Medicine has got itself into a tangle over vitamin D. Research studies have found associations between low levels of vitamin D and around 130 different health problems, including heart disease, depression, various cancers and susceptibility to flu. Patients are increasingly likely to have their levels measured by a blood test, and to be prescribed high doses if found to be “deficient”. And in 2016, Public Health England published recommendations that everyone in the country should take supplements, at least for autumn and winter. You would think vitamin D must be a panacea, yet there is increasing evidence that this frenzied testing and supplementing is achieving little in the way of actual health benefits.
The distinction between association and causation is important. Just because vitamin D levels are lower in patients with a wide variety of illnesses it does not necessarily mean that the vitamin “deficiency” is the cause. Several established risk factors – such as obesity, unhealthy diets and a sedentary lifestyle – simultaneously deplete vitamin D. Someone with a heart attack brought on by excess weight and lack of exercise will, if tested, have low vitamin D, but that is an incidental finding. Additionally, people in poor health tend not to get outside so much, reducing their synthesis of vitamin D through the action of sunlight on skin, so increasing the chance that their levels will be low. If all that wasn’t enough, there is a seasonal variation in many diseases – heart attacks, arthritic pain and depression are all more common in winter, when short dark days put our reserves of vitamin D at a natural, but unrelated, low ebb.
If vitamin D deficiency were an important cause of disease then giving people supplements ought to be preventative. But aside from rickets and osteomalacia – two bone-softening conditions long known to be caused by a severe sustained deficiency – it seems from trial data that you can dole out as much vitamin D as you like and it won’t prevent people from falling ill.
Even the cherished practice of prescribing vitamin D to prevent or treat osteoporosis appears from recent research to have little meaningful impact on the risk of future fractures.
How has the vitamin D bandwagon gathered such momentum? In part, it is wishful thinking: we’d all like some natural remedy that will solve multiple ills. And in part it arises from the way doctors are used to interpreting tests. If we get an abnormal blood result, it usually means something. But the case of vitamin D is likely to be different. Take any individual and measure their vitamin D level in, say, February and August, and you’ll get wildly different readings – low in winter, high in summer – simply as a function of sunlight exposure. It is implausible that our physiology would be reliant for short-term effects on a vitamin that is subject to such natural peaks and troughs. It is far more likely that vitamin D principally influences processes such as bone health that fluctuate only very slowly over time, so negating any impact from normal seasonal variation.
As ever there are caveats. Widespread migration means that large numbers of individuals now live at latitudes with which their skin type is ill-equipped to cope: vitamin D synthesis is much slower in darkly pigmented skin, which evolved over millennia in equatorial conditions. And religious-cultural factors such as full-body clothing can compound this problem. Rates of rickets and osteomalacia are at a 50-year high in the UK as a result, and supplementation is of definite help to these patients. There is also evidence that vitamin D supplementation in pregnancy and infancy is beneficial – though this is no more surprising than the idea that ensuring adequate levels of any nutrient is advantageous to developing young bodies.
As for the hope that vitamin D supplements will keep the general population in better fettle, that looks increasingly dubious. Expect new guidance from Public Health England sometime soon.
This article appears in the 27 Feb 2019 issue of the New Statesman, How Brexit broke politics