People born in winter and people born in regions with less sunlight and shorter days may have a lower risk of developing celiac disease (CD) than those born in warmer regions or seasons, a new Swedish study suggests.
What is Celiac Disease?
The Celiac Disease Foundation states that “Celiac disease is a genetic autoimmune disorder where the digestion of gluten leads to damage in the small intestine.”
About 1 in 100 people worldwide have celiac disease, with 2.5 million Americans having the disease. But these individuals are largely undiagnosed, which puts them at risk for long-term health complications, including malnutrition, low bone density, lactose intolerance, and infertility.  
What is Gluten?
Gluten is a protein found in wheat, rye, barley, and triticale (a cross between wheat and rye). Gluten helps foods maintain their shape, sort of like a glue. 
The Study and the Findings
Scientists haven’t nailed down exactly what causes celiac disease, but previous studies have suggested that the season a person is born in may be one of many environmental factors that could influence risk. 
For the study, researchers at Umea University in Sweden examined data on nearly 2 million children up to age 15 born in that country from 1991 to 2009, including about 6,600 diagnosed with celiac disease. Lead study author Fredinah Namatovu, a public health researcher at Umea University, and his colleagues looked at the relationship between CD diagnosis and season, region, and year of birth. 
The team found that the risk of diagnosis was approximately 10% greater in children born in spring (March-May), summer (June-August), and autumn (September-November) than it was in people born in winter (December-February).
Children born in the south of Sweden, where there is intense sunlight in spring and summer, had a higher risk of being diagnosed with celiac disease than children born in the north of the country, where it is colder in the spring and summers are shorter.
The researchers also discovered that children diagnosed with CD before 2 years of age appeared to have a greater risk of CD if they were born in the spring, while older kids appeared to have an increased risk if they were born in summer or fall.
Why do Season and Region of Birth Affect CD Risk?
The researchers wrote:
“One hypothesis for increased (celiac disease) 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.” 
In a news release that accompanied the study, the team explained that viral infections trigger changes in the intestines that could lead to development of CD.
The authors also theorize that low levels of vitamin D in pregnant women affect development of the fetal immune system.
Dr. Alex Krist, a researcher at Virginia Commonwealth University in Richmond who wasn’t involved in the study, said:
“Many factors are associated with the risk of contracting celiac disease and thus far, research has not led to an understanding of the potential cause or origin of the disease. Our current understanding of who is at risk is limited to people who have a family history or other autoimmune conditions, as well as those belonging to certain racial and ethnic groups.
“Learning more about other potential associations, like birth season, may prove useful in better understanding both the cause of and who is at risk for celiac disease.”
The study is published in Archives of Disease in Childhood.