Researchers analyzing the health and incidence of celiac disease in almost 2 million children in Sweden between 1991 and 2009 found that children born in the spring are 8% more likely develop celiac disease, while children born in the summer and autumn are 10% more likely to develop the disorder than those who remain healthy.
The nationwide prospective cohort longitudinal study also found that the risk of developing celiac disease varied regionally, with children in southern Sweden more likely to develop the autoimmune disorder than those in central or northern Sweden.
The researchers hypothesize that seasonal viral infections during a vulnerable period of immune development are the most likely explanation for the observed seasonality of celiac disease development.
“One hypothesis for increased CD risk and spring/summer birth is that, those infants are more likely to be weaned and introduced to gluten during autumn/winter, a time characterized by exposure to seasonal viral infections,” the researchers suggest. They explain that viral infections, such as respiratory syncytial virus, rotavirus and influence could “influence the intestinal microbiota, the barrier function and increase mucosal permeability, which might be the mechanism of action in CD pathogenesis.”
However, they note that their study did not analyze corresponding exposure to viral infections, and so this theory is speculative and requires further investigation.
This theory could also explain the geographic differences in the rate of celiac disease development, although the researchers acknowledge that the connection is a bit more tenuous. They explain that yearly epidemics of respiratory syncytial virus, rotavirus and the flu start in southern Sweden and spread northwards over a couple of months, which could account for the slight shift in seasonal associations.
The potential role of vitamin D
Another possible explanation for the seasonality of CD development could be vitamin D deficiency in mothers during late gestation when “important programming and development of the fetal immune system takes place,” according to the study.
It explains that pregnant women who give birth in spring likely in the months leading up to giving birth had less exposure to sunlight, which is needed to synthesize vitamin D, thus increasing their chances of having vitamin D deficiency.
Alternatively, the researchers note “another hypothesis suggests increased CD risk due to high levels of vitamin D,” which would fit the situation in south Sweden. But, they caution, “this hypothesis is still new and is based on comparisons between countries.”
Given these conflicting hypotheses, a recommendation related to supplementation is difficult without further research.
The researchers dismiss the idea that vitamin D deficiency in the children could be related to CD development, even though suboptimal levels of vitamin D during early childhood increase the risk for immune-mediated diseases.
They explain that children in different regions have different levels of exposure to sunlight for vitamin D synthesis during the different seasons, which if related would have altered the seasonal pattern of CD development.
“Moreover, in Sweden every child is recommended and offered government-funded vitamin D supplementation from 1 week to 2 years of age, ensuring adequate levels,” they add.