New insights into how the 'magic pill' works
The role of vitamin D in carcinomas could explain one of the biggest mysteries about the cause of cancer: why so many people who develop the disease have no known risk factors, such as a family history of the illness.
The simple answer may be that Vitamin D interacts with an unusually large number of our genes, working like a master switch to turn them on or off. Researchers believe a deficiency of the vitamin leads to a deficiency of the proteins manufactured under the direction of these genes, which then undermines key defences against seemingly unrelated diseases such as cancer, diabetes and multiple sclerosis.
John White, who has been studying the antimicrobial activities of vitamin D at McGill University in Montreal, says that "virtually every cell" in the human body has receptors for vitamin D and that hundreds of different genes may be regulated by it.
Vitamin D's most profound gene-influenced activity appears to be in keeping healthy the broad category of cells known as epithelium, which line the outsides of our organs and the surfaces of the structures in our body.
Even though these lining tissues amount to only about 2 per cent of the weight of our bodies, they are the source of about 85 per cent of cancers, those known as carcinomas.
These include cancer of the colon, prostate, pancreas and uterus, along with the most common type of breast cancer, ductal carcinoma, which develops on milk-duct lining. (The other main type of cancer, sarcomas, appear in muscles and connective tissue, and don't have a strong association with vitamin D insufficiency.)
"Vitamin D is a particularly effective agent in inhibiting abnormal growth or development of malignancies in epithelial tissues," says Cedric Garland, a professor of preventive medicine at the University of California, San Diego.
Although many researchers view cancer as a hopelessly complex disease with different causes for each tumour type, Dr. Garland, who has been studying vitamin D for more than three decades, believes the carcinomas have a common origin in low levels of the vitamin. By his estimate, up to 75 per cent of these cancers could be prevented if vitamin D levels were raised through supplements. "I'm convinced that cancer is largely a vitamin D deficiency disease," he says.
One important function of vitamin D at the gene level that may explain its anti-cancer properties is that it helps to regulate the production of E-Cadherin, a type of biological glue that holds cells together. When this glue is in short supply, it allows epithelial cells to lose adhesion to one another, permitting some to escape from the tissue they are supposed to be embedded in. Unconstrained, these cells start to multiply at a greater rate than they otherwise would and begin forming the lesions that ultimately turn into cancers.
Vitamin D plays a role in telling cells when to die, thus helping to prevent uncontrolled proliferation and curbing the growth of new blood vessels that nourish growing tumours.
It may also play a role in diseases unrelated to cancer. A main biological function of epithelial cells is to be a barrier against viruses and bacteria that cause infections.
Scientists speculate that when low vitamin D status weakens epithelial cells, the barrier function is compromised, exposing tissues to attack from disease-causing agents - in diabetes, for example, by weakening islet cells; in multiple sclerosis, by weakening glial cells in the nervous system; and in tuberculosis, by reducing the ability of the lung lining to repulse bacteria, according to Dr. Garland.
Some medical researchers have even begun to suspect a link between vitamin D insufficiency and schizophrenia, which occurs 10 per cent more often among those born in winter and early spring, when vitamin D from sunshine is less available.
Researchers in Australia are testing this hypothesis by studying the brains of rats born to pregnant mothers deprived of vitamin D - with alarming results. The vitamin-D-deprived rodent brains had more cell proliferation, enlarged ventricles and less of a protein necessary for nerve growth.
"What we see is that when you take [vitamin]D out of the brain in the rodent, you can break their brain basically," says John McGrath, a professor at the Queensland Brain Institute at the University of Queensland in Brisbane. "We can change the way their brain develops."
Dr. McGrath says it is too early to say whether the rodent-brain research applies to humans. But he adds that "even if only a small fraction of [the cases of ]schizophrenia could be averted by optimizing maternal nutrition, that is going to be a really important outcome."