Can Probiotics Help Climbers Better Adjust to High Altitude?

One afternoon in August 2024, high in California’s White Mountains, Tatum Simonson, an associate professor and physiologist with expertise in altitude adaptation at the University of California, San Diego, and her volunteers shuffled into Barcroft Station. The off-grid research outpost sits at 12,470 feet amid jagged peaks and stark scree slopes along the eastern edge of the Sierra Nevada. Within hours of arrival, some participants felt the physical impacts of reaching the high-elevation lab. Pounding headaches, the nausea creeping up, the restless sleep—all of which are telltale signs of altitude sickness (a condition that occurs when you rapidly ascend without giving your body enough time to adjust to lower amounts of oxygen in the atmosphere as your elevation increases).

For decades, mountaineers have relied on little more than slow ascents and sometimes a prescription drug called acetazolamide, better known as Diamox. But Simonson wondered if they were overlooking another factor: the gut.

Bacteria, fungi, and other microbes make up roughly half of the cells in our bodies, and some estimates suggest their genes outnumber our own by about 100 to one. When oxygen runs low at altitude, intestinal microbiota can feel the stress too, potentially shaping how the body senses and responds to low oxygen.

“Gut bacteria are constantly sending signals to each other and to the body’s cells. Some of these signals can be sent far beyond the gut, even to the brain,” Simonson told Outside.

When Simonson experienced the stress of altitude sickness, she wondered: Could these signals help explain why some people feel so sick at altitude?

Simonson’s curiosity with high-altitude physiology led her to the Tibetan Plateau 15 years ago. She was doing fieldwork in Madou County, Quinghai province, at an elevation of 14,241 feet, scanning the genomes of Tibetan, Han Chinese, and Japanese populations to understand how Tibetans have adapted to life at extreme altitudes. It was there that she experienced firsthand how elevation affects the body. “I was trying to lift heavy equipment at a small village hospital, and the altitude just took my breath away,” she recalls. She was told to take Diamox, but a common side effect of the medication, temporary hand tingling, was unpleasant.

Simonson began studying the genetics of Tibetan populations, people who have lived at extreme altitude for millennia. Over time, she expanded her work into the Andes and incorporated the microbiome (all of the bacteria, fungi, and other microscopic organisms that exist on and in the human body) into her research, collecting stool samples from high-altitude communities.

The gut microbiome affects digestion and immunity and may even be linked to mental health. With this knowledge, Simonson’s question shifted: Could she influence a person’s microbiome to help them cope with high altitude?

How Low Oxygen Impacts the Gut at High Altitudes

Even the hardiest climbers can feel the toll of altitude sickness. Nearly 80 percent of people with acute mountain sickness experience at least one gastrointestinal symptom because, at altitude, reduced pressure and oxygen levels alter gut bacteria. These gut issues are referred to as high-altitude flatus expulsion. Among climbers on Everest, diarrhea is commonly reported, and nausea and vomiting are frequently observed in trekkers ascending to high elevations, even with access to clean water.

For mountain guide Emilie Drinkwater, those symptoms are all too familiar. Drinkwater has been guiding professionally since 2001, and is the 9th American woman to hold IFMGA certification, the highest credential a professional mountain guide can earn. She guides heli-skiing in Utah in winter and alpine climbing in Wyoming in summer, and has made first ascents in Alaska, the Karakoram, and the Hindu Kush. “My body doesn’t feel good at altitude,” she says. “The number one symptom is nausea and a near inability to eat above 15,000 feet. I’ve always attributed this to intestinal hypoxia (low oxygen in bodily tissues). Other symptoms include deep fatigue, headaches, and poor sleep.” She manages the headaches and poor sleep with hydration and Diamox, but nothing seems to ease the nausea or low appetite, and she reports weight loss on longer expeditions.

One possible explanation for the nausea and loss of appetite could be a disruption of the intestinal barrier, the lining that keeps bacteria and their byproducts contained. At altitude, that barrier may weaken.

“When the body is hypoxic, the intestinal barrier begins to ‘break down,’ and tiny fragments of bacteria can slip into the bloodstream and interact with the immune system, potentially triggering inflammation,” says Zach McKenna, an assistant professor of exercise science at the University of Arkansas. McKenna studies the effects of altitude and other environmental stressors on the intestinal barrier and was not involved with Simonson’s research.

The brain, meanwhile, responds to the drop in oxygen by acting as the control center, signaling the body to breathe faster and deeper at higher elevations. But the gut might also be sending signals to the brain through the gut-brain axis, a two-way network of nerves, hormones, and immune signals. Keeping the gut healthy with probiotics may help support these signals.

Still, experts say the gut remains something of a mystery. “We don’t fully know what causes gastrointestinal issues during high-altitude exposure,” McKenna adds. “The reasonable culprits include oxidative stress (cell damage), damage to the microvasculature (blood vessels), and intestinal barrier dysfunction. All of these are tied to low oxygen levels, as well as the way blood is redistributed away from the gut during altitude exposure and activity or exercise.”

If that’s the case, could probiotics help?

Climbers Tested Probiotics at a High-Altitude Lab

To test this idea, Simonson and her team brought 17 volunteers to Barcroft Station, an ideal high-altitude laboratory, to examine how a probiotic might influence oxygen levels, sleep, and gut-brain signaling in real-world conditions. Study participants received either a probiotic containing multiple bacterial strains or a placebo, administered prior to ascent and up to three times per day during the climb.

“Within a few hours and during the next few days, we measured their oxygen saturation,” says. “Those taking the probiotic had higher oxygen saturation and felt better overall. Sleep is often disrupted at altitude, but they maintained higher oxygen even as they slept.” According to Simonson, these findings are important because the “results suggest there could be signals traveling from the gut to the brain, helping trigger these responses.”

The findings, published in the journal iScience, are encouraging, though still early. Simonson describes the study as exploratory and says future research will test different probiotic strains and timing during the trek to assess how the gut responds at high altitudes. “People may respond differently to probiotics, and long-term studies with large groups of people are needed,” she adds.

“The study presents some promising results,” says McKenna. He agrees that Simonson’s study needs to be replicated in a larger group and with more direct measures. “While the connection between gut barrier function and acute mountain sickness isn’t fully proven, probiotics may offer benefits on multiple fronts,” he says.

For instance, research published in 2025 by the journal Frontiers in Nutrition suggests probiotics may help protect cells from oxidative stress. Other recent research shows that probiotics can be helpful in reducing cognitive decline in people with neurodegenerative diseases such as Parkinson’s and Alzheimer’s disease.

What Does Future Gut Microbiome Research Mean for Climbers?

If the gut does play a role in coping with altitude, it could have real implications for mountain safety. Maintaining good judgment can be the difference between a safe expedition and a risky one. “I need to be capable, competent, and strong enough to keep making good decisions and managing risk for myself and the team,” Drinkwater says. “To get through this, I prioritize appropriate acclimatization, hydration, and rest.” She also routinely checks in with team members to see how they’re feeling.

Understanding how probiotics and the gut impact altitude sickness is an exciting frontier that scientists say could redefine what we know about the trillions of microbes within us and how that relationship changes at high elevations.

When asked if she’d take a probiotic herself, Simonson laughs: “I absolutely would.” And since altitude affects her husband more, she’d make sure he took one too, just in case.

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