Could some cases of schizophrenia boil down to something as simple as vitamin D deficiency? The idea was first put forth more than a decade ago by schizophrenia researcher John McGrath of the University of Queensland in Australia. The circumstantial evidence fit: people born in winter or spring or at high latitudes are at slightly increased risk of developing schizophrenia, and vitamin D deficiency is also more common in winter months and at high latitudes because of lack of sunlight. It may be that a deficit of vitamin D leaves expecting mothers more vulnerable to illnesses such as influenza, which could in turn sensitize the maturing brain to stress-related damage later in life. [For more on how prenatal infections can lead to mental illness, see “Infected with Insanity,” by Melinda Wenner; Scientific American Mind, April/May 2008.]

Now McGrath and his colleagues have put the hypothesis to the test. They analyzed blood samples taken from 424 Danish newborns who went on to develop schizophrenia as well as an equal number of babies who never acquired the disease. In each sample, they measured the amount of the chemical 25OHD, which the body converts into vitamin D. The researchers found that infants who had low levels of 25OHD in their blood—and therefore mothers who were deficient in vitamin D while they were pregnant—were at a higher risk of developing schizophrenia when they grew up.

The result, published in the September issue of Archives of General Psychiatry, could be especially interesting for communities of black immigrants living in northern countries. Researchers have found a striking increase in schizophrenia risk for the children of dark-skinned migrants living at high latitudes—a finding neatly explained if vitamin D plays a role, because dark skin blocks ultraviolet B radiation, the component of sunlight necessary for the body to synthesize vitamin D.

There are some loose ends to tie up, however, before recommending vitamin D supplements for at-risk mothers. The group found that infants with high levels of 25OHD were also at increased schizophrenia risk. McGrath speculates that these infants might have been relatively incapable of generating vitamin D, leading to a buildup of the precursor in their blood—but more research is necessary to say for sure. All told, 44 percent of the schizophrenia cases in the study were attributable to either low or high vitamin D levels. “Even if vitamin D supplements can prevent only a small fraction of schizophrenia,” McGrath says, “it will be a fantastic outcome.”