Researchers have found that vitamin D treatments during pregnancy appear to prevent the development of autism in mice, and are now planning to investigate if similar effects can be achieved in humans using vitamin D supplements.
The research is still in its very early stages, but it's thought that vitamin D plays a big role in early brain development, and previous studies have suggested that vitamin D deficiency could influence the increased size and unique shape observed in the brains of people with autism spectrum disorder (ASD).
"Our study used the most widely accepted developmental model of autism, in which affected mice behave abnormally and show deficits in social interaction, basic learning, and stereotyped behaviours," says one of the team, Darryl Eyles from the University of Queensland in Australia.
"We found that pregnant females treated with active vitamin D (a different form than in supplements) in the equivalent of the first trimester of pregnancy produced offspring that did not develop these deficits."
For some background into the extensive research that's been done on vitamin D and autism in the past, for more than a decade, scientists have been trying to figure out the significance of animal studies that have linked severe vitamin D deficiency to increased brain size and enlarged ventricles - characteristics similar to those found in children with ASD.
With ASD being such a complex condition, and thought to be affected by a range of risk factors, including genetics and perhaps even environmental conditions such as air pollutants and viral infections, this has been particularly difficult to study in humans.
But we have seen hints that there could be something to this hypothesis, not least of which is the fact that vitamin D deficiency during pregnancy has been linked to an array of physical and psychological conditions including schizophrenia, asthma, and reduced bone density.
Then there was that 2008 study by Swedish researchers that found the prevalence of autism and related disorders was three to four times higher among Somali immigrants in Stockholm than non-Somalis.
The trend seemed to be occurring in Minnesota too, with a separate group of researchers finding that the rate of ASD in the 60,000 Somali immigrants living in the state was comparable to those who had migrated to Sweden.
And it wasn't because Somalis are genetically pre-disposed to the condition - in fact, it appeared to the opposite.
"It has shocked the community. We never saw such a disease in Somalia. We do not even have a word for it," Huda Farah, a Somali-born molecular biologist, told Gabrielle Glaser at Scientific American at the time.
The researchers suspected that in both cases, the unusually high rate of ASD in Somali immigrants was down to the fact that they were getting less sun in their new homes than they were in their native country, which would have lowered their vitamin D levels.
"At northern latitudes in the summertime, light-skinned people produce about 1,000 international units (IUs) of vitamin D per minute, but those with darker skin synthesise it more slowly, says Adit Ginde, an assistant professor at the University of Colorado Denver School of Medicine. Ginde recommends between 1,000 to 2,000 IUs per day."
Fast-forward to 2016, when an international team of researchers published a paper that also highlighted a link between vitamin d deficiency during pregnancy and autism.
Looking at blood samples from 4,229 pregnant women and their children, the researchers found that those with lower than average vitamin D levels at 20 weeks' gestation were more likely to have a child with autistic traits by the age of six.
The team reported that pregnant women who were deficient in vitamin D had "significantly higher" scores on established autism scales than those with average vitamin D levels.
"Just as taking folate in pregnancy has reduced the incidence of spina bifida, the results of this study suggest that prenatal vitamin D supplements may reduce the incidence of autism," lead researcher John McGrath told ABC at the time.
Now, the University of Queensland team has similar results, this time based on mouse models.
They treated pregnant mice in the equivalent of their first trimester with the active hormonal form of vitamin D, and tested the behaviour of their offspring as they developed.
Behavioural studies, maze tests, social interactions, fear conditioning, and marble burying activities were all administered to assess the mouse pups' anxiety, sociability, stereotyped behaviour (repetitive movements like pacing and rocking) and emotional learning and memory.
They found no signs of behaviours that would be linked to ASD in any of the mice.
"The present study tested the hypothesis that maternal administration of the active vitamin D hormone ... would prevent autism-relevant behavioural abnormalities," the team reports in their paper.
"Our data support this hypothesis by showing that maternal vitamin D co-administration blocked the emergence of the ASD-relevant deficits in social interaction, stereotyped behaviour, and emotional learning and memory."
The team also disproved a promising hypothesis for why lower vitamin D levels appear to be linked to higher autism risk - they found no evidence that the vitamin had a protective anti-inflammatory effect during brain development in the womb.
Of course, attempting to replicate the results in humans is going to be complicated. For one thing, the active vitamin D hormone used by the team cannot be given to pregnant women, because it could risk the skeletal development of the foetus.
But they suggest that cholecalciferol - the vitamin D supplement form that is safe for pregnant women to take - could have similar preventative effects.
"Recent funding will now allow us to determine how much cholecalciferol ... is needed to achieve the same levels of active hormonal vitamin D in the bloodstream," says one of the team, Wei Luan.
"This new information will allow us to further investigate the ideal dose and timing of vitamin D supplementation for pregnant women."
To be clear, we're still way too early on in the research process for anyone to be changing their behaviour during pregnancy based on these results - if you're worried about your vitamin D levels, go see your doctor for personalised medical advice.
But the study is yet another piece of the puzzle that could help us understand at least one of the potential risk factors for a very complicated condition, and that could make all the difference in the future.
As autism expert Andrew Whitehouse from the Telethon Kids Institute, who was not involved in the 2016 study, told The Guardian:
"There are likely dozens, if not hundreds, of different mechanisms that can lead to autism. Now this study gives us an inkling of one possible mechanism, but before we think about anything, we need to see a replication of this finding.
What we know is that vitamin D during pregnancy is very important for how the baby develops."
The latest study has been published in Molecular Autism.