Extremely hot weather makes us sweat and frizzes some people's hair, but a new study reports for the first time that it can also worsen some gastrointestinal (GI) problems.
During a heat wave, there's an increased risk for inflammatory bowel disease (IBD) flare-ups, and a greater risk of infectious gastroenteritis (IG), Swiss researchers found.
"This is something very new," said study researcher Dr. Christine Manser, a gastroenterologist at University Hospital in Zurich, Switzerland.
"There has never before been a study investigating the impact of climate change — represented by an increase in heat waves — on IBD and IG flares," Manser said.
The researchers defined a heat wave as any period of six or more days with high temperatures rising above the average daily high by more than 9 degrees Fahrenheit (5 degrees Celsius).
The study showed that prolonged periods of hot weather led to a 4.6 percent increase in risk of people needing to be hospitalized with a relapse of inflammatory bowel disease for every additional day that a heat wave lasted.
Manser explained that if extreme heat began on a Monday, and continued all week, by Saturday (day six) it would be classified as a heat wave, and by Sunday (day seven) the risk for IBD flares would increase by 4.6 percent a day.
Crohn's disease and ulcerative colitis are two types of inflammatory bowel disease, and have symptoms including stomach pain, diarrhea, and bleeding.
The study also found the risk of needing to be hospitalized was 4.7 percent higher in people sickened by infectious gastroenteritis for each additional day of a heat wave. [5 Ways Climate Change Will Affect Your Health]
Infectious gastroenteritis may result in vomiting and stomach cramps, and can be caused by a virus, such as norovirus; a bacteria, such as salmonella; or a parasite, such as giardia.
The study is published online today (Aug. 13) in the American Journal of Gastroenterology.
To find out whether digestive symptoms were linked with heat waves, researchers looked at the University Hospital of Zurich's admission records over a five-year period, which included 17 heat waves.
A total of 738 people with inflammatory bowel disease, and 786 people with infectious gastroenteritis were admitted to the hospital during these hot spells. Researchers also looked at a control group of 506 people hospitalized for noninfectious GI problems, but found no evidence of a heat wave effect.
The data revealed that when heat waves occurred, they had an immediate impact on the risk for IBD flare-ups. But the highest risk for developing infectious gastroenteritis occurred on the seventh day of a heat wave.
One possible reason for the one-week delay in IG flares is that heat waves change the bacterial composition of the gastrointestinal tract, Manser said. But this change in gut bacteria takes time, which may explain the seven-day time lag in developing intestinal symptoms, she said.
Manser said several potential mechanisms may explain why IBD flares in hot weather. One possibility is "that heat waves induce physical stress, which has been shown to cause flares of inflammatory bowel disease," Manswer said.
Heat as a tipping point
"I think the study presents an interesting observation, but a heat wave's overall impact in terms of hospital admissions is a relatively small one," said Dr. Alan Moss, a gastroenterologist at Beth Israel Deaconess Medical Center in Boston, who was not involved in the research.
A 4- to 5-percent increase in hospitalizations may amount to one or two IBD or IG patients a year, Moss explained.
He noted that air conditioning is more prevalent in the United States than it is in some places in Europe, perhaps exposing the Swiss patients to higher outdoor temperatures.
During a heat wave, there's probably a physiological stressor, or some aspect of diet that changes for IBD patients, that may also be contributing to flare-ups, Moss said.
"The heat wave may well be a tipping point for IBD and infectious gastroenteritis," Moss said.
In other words, people with an inflammatory bowel disease, for example, may experience more bouts of diarrhea as the mercury outdoor rises. And because hot weather also makes someone likely to sweat more, that individual could become dehydrated.
"If you're not keeping up with replacing these fluid losses, it's the dehydration that's making you feel lousier," Moss explained, prompting some GI patients to head to the emergency room during a heat wave.
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