(Reuters Health) – Flare-ups of inflammatory bowel disease and “stomach bugs” may be more common during and immediately after heat waves, a new study suggests.

Swiss researchers looked at five years of records from one hospital and found more admissions related to inflammatory bowel disease (IBD) during long stretches of hot days.

Hospitalizations for so-called infectious gastroenteritis, marked by vomiting and watery diarrhea, also increased, lagging behind IBD admissions by about a week.

The study “shows clearly that climate change might have serious effects on human health that have not been understood until now,” Thomas Frei, a climatologist who worked on the study while at the Swiss National Weather and Climate Service, said.

However, one researcher not involved in the study said the findings should be interpreted cautiously.

IBD is a chronic condition that occurs when the immune system attacks the digestive tract, leading to chronic inflammation of the intestines and sometimes other parts of the digestive system like the liver. The Centers for Disease Control and Prevention estimates that over one million Americans have IBD.

Gastroenteritis affects about 179 million Americans each year. Infectious forms can be caused by viruses, bacteria or parasites. Children and the elderly are at higher risk for health complications caused by intestinal infections.

For their study, the researchers looked at the records of 738 people admitted to the University Hospital of Zurich with symptoms of IBD – including Crohn’s disease and ulcerative colitis – between 2001 and 2005, and 786 admitted for intestinal infections.

Temperature data was collected from the meteorological station in Zurich.

There were 17 heat waves during that time, lasting up to 19 days. During a heat wave, admissions for both IBD and intestinal infections increased by close to five percent for every additional day the heat wave went on, according to findings published in The American Journal of Gastroenterology.

Admissions for intestinal infections seemed to lag a week behind the heat wave. When that was taken into account, the researchers found those hospitalizations rose by about seven percent for every day a heat wave lasted.

There was no change in hospital admissions for a comparison group of 506 people with non-infectious chronic intestinal inflammation, such as those with colitis or celiac disease, during heat waves.

Prior studies linking IBD flare-ups with weather patterns have come to conflicting conclusions, the researchers said. Some suggest symptoms worsen during winter months; other studies show just the opposite.

It’s unknown why digestive illnesses might cause more problems during a heat wave, Frei, currently an independent research contractor, told Reuters Health in an email.

The researchers suggest that heat waves may alter bacterial growth and produce stress on the body, which could trigger a flare-up of digestive symptoms. The most likely explanation for the time lag between heat waves and admissions for vomiting and diarrhea, they said, is that bacteria require time to grow, so they don’t usually cause symptoms right away.

“This is a novel study examining a unique environmental variable,” Dr. Ashwin Ananthakrishnan, who studies environmental influences on Crohn’s disease at Harvard Medical School in Boston, said.

“However, I think much of the conclusions from the manuscript are highly speculative, and I would be very reluctant to tie in ‘climate change’ as the authors have to IBD.”

Ananthakrishnan told Reuters Health in an email the results could be influenced by missing information about people’s history of IBD, treatment, medication adherence, reasons for visiting the hospital and other factors such as smoking and use of aspirin, which can affect flare-ups.

Another limitation, also mentioned by the authors, is that the comparison group may have included people with mild or misdiagnosed IBD.

“It is somewhat difficult to presume that just one day of a heat wave is sufficient to influence somebody with (disease that is in remission) to go into a flare requiring hospitalization,” Ananthakrishnan, who wasn’t involved in the new research, said.

“I would be concerned about over interpretation of the results given the significant and concerning limitations,” he said.

SOURCE: The American Journal of Gastroenterology, online August 13, 2013.