If you’re like a lot of people, you’re finding it harder and harder to stomach climate change—literally. A warming world leads to all manner of health problems, including increased risk of cardiovascular disease, exacerbation of pulmonary conditions like asthma and COPD, and mental health problems including depression and anxiety. Increasingly, however, climate change is being implicated in a range of illnesses of the gut, such as diarrheal diseases, irritable bowel syndrome, intestinal infection, and more. While the mechanism behind the increase in pulmonary disease in a warmer world is more or less direct—breathing hot, dirty, sooty air isn’t good for anyone’s lungs—the gut connection is more nuanced and multifactorial, involving crop growth, contaminated water supplies, droughts, heat waves, malnutrition, and the microbiome of the soil. None of this is good for us; all of it can affect any of us. Here’s what you need to know about the climate-gut connection.

How high temperatures directly affect the gut

The body is an exquisitely balanced system. We operate optimally at 98.6°F; nudge us up to just 99°F and we already start feeling unwell. It’s no wonder then that if the planet runs a fever we will pay a price. “Higher temperatures can increase stress hormones in the body, and that really affects gut physiology,” says Elena Litchman, professor of aquatic ecology at Michigan State University. 

The principal stress hormone is cortisol, which is produced by the adrenal gland. Cortisol affects multiple parts of the body, but can have an especially powerful impact in the gut, which is lined with immune system cells; epithelial cells, which form a barrier between the intestines and the rest of the body; and enteroendocrine cells, which help regulate the hormonal environment of the gut. All of these cells have cortisol receptors, and all of them may become dysregulated if cortisol levels climb too high. Cortisol can also speed or slow the time it takes for food to transit through the intestines, which can lead to what’s known as dysbiosis—or an imbalance in the number, type, and distribution of the trillions of bacteria, viruses, and fungi that make up the microbiome inhabiting the digestive tract.

High temperatures are also known to increase the permeability of the intestinal lining, leading to so-called leaky gut. “Temperature has a direct effect on the intestines,” says Desmond Leddin, professor of medicine at Dalhousie University in Canada. “One of the causes of heat stroke is thought to relate to intestinal permeability.”

Leaky gut can also allow organisms that make up the intestinal microbiome—which are supposed to remain in the intestines—to migrate into the bloodstream and spread infection. The microbes that remain behind, meantime, can be thrown entirely out of balance.

“When the connections [in the intestinal lining] become less tight, you can have more oxygen getting into the gut,” says Litchman. “That may stimulate bacteria or other gut microbes that are not necessarily beneficial.”