Vitamin D has always been a little mysterious. Every schoolchild knows its alphabetical neighbour, vitamin C: if you don’t eat enough oranges, your gums will bleed and you’ll get scurvy like anyone unlucky enough not to have sailed with Captain Cook. As is often the case, we’re better informed about all the diseases we’re never likely to get (bubonic plague, smallpox, anthrax) than about those we are – whoever learnt about type II diabetes at primary school? Until recently, vitamin D had a lower profile than other vitamins and minerals. What it is in, or what it does, or what happens if you don’t have enough were all a bit hazy.
Vitamin D is not, in fact, even a vitamin – it’s a hormone (vitamins are substances that we cannot make in our own bodies). It’s found in relatively few foods – oily fish (hence the postwar obsession with cod-liver oil), egg yolks, cheese and some vegetables. Margarine and breakfast cereals are often fortified: Kellogg’s Coco Pops, for example, now come with “added sunshine”. We can make vitamin D ourselves if our skin is exposed to sunlight, but we need a reasonable amount of quite strong sunshine, and it can’t be filtered through a window or sunscreen.
The exact amount is different for everyone; one estimate suggested 20 minutes a day, three times a week might be enough to top up Caucasians during the summer months. If your skin is pigmented, you need more than this. Between November and March the light levels in Britain are not high enough for anyone to make much vitamin D; we tend to rely on what we’ve stockpiled. A poor summer, like the one we have just had, means reduced stores; it is probable that everyone in Britain is “suboptimal” during the winter months. One study estimated that 90 per cent of British South Asians and 50 per cent of white Britons have insufficient or, frankly, deficient levels of vitamin D.
Vitamin D allows you to absorb calcium from your gut; therefore it helps with all the things that calcium does, primarily maintaining bones. Without it, your bones become soft: children get rickets and adults osteomalacia. Vitamin D is also involved in many other processes, including the regulation of immunity and inflammation. There are studies investigating its role in diabetes, high blood pressure, multiple sclerosis and cancer, but no clear causal relationships have yet emerged. So there are no national guidelines recommending supplementation, except for in pregnant or breastfeeding women (breastfed infants are likely to lack vitamin D; bottle-fed infants are less so, as formula is fortified).
Just as the importance of vitamin D remains uncertain, the effects of deficiency can present insidiously. Osteomalacia produces a series of hopelessly non-specific symptoms: fatigue, aching bones, generalised body pain. When I asked a Bangladeshi patient to tell me what was wrong, she simply stated, “Too much pain”. When I tried to get her to tell me where the pain was, she reiterated, “Too much pain!” and made a sweeping gesture with her hand to reinforce the fact it was everywhere. As osteomalacia worsens, it can cause proximal muscle weakness – that is weakness in the muscles nearer to your middle. This makes it difficult to climb stairs or get up from a chair. As your bones get softer, you may develop a waddling gait, or pain from microscopic fractures.
We treat with a course of high-dose vitamin D, followed by lower dose supplementation. This is likely to be life-long, given that no one can engineer sunshine.
Sophie Harrison is a hospital doctor in South Yorkshire