In addition to imposing a systemic hypoxic stressor, high altitude ascent elicits a plasma volume contraction and acid-base dysregulation, and subsequent renal acclimatization. High altitude research expeditions often take place in developing countries, where traveler’s diarrhea (TD) is common, potentially contributing to systemic dehydration and arterial bicarbonate elimination. Unfortunately, TD is rarely accounted for in the assessment of physiological measures related to acclimatization at high altitude, and it may confound these measures. We aimed to assess the extent that the presence of self-reported TD, through its potential effect on dehydration and insensible bicarbonate elimination of pancreatic origin, affects hydration, renal function and blood acid-base homeostasis during ascent and residence at high altitude. Specifically, we hypothesized that those sojourners with self-reported TD (TD+) would have larger magnitude dehydration, reduced renal function and lower arterial bicarbonate with subsequent effects on acid-base variables, compared to those without TD (TD-). We included 33 lowland participants (20M/13F) before (1,100m) and after rapid ascent with subsequent residence at 3,500m for 9-10 days. We performed radial arterial blood draws (ABGs) to assess markers of hydration (measured via %-plasma volume %-PV reduction and osmolality), renal function (via creatinine and estimated eGFR) and acid-base variables both before (1,100m; day 0) and following ascent to 3,500m (days 9/10). On ABG testing days at 3,500m on days 9/10, we grouped data between participants with self-reported TD+ (n=16; 35±11.3yrs; 11M/5F) and TD- (n=17; 34.6±10.2yrs; 8M/9F) for comparison between groups. With respect to hydration status, %-PV reduction and osmolality were not different between TD+ vs. TD- groups (P=0.49; P=0.64), suggesting that TD did not exacerbate relative dehydration associated with ascent. With respect to renal function, creatinine and eGFR were not different between TD+ vs. TD- groups (P=0.42; P=0.77), suggesting that TD did not negatively-affect renal function associated with ascent. With respect to acid-base homeostasis, arterial PCO 2 , HCO 3 - and pH were not different between TD+ vs. TD- groups (P=0.51; P=0.45; P=0.98), suggesting that TD did not affect the assessment of renal acid-base acclimatization with ascent. Upon ascent to and residence at 3,500m over 9-10 days, we demonstrated no differences in arterial blood markers of hydration status, renal function and acid-base acclimatization between self-reported TD+ and TD- groups. Our results suggest that in participants on high altitude research expeditions, who are eating and drinking fluids ad libitum, and treating TD symptoms with over-the-counter medications as needed, the common occurrence of TD on these research expeditions does not appear to confound physiological measures related to acclimatization. ACKNOWLEDGEMENT OF FUNDING Natural Science and Engineering Research Council of Canada Discovery grants, Hotchkiss Brain Institute RHISE and University of Calgary Transdisciplinary Connector Grants This abstract was presented at the American Physiology Summit 2026 and is only available in HTML format. There is no downloadable file or PDF version. The Physiology editorial board was not involved in the peer review process.
Meier et al. (Fri,) studied this question.