The causes of schizophrenia have long been a mystery, but new research has just revealed an intriguing clue. A long-term study of 2,602 individuals has confirmed a link between vitamin D deficiency at birth and developing schizophrenia later in life.

Although most patients don't show signs of the disorder until the age of 15, neurologists have previously hypothesised that it starts in the womb. There's a growing body of evidence that supports this idea - but the exact mechanisms are still unknown.

There's pretty solid evidence that it's genetic, and it's also possible that there's more than one factor at play, at least in some cases.

We know that vitamin D is vital to the absorption of calcium in the bones, and a deficiency can lead to disorders such as osteoporosis and rickets, but we still haven't accounted for its full range of functions in the body.

Now, based on this recent data, it looks like this nutrient could play a role in the development of schizophrenia.

"Schizophrenia is a group of poorly understood brain disorders characterised by symptoms such as hallucinations, delusions and cognitive impairment," said neuroscientist John McGrath of the Queensland Brain Institute in Australia and Aarhus University in Denmark, senior author on the paper.

"The new study that was published this week shows that in a very large sample of Danish babies ... those that have a vitamin D deficiency at birth have a 44 percent increased risk of getting schizophrenia as an adult."

The study, conducted in Denmark, made use of the Danish National Register, an anonymised database of health information that is used to study health trends and statistics.

The researchers identified 2,602 individuals born between 1981 and 2001 who had been diagnosed with schizophrenia as young adults, and looked at the concentration of vitamin D in blood samples taken from these individuals as newborn babies.

They then compared this to age- and sex-matched controls who did not go on to develop schizophrenia. The result was a clear difference - vitamin D could account for as much as 8 percent of Denmark's schizophrenia cases, the researchers said.

The clue was a previously identified link between being born in winter or spring in high-latitude regions such as Denmark - a time when there is considerably less sunlight, which promotes vitamin D production in the body - and an increased risk of schizophrenia.

They hypothesised that the reason might be a vitamin D deficiency, and used the Denmark database to investigate.

"This is too serious a disorder to waste any clue. We have to follow every clue as hard as we can, even though some clues may be implausible or unusual," McGrath said.

"The holy grail is to prevent individuals going on to get schizophrenia in the first place, and I think the experience with the links between folate and spina bifida is a good example that sometimes safe, simple, cheap public health interventions can prevent brain disorders."

Previous studies have also identified a genetic link between schizophrenia and autism, and a link between autism and prenatal vitamin D deficiency.

It's likely that this important finding is not the entire explanation. Schizophrenia is still prevalent in regions with relatively high levels of sunlight, and can also develop in individuals with normal levels of vitamin D at birth.

But now that they've observed this link, the research team plans to pursue this hypothesis further, in the hopes of leading to a very simple and effective method of preventing at least some cases of schizophrenia.

"The next step is to conduct randomised clinical trials of vitamin D supplements in pregnant women who are vitamin D deficient, in order to examine the impact on child brain development and risk of neurodevelopmental disorders such as autism and schizophrenia," McGrath said.

The team's research has been published in the journal Scientific Reports.