A number of studies have suggested an association between an increased risk of schizophrenia and being born in winter months. However, it is not clear why this is. There are a number of possible explanations for the association, such as an increase in maternal infections during the winter, seasonal availability of fruits and vegetables during pregnancy, or an increase in environmental toxins during the summer. Another explanation put forward is exposure to and levels of prenatal vitamin D. A research team from Australia and Denmark  looked at archived dried blood spots taken at the birth of 1,301 Danes who had received a schizophrenia diagnosis, and compared them to those taken from matched controls. They found that those in the lowest quintile for vitamin D concentration had a significantly increased risk of schizophrenia (incidence rate ratio = 1.44, 95%CI 1.12-1.85, p=0.004). Their level of vitamin D was consistent with the standard definition of being vitamin D deficient. Further analyses also showed that there were seasonal fluctuations in vitamin D levels, with the lowest being found in people born in winter or spring. The authors suggest conducting an RCT to provide vitamin D supplements to pregnant women should be the next step, though the CLAHRC WM Director does not think the sample size calculation would ‘stand up’.
— Peter Chilton, Research Fellow
- Eyles DW, Trzaskowski M, Vinkhuyzen AAE, et al. The association between neonatal vitamin D status and risk of schizophrenia. Nature Scientific Reports. 2018; 8: 17692.