Yet little is known about the causes of these diseases, which are thought to involve a dysfunction of the immune system.
For the new study, researchers combed through data on more than 238,000 women taking part in the long-running Nurses’ Health Study, which began in 1976.
The study collected information on where the women were living at birth, age 15, and age 30. It also recorded any diagnosis of an inflammatory bowel disease up to 2003.
Researchers also followed up with women who reported having inflammatory bowel disease and verified their diagnoses through medical records.
They found that women who lived in Southern regions that got a lot of sunlight had a 52% lower risk of being diagnosed with Crohn’s disease by age 30 and a 38% lower risk of getting ulcerative colitis than those who lived in Northern regions.
That result held up even when researchers tried to rule out other things that might increase a person’s risk for an inflammatory bowel disease, like having a family history.
“The differences are pretty drastic. That’s what surprised us the most. Especially when it comes to Crohn’s disease. We’re seeing a 40% to 50% reduction in risk,” says researcher Hamed Khalili, MD, a gastroenterologist at Massachusetts General Hospital in Boston.
This study confirms previous research from Europe, and it suggests that the amount of UV light exposure from sunlight may play an important role in the development of inflammatory bowel disease, though researchers aren’t sure why.
One theory is that people in sunnier states may have higher exposure to UV light, leading to higher vitamin D levels. Vitamin D is known to help regulate immunity and inflammation.