As Dr. Cannell makes clear in his 2015 book, the autism spectrum disorders (ASD or autism) are a group of developmental disabilities, which can range from mild to severe. Intellectual disabilities are common with severe autism; while individuals with mild autism may have exceptional intellectual abilities. However, all individuals with autism have repetitive behaviors and impaired communication.
The Centers for Disease Control and Prevention reported in 2010 that 1 in every 110 children born in 2002 was diagnosed with ASD by age 8. Recently this number increased to 1 in every 68; that is one out of every 68 eight-year-olds in the USA has ASD.  The reasons for the dramatically increased prevalence of ASD in the United States are not known.
Risk factors for autism include:
There are numerous other risk factors, both genetic and environmental, but most researchers now believe there is an unidentified factor exists that interacts with both genes and the environment.  In his book, Dr. Cannell makes the case that vitamin D is that factor.
In 2008, Dr. John Cannell of the Vitamin D Council published a paper on the link between autism and vitamin D.  This was the first published paper suggesting that vitamin D was associated with autism. The paper provided evidence for this relationship, such as an increased prevalence of autism in the USA in regions of greater cloud cover and rainfall. Also, in the United States, the rate of autism for children ages 6 to 17 years is highest in the regions of the country with the lowest solar UVB doses.  Whether this variation is due solely to vitamin D production or other factors of sunlight in addition to vitamin D production cannot be determined from this geographical ecological study.
Autism is also more common in areas with impaired UVB production, such as areas with high air pollution. Also, people who are dark skinned experience an increased risk for both vitamin D deficiency and autism.
A study conducted in the Faroe Islands found that those with ASD had lower vitamin D levels than their parents or siblings.  Most, but not all, studies have found children with ASD have lower levels of vitamin D than do typically developing controls.  According to several studies, more children with autism are born during the spring, which is the time of year with the lowest vitamin D levels in northern latitudes. 
In a groundbreaking discovery, researchers in Sweden have determined that children who later develop ASD had lower vitamin D levels at birth than their typically developing siblings do.  This helps rules out, but doesn’t entirely disprove, environmental factors as a cause of lower 25(OH)D levels in ASD children; it does suggest that heritability in vitamin D metabolism may cause the lower 25(OH)D levels at birth. Vitamin D levels at birth are dependent on the mother’s vitamin D levels, and one would think that a mother’s vitamin D levels would not drastically change from one pregnancy to the next. Therefore, this study suggests that the heritability of 25(OH)D, which ranges from 30% to 70% in different studies, is the genetic/environmental factor scientists have long sought in ASD.
Also, in another innovative study, Schmidt et al discovered that certain genes associated with lowered vitamin D blood levels are also associated with autism.  However, a Mendelian randomization study (where scientists study thousands of people for genetic defects in vitamin D metabolism to see if those genes are associated with autism) has not yet been done.
Several findings suggest that low vitamin D levels (in mothers during pregnancy and in infants) may affect the risk of autism:
These and other findings do not prove that vitamin D reduces the risk of autism. However, the theory is strong enough that Dr. Cannell states, “The possibility deserves immediate attempts to disprove it.”
In Dr. Cannell’s recently published book, he shares his experience that vitamin D supplementation may even help treat some children with autism. Two recent studies support his contention. In the first published open label trial of vitamin D in autism, which Dr. Cannell co-authored, 83 autistic children completed 3 months of daily high dose vitamin D treatment (5,000 IU/day).  Collectively, 80% (67/83) of subjects who received vitamin D3 treatment had significantly improved outcome, mainly in the sections of standardized rating scales that measure behavior, stereotypy, eye contact and attention span. Also, a case report in the influential journal, Pediatrics, found that high dose vitamin D supplementation remarkably reduced the core symptoms of ASD in a 32-month old toddler.
Randomized controlled trials of high dose vitamin D supplementation both in pregnancy, infancy and toddlerhood are warranted to further investigate this theory. One retrospective study that could easily be done is to see if parents of children with ASD were less likely to follow the recommendation of the American Academy of Pediatrics concerning vitamin D supplementation during infancy and early childhood than were parents of typically developing children.
Research continues to support the theory that vitamin D, especially during pregnancy, infancy and toddlerhood, reduces the risk of developing autism. There is also some evidence that Markedly increasing vitamin D levels in those with autism will reduce the core symptoms of the condition. Men and women planning pregnancy should measure their vitamin D levels to ensure they are sufficient.
We recommend that women who are pregnant or breastfeeding maintain vitamin D levels between 40-80 ng/mL (100-200 nmol/L). This may be achieved by supplementing with 5,000 – 10,000 IU/day of vitamin D3 and/or receiving safe sun exposure when the sun is high enough that one’s shadow is shorter than one’s height. Also normal infants and young children need either sunlight or adequate supplementation (1,000 IU/day/25 pounds of body weight) to help prevent autism. Children with autism need between 5,000 to 15,000 IU/day depending on their vitamin D blood levels. As Dr. Cannell makes clear in his book, vitamin D’s co-factors are also important.
All individuals – with or without autism – should maintain sufficient levels. Sunlight may have additional health benefits beyond that of vitamin D production.
 Kočovská E, Andorsdóttir G, Weihe P, Halling J, Fernell E, Stóra T, Biskupstø R, Gillberg IC, Shea R, Billstedt E, Bourgeron T, Minnis H, Gillberg C. Vitamin D in the general population of young adults with autism in the faroe islands. J Autism Dev Disord. 2014 Dec;44(12):2996-3005.
 Elisabeth Fernell, Susanne Bejerot, Joakim Westerlund, Carmela Miniscalco, Henry Simila,Darryl Eyles, Christopher Gillberg and Mats B Humble. Autism spectrum disorder and low vitamin D at birth: a sibling control study. Molecular Autism 2015, 6:3 doi:10.1186/2040-2392-6-3. Published: 14 January 2015.
 Schmidt RJ, Hansen RL, Hartiala J, Allayee H, Sconberg JL, Schmidt LC, Volk HE, Tassone F. Selected vitamin D metabolic gene variants and risk for autism spectrum disorder in the CHARGE Study. Early Hum Dev. 2015 Aug;91(8):483-9.
 Schieve, L.A., et al., Population attributable fractions for three perinatal risk factors for autism spectrum disorders, 2002 and 2008 autism and developmental disabilities monitoring network. Ann Epidemiol, 2014. 24(4): p. 260-6.
 Cannell, J.J., Autism Causes, Prevention and Treatment: Vitamin D Deficiency and the Explosive Rise of Autism Spectrum Disorder, Sunrise River Press, 208 pp, ISBN-13: 9781934716465, April 1, 2015.
This page was last updated October 28, 2015.