on an analysis of NHS data from over 36,000 participants in the UK Biobank. Pneumonia and bronchitis are examples of lower respiratory infections that are serious problems. They are among the top ten causes of death worldwide for individuals over 75. Such a statistic is worthy of more than a footnote.
The numbers in this study move in a consistent direction, which sets it apart from the typical nutritional research, which can feel frustratingly theoretical and ambiguous. The hospitalization rate for respiratory infections decreased by 4% for every 10 nmol/L increase in vitamin D levels. It’s not a coincidence, but a pattern. It provides actual data to support a theory that has been around for years in medical circles but lacks sufficient proof to be taken seriously.
However, a companion study that was published in Nutrition Reviews contains the more unsettling finding. The effects of different vitamin D supplements vary. The body knows how to use vitamin D3, which it naturally produces from sunlight. Vitamin D2 is derived from plants and has long been regarded as being equivalent to vitamin D3. It can be found on the label of many supplements that are sold in pharmacies and supermarkets. According to researchers, that is incorrect. In addition to not increasing total vitamin D levels as well, taking D2 can actually lower the body’s concentration of D3. D3 levels decreased less in participants taking D2 supplements than in the control group, or those taking nothing at all, in a number of the trials that were looked at.
It’s difficult to ignore that for a little while. People who follow government guidelines and purchase supplements in good faith may wind up worse off than if they had completely avoided the aisle. That’s a subtly unsettling conclusion, but it’s not particularly dramatic.
This has a social component that is also worth considering. Because darker skin produces less vitamin D from sunlight, especially in northern latitudes where meaningful sun exposure is limited for the majority of the year, ethnic minority communities in the UK are more likely to suffer from vitamin D deficiency. Individuals who are more susceptible to severe respiratory infections include older adults, those who have little access to the outdoors, and those who live in more northern areas. These groups are also frequently the least able, either financially or biologically, to maintain sufficient levels of vitamin D. This overlap is not coincidental. It indicates a structural issue.
The researchers take care to avoid exaggerating their findings. Supplementation cannot take the place of a more comprehensive public health approach; more research is required. However, it seems more and more obvious where the evidence is going. There are good reasons to take vitamin D this winter, so it’s probably worthwhile to find out what kind you’re taking. In the fine print of a supplement label, the distinction between D2 and D3 may appear to be a small detail. It might not be minor at all, according to what Surrey’s researchers have discovered.⁖※
