W hen Xand van Tulleken set off on a journey into the wild west of wellness, he didn’t expect to discover anything new about vitamin D. Of all the supplements that line our shelves and social media feeds, it ought to be the least controversial; even the NHS website, which is hardly a wellspring of quackery, advises that everyone should consider taking a daily dose during autumn and winter, when we risk becoming deficient (D is the vitamin we get mostly from sunlight, and which helps keep our bones strong).

The tiggerish TV doctor and twin brother to Chris (he’s the one who wrote Ultra-Processed People about the food industry) qualified at a time of high excitement in vitamin D circles; in the early 2000s, big studies were linking it not just to bone health but cancer, heart disease and dementia.…