This essential nutrient is called a vitamin, but dietary vitamin D is actually a precursor hormone — the building block of a powerful steroid hormone in your body called calcitriol. It’s been known for many years that vitamin D is critical to the health of our bones and teeth, but deeper insight into D’s wider role in our health is quite new.

Vitamin D works in concert with other nutrients and hormones in your body to support healthy bone renewal — an ongoing process of mineralization and demineralization which, when awry, shows up as rickets in children and osteomalacia (“soft bones”) or osteoporosis (“porous bones”) in adults.

Researchers are discovering that D also promotes normal cell growth and differentiation throughout the body, working as a key factor in maintaining hormonal balance and a healthy immune system. It appears that calcitriol actually becomes part of the physical composition of cells, assisting in the buildup and breakdown of healthy tissue — in other words, regulating the processes that keep you well.

And it is not just dietary vitamin D that seems to ward off cancer. William Grant, a relative outsider to the cancer research community, says the effect extends to vitamin D made by the skin. Grant's day job is as an atmospheric ozone physicist at NASA's Langley Research Center in Hampton, Virginia. While analysing correlations between atmospheric ozone and tree health in the late 1990s, he stumbled upon National Cancer Institute maps of cancer death rates across the US. He noticed that mortality from many common cancers, including that of colon, breast and prostate, was noticeably higher in the north-east than most of the rest of the country (see Map). The accepted reason was that people in the colder north have a fattier diet, which is known to be a risk factor for cancer. But Grant found that diet only varied by about 10 to 20 per cent between the north-east and south-west, while cancer incidence varied by about 150 per cent. "It was way beyond what can be explained by diet," he says. "The American diet is very homogeneous now." Grant considered another possibility - that people in the southern states, and those who live in rural areas and spend more time outside, are making more cancer-preventing vitamin D. He carried out a huge analysis, comparing the cancer maps with regional UV intensity. There was a statistically significant correlation between low UV and deaths from 13 cancers. He published the research last year, concluding that every year at least 23,600 Americans die of cancer through lack of sunshine (Cancer, vol 94, p 1867). To put that into context, about 9800 die each year from skin cancer.