Published April 7, 2026 01:24PM<\/p>\n<\/div>\n
One afternoon in August 2024, high in California\u2019s White Mountains, Tatum Simonson<\/a>, 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\u2014all of which are telltale signs of altitude sickness (a condition that occurs when you rapidly ascend without giving your body enough<\/a> time to adjust to lower amounts of oxygen in the atmosphere as your elevation increases).<\/p>\n For decades, mountaineers have relied on little more than slow ascents and sometimes a prescription drug called acetazolamide<\/a>, better known as Diamox. But Simonson wondered if they were overlooking another factor: the gut.<\/p>\n Bacteria, fungi, and other microbes make up roughly half<\/a> of the cells in our bodies, and some estimates suggest their genes outnumber our own by about 100 to one<\/a>. When oxygen runs low at altitude, intestinal microbiota can feel the stress too<\/a>, potentially shaping how the body senses and responds to low oxygen.<\/p>\n \u201cGut bacteria are constantly sending signals to each other and to the body\u2019s cells. Some of these signals can be sent far beyond the gut, even to the brain,\u201d Simonson told Outside<\/i>.<\/p>\n When Simonson experienced the stress of altitude sickness, she wondered: Could these signals help explain why some people feel so sick at altitude?<\/i><\/p>\n <\/p>\n <\/section>\n Simonson\u2019s curiosity with high-altitude physiology led her to the Tibetan Plateau 15 years ago. She was doing fieldwork<\/a> 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. \u201cI was trying to lift heavy equipment at a small village hospital, and the altitude just took my breath away,\u201d she recalls. She was told to take Diamox, but a common side effect of the medication, temporary hand tingling, was unpleasant.<\/p>\n 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<\/a> (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.<\/p>\n The gut microbiome<\/a> affects digestion and immunity and may even be linked to mental health. With this knowledge, Simonson\u2019s question shifted: Could she<\/i> influence a person\u2019s microbiome<\/i> to help them cope with high altitude?<\/i><\/p>\n Even the hardiest climbers can feel the toll of altitude sickness. Nearly 80 percent<\/a> 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<\/a>. 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<\/a>.<\/p>\n For mountain guide Emilie Drinkwater<\/a>, 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. \u201cMy body doesn\u2019t feel good at altitude,\u201d she says. \u201cThe number one symptom is nausea and a near inability to eat above 15,000 feet. I\u2019ve always attributed this to intestinal hypoxia (low oxygen in bodily tissues). Other symptoms include deep fatigue, headaches, and poor sleep.\u201d 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.<\/p>\n 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.<\/p>\n \u201cWhen the body is hypoxic, the intestinal barrier begins to \u2018break down,\u2019 and tiny fragments of bacteria can slip into the bloodstream and interact with the immune system, potentially triggering inflammation,\u201d says Zach McKenna<\/a>, 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\u2019s research.<\/p>\n 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<\/a>, hormones, and immune signals. Keeping the gut healthy with probiotics may help support these signals<\/a>.<\/p>\n Still, experts say the gut remains something of a mystery. \u201cWe don\u2019t fully know what causes gastrointestinal issues during high-altitude exposure,\u201d McKenna adds. \u201cThe 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.\u201d<\/p>\n If that\u2019s the case, could probiotics help?<\/p>\n 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.<\/p>\n \u201cWithin a few hours and during the next few days, we measured their oxygen saturation,\u201d says. \u201cThose 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.\u201d According to Simonson, these findings are important because the \u201cresults suggest there could be signals traveling from the gut to the brain, helping trigger these responses.\u201d<\/p>\n The findings, published in the journal iScience<\/i>,<\/a> 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. \u201cPeople may respond differently to probiotics, and long-term studies with large groups of people are needed,\u201d she adds.<\/p>\n \u201cThe study presents some promising results,\u201d says McKenna. He agrees that Simonson\u2019s study needs to be replicated in a larger group and with more direct measures. \u201cWhile the connection between gut barrier function and acute mountain sickness isn\u2019t fully proven, probiotics may offer benefits on multiple fronts,\u201d he says.<\/p>\n For instance, research published in 2025<\/a> by the journal Frontiers in Nutrition<\/i> suggests probiotics may help protect cells from oxidative stress. Other recent research shows<\/a> that probiotics can be helpful in reducing cognitive decline in people with neurodegenerative diseases such as Parkinson\u2019s and Alzheimer\u2019s disease.<\/p>\n 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. \u201cI need to be capable, competent, and strong enough to keep making good decisions and managing risk for myself and the team,\u201d Drinkwater says. \u201cTo get through this, I prioritize appropriate acclimatization, hydration, and rest.\u201d She also routinely checks in with team members to see how they\u2019re feeling.<\/p>\n 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.<\/p>\n When asked if she\u2019d take a probiotic herself, Simonson laughs: \u201cI absolutely would.\u201d And since altitude affects her husband more, she\u2019d make sure he took one too, just in case.<\/p>\n Want more <\/b>Outside<\/i><\/b> health stories? <\/b>Sign up for the Bodywork newsletter<\/b><\/a>.<\/b><\/p>\n <\/div>\nWhat Are the Stages of Altitude Sickness?<\/b><\/h2>\n
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How Low Oxygen Impacts the Gut at High Altitudes<\/b><\/h2>\n
Climbers Tested Probiotics at a High-Altitude Lab<\/b><\/h2>\n
What Does Future Gut Microbiome Research Mean for Climbers?<\/b><\/h2>\n