Since solar UVB is the most important source of vitamin D for most people, this fact can be used as the basis of a vitamin D index for use in ecological studies. Thus, looking for geographical variations in prevalence or seasonal variations in disease incidence or birth rates can be done. Cannell1 listed the evidence linking autism to lower levels of sunlight including increased prevalence of autism in regions of greater precipitation and cloud cover2.
It has been known since about 1978 that there is an excess birth rate for those with autism for those born in spring 456789. Grant and Soles10 looked at the seasonality of excess birth rates for autism reported in a number of studies. It was found that the month of the peak was near June for latitudes from 32º N to 45º N, changing to February at 58º N. Since March is the time of lowest serum 25(OH)D in northern midlatitudes, this corresponds to brain damage around the sixth month of pregnancy.
A study investigating the latitudinal variation in autism prevalence rates of autistic children was conducted by Grant and Soles10. Using data on infantile autism prevalence in European countries, Israel and states in the United States prior to 1985, it was found that there was a strong latitudinal gradient, rising from 1.4/100,000 at 32º N to 6.3/100,000 at 58º N. However, the latitudinal gradient was not apparent in data for those born between 1988 and 1995.
There are a number of hypotheses to explain the etiology of autism involving environmental factors, many of which may have changed in magnitude starting in the 1980s. These factors include mercury exposure from all sources11, changes in diagnostic criteria121314, autoimmune reaction to a viral infection15, and replacement of aspirin with Acetaminophen (paracetamol)161718. These factors are reviewed in a recent review19.
Thus, there is now additional support for the finding by Grant and Soles10 for a changing epidemiology of autism after the late 1980s, including reduced vitamin D levels at the population level due to increased use of sunscreen and avoidance of sun exposure2021.
Kinney et al22 reviewed evidence for the hypothesis that a number of environmental risk factors may help cause autism by leading to de novo mutations that several groups reported are associated with increased risk for the disorder. Kinney et al note that several environmental risk factors for autism are also established mutagens. These factors include several heavy metals and several conditions associated with vitamin D deficiency, which is mutagenic because vitamin D is important for protection against DNA damage as well as repair of damage once it occurs.
Page last edited: 17 May 2011
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