Vitamin D plays a pivotal role in a number of disorders. Now scientists are investigating whether the "sunshine vitamin" could be implicated in autism.
With autism rates climbing and levels of vitamin D declining because of more sunscreen use and less time spent outdoors, scientists have begun to look into a possible link. A recent study was the first to discover that children with autism had significantly lower levels of vitamin D in their bloodstreams than non-autistic children. Yet many questions remain, and experts say none of the research so far has shown a convincing link.
Vitamin D is one of many environmental factors eyed by researchers seeking to understand why autism rates have continued their uninterrupted climb over the past several decades. One in every 88 U.S. kids by the age of 8 has been diagnosed with an autism spectrum disorder, according to the Centers for Disease Control and Prevention.
Some of this increase is attributed to heightened awareness of the brain disorders, which are marked by social impairment, difficulty with communication and repetitive behaviors. Still, greater diagnosis alone cannot explain it, leading researchers to increasingly look for environmental answers.
In recent years, limited evidence has emerged that suggests a link between autism and pregnant women's exposure to pesticides, air pollutants, phthalates and heavy metals like mercury. Now, in a Saudi Arabian study, a possible association between vitamin D and autism is gaining some traction.
Children with autism had lower levels of vitamin D than non-autistic kids, according to the study by researchers at King Saud University. And the lower the vitamin D levels, the higher the children's scores on the Childhood Autism Rating Scale, which measures autism severity.
Fifty children with autism, ages 5 to 12, were recruited and matched with 30 children without autism. Forty percent of the autistic children were vitamin D deficient; none of the control children were.
The study also revealed another first-of-its-kind finding—70 percent of the children with autism had elevated levels of antibodies that can disrupt the signaling of neurons in the brain. These antibodies could trigger an autoimmune response causing brain inflammation, and perhaps autistic traits, in genetically susceptible kids. And the study found that the less vitamin D a child had, the greater the level of the antibodies, and the more severe the autism symptoms.
"There is a growing body of literature linking vitamin D to various immune-related conditions, including allergy and autoimmunity," said Laila Y. AL-Ayadhi, a professor of neurophysiology at King Saud University and one of the study's lead researchers.
One study in 2009 found that autistic patients had significant levels of brain inflammation, indicating an autoimmune disorder—when the body's immune system begins attacking healthy tissues, in this case, brain tissues. Vitamin D deficiency, in turn, has been identified as a possible environmental trigger for other autoimmune disorders, including multiple sclerosis and lupus.
Vitamin D deficiency in the study apparently was unrelated to sun exposure, which was about the same for the autistic and non-autistic kids. Instead, AL-Ayadhi said "the most probable explanation" for the children's lowered vitamin D was "an abnormality in the liver," where vitamin D is converted into its usable form, although that hypothesis was not fully explored.
The study has limitations, said Heather Volk, an assistant professor at the University of Southern California who studies gene-environment interactions in relation to autism. "While they can find differences of circulating vitamin D, it's difficult to really say this is potentially a causal factor because we're looking at things in one point in time in these individuals," she said. "We don't really know if [the lowered] vitamin D levels preceded the autism."
"But," she added, "it is important and interesting and the more environmental factors that can be modified that we manage to study for autism, the more insights that we start to get."
Vitamin D deficiency in mothers is thought to be a risk factor for autism, too, and has been supported by studies examining where children are born and during what season in relation to autism rates.
"Vitamin D performs a number of biological functions that are important for neurodevelopment, including promoting cell division and protecting against neurotoxins. A research goal now is to hone in on what the exact biological mechanisms are that may link maternal vitamin D insufficiency and atypical brain development," said Andrew Whitehouse, head of the Developmental Disorders Research Group at the Telethon Institute for Child Health Research in Australia.
In rats, vitamin D deficiency during gestation has resulted in permanent changes in developing brains. Also, babies born with low vitamin D levels have twice the risk of developing schizophrenia, and deficiency in pregnant mothers has been associated with language impairment in children.
Results of autism studies, however, have been mixed. Whitehouse led a study of more than 900 pregnant women that measured Vitamin D at 18 weeks gestation; their children were later assessed for autism. It found no correlation between children's scores on an autism rating scale and maternal vitamin D levels.
But research at the University of California, Davis found an increased risk for children with autism among 700 mothers who did not take prenatal vitamins during the three months before and the first month of pregnancy.
While that study did not specifically call out vitamin D as a suspect, UC Davis researcher Rebecca Schmidt is currently working on a similarly large study that will focus on the importance of maternal vitamin D intake on autism rates.
"This is the kind of study I think needs to be done where we go back and look at the vitamin D levels in the maternal blood during pregnancy and we follow these children up to see if they've developed autism," Schmidt said.
Volk's research has focused on a possible link between maternal air pollution exposure and autism, but she notes that her findings also could implicate vitamin D.
Her study, published in January, found that children with autism were more likely to live in homes that had the highest percent of traffic-related air pollution during their mothers' pregnancy and the first year of life compared to control children. One explanation is that pollution may induce inflammation in the body that raises the risk of autism.
"Air pollution exposure can increase systemic inflammation in the body that really might be affecting the brain," Volk said, adding that "vitamin D helps your body deal with inflammation. It helps turn on the body's responses."
Air pollution, studies show, could be acting as a double-edged sword: increasing inflammation and contributing to maternal vitamin D deficiency.
Of course pollution isn't the only reason women are getting less sunlight, and less vitamin D. For decades, women have been avoiding the sun with the help of sunscreens and more time spent indoors. And those decades track with the rising incidence of autism disorders. A 2007 study of 400 pregnant women found that 42.1 percent of white women and 54.1 percent of black women were vitamin D insufficient even though 90 percent of these women took prenatal vitamins.
One study published last year found that children in states with the highest exposure to UVB rays in summer and fall had about half the autism rates of states with lower UVB exposures. The study was conducted by John Cannell of the Vitamin D Council and another researcher; both reported that they received funding from vitamin manufacturers.
Such studies linking sunshine exposure and autism remain highly speculative, Schmidt said. "There are a lot of things that could be explaining those types of associations besides vitamin D," Schmidt said. "You have to be careful about making those kind of big links until there's better data."
For instance, experts originally suggested a link between sunshine and autism in relation to immigrants with autism, but now other factors are considered more likely. Somali immigrants moving to northern latitudes in Sweden and Minnesota had higher-than-average rates of autism than surrounding communities, according to government officials and the Minnesota Health Department. Vitamin D quickly became a suspect. But on closer examination, the theory didn't hold. Instead, it was found that the stress of migrating may have been the more critical factor.
So is it worthwhile to begin preventive measures, such as five to 30 minutes of sun exposure a couple times a week? Schmidt says supplements alone are not enough.
The levels in those supplements are so low compared to what you get from the sunlight that they may not be making as much of a difference as they need to," she said.
In 2007, the Canadian Pediatric Society recommended a sharp increase in supplementation during pregnancy and breastfeeding, from 400 international units (IU) per day, which are typically found in prenatal vitamins, to 4,000 IU/day. The American College of Obstetricians and Gynecologists advises that 1,000-2,000 IUs of vitamin D during pregnancy can be considered safe for women with an identified vitamin D deficiency although they add that "most experts agree that supplemental vitamin D is safe in dosages up to 4,000 international units per day during pregnancy or lactation."
"We don't want to get people too concerned before we need to," Schmidt said, "but it is something we can do something about."
Brita Belli is the author of The Autism Puzzle: Connecting the Dots Between Environmental Toxins and Rising Autism Rates and the editor of E—The Environmental Magazine http://www.emagazine.com/.