The Doctor’s Surgery

Is Vitamin D good for you?

Not always – and avoid the Vitamin A in polar-bear livers, too theodore dalrymple

What stands to reason depends critically on the reasoner, of course.

For many people – probably most of them – if a deficiency of x in the blood is associated with an increased death rate, then correcting that deficiency will reduce the death rate. After all, it stands to reason that it should.

Such reasoning has in the past led to runs on products in supermarkets. I remember a run on Brazil nuts subsequent to a report that low levels of selenium were associated with high rates of heart attack. Brazil nuts had also been reported to contain high levels of selenium.

Nature, however, is not always reasonable; at least not by our – sometimes not very high – standards. She fails to co-operate as she should.

By now it is tolerably certain that people with a relatively low level of Vitamin D in their blood have higher rates of mortality from practically everything from cancer to heart diseases, as well as an increased risk of developing non-fatal auto-immune diseases.

A recent study found, not for the first time, that those with the lowest levels of Vitamin D and who were either diabetic or pre-diabetic (a category that has recently come under sceptical examination) were considerably more likely to suffer from what are sometimes elliptically called ‘events’ – heart attacks or strokes – than those with the highest level.

The authors of the study concluded, ‘These findings highlight the importance of monitoring and correcting Vitamin D deficiency in the prevention of CVD and mortality among adults with pre-diabetes and diabetes.’

This conclusion, which stands to a certain kind of reason, would in fact be reasonable if the deficiency were causative rather than merely a statistical association. Various biological causative explanations for the association have been put forward, but people rarely differ only in their levels of Vitamin D in the blood. For example, those with low levels tend to have poorer lifestyles (according to the current conceptions of good – which is to say healthy – lifestyles) than those with higher levels.

The proof of the pudding, or hypothesis, is in the experimentation. Does Vitamin D supplementation reduce death rates from the various diseases with which low levels are associated; and, if so, is it by an amount that makes taking Vitamin D supplementation for a long time worthwhile? Any bad effects of such supplementation, apart from the bother of taking it, must be weighed in the balance.

Fortunately, there do not seem to be any bad effects of supplementation when taken in the prescribed quantities – though there are always people to be found who think that if x is good for you, then 2x must be twice as good (after all, it stands to reason). Vitamins in excess can be dangerous, however, as those who have eaten polar-bear liver, exceptionally high in Vitamin A, have discovered to their cost. Do not eat it if offered.

Unfortunately, the many trials in and publications on Vitamin D supplementation give different results.

One complication is that Vitamin D is not a single or simple compound, and different forms of it give different results. On the whole, Vitamin D3, cholecalciferol, derived from animal sources, is superior to Vitamin D2, ergocalciferol, derived from vegetable sources.

Moreover, some very large metaanalyses (the amalgamation of the results of many trials) have failed to show any effect of supplementation on all-cause mortality – all-cause mortality being a reliable endpoint of an experiment, as we doctors call death.

On the other hand, other controlled trials have shown a reduction in death from cancer and heart disease, especially with D3. I confess that all this leaves me a little muddled. In summary, some trials have shown benefit and none harm.

Whether a reduction of 15 per cent in cancer death over five years, when the risk of such death is, say, 5 per cent, makes it worthwhile taking pills every day is for the patient to decide.

My advice is to do what you think is right – for you.

‘The Emperor bought his new clothes with non-fungible tokens’