For 20 years doctors in Australia and Denmark have been chasing a theory that vitamin D deficiency during pregnancy raises the risk of schizophrenia. A new study has provided the strongest support so far. Although the proportion of cases of the mental disorder implicated by this research is small, it may turn out to be the tip of an iceberg.

“Schizophrenia is a group of poorly understood brain disorders characterized by symptoms such as hallucinations, delusions, and cognitive impairment," Professor John McGrath of the University of Queensland said in a statement.

Two decades ago McGrath noted schizophrenia is more common among people born in later winter or early spring than those born the rest of the year. He also observed the greater frequency of the condition in Tasmania, far from the equator, than his famously sunny home state. Combining these facts, McGrath wondered if reduced exposure to sunlight might be a contributing factor, most likely through vitamin D deficiency, a theory he has been testing ever since.

McGrath's latest work takes advantage of the unusually long period in which Scandinavian countries hold onto the blood samples taken from newborn children. McGrath compared the rates of schizophrenia among 2,602 Danish children born between 1981 and 2000 with their vitamin D levels shortly after birth.

In Scientific Reports, McGrath announced children with vitamin D deficiency were 44 percent more likely than those above recommended minimums to be diagnosed with schizophrenia as adults. Substantial as this sounds, McGrath estimates just 8 percent of cases of schizophrenia in Denmark occur because of vitamin D deficiency in utero. He noted to IFLScience the figure is likely to be considerably lower globally, since winters as dark as Denmark's are rare.

On the other hand, McGrath told IFLScience, the issue may not stop at birth. He previously looked at a survey from northern Finland, and found children who were not given vitamin D supplements in their early years were also more likely to develop schizophrenia.

Moreover, there is less advanced work linking vitamin D deficiency before birth or during early childhood to autism and multiple sclerosis.

McGrath is very cautious about the implications of his research, and told IFLScience public health recommendations are probably several years away. The highest standard of medical evidence is hard to achieve in this case because, he explained, “It is not ethical to leave vitamin D deficient pregnant women untreated for randomized control trials.”

McGrath also pointed out it is possible to take too many vitamin D supplements. Nevertheless, with the effects of deficiency on bone health already well established, his work provides a strong case for extra testing, particularly among those whose sunlight exposure is limited, and supplements where they are found to be appropriate.