Short-term exposure to high relative humidity increases blood urea and influences colonic urea-nitrogen metabolism by altering the gut microbiota

Colonic urea-nitrogen metabolites have been implicated in the pathogenesis of certain diseases which can be affected by environmental factors. We aimed to explore the influence of ambient humidity on colonic urea-nitrogen metabolism. Blood biochemical indexes, metabolites of intestinal tract, and gut microbiota composition of mice (n = 10/group) exposed to high relative humidity (RH, 90 ± 2%) were analyzed during the 14-day exposure. After 12-h exposure, plasma blood urea nitrogen (BUN) level increased along with a decrease in the activity of erythrocyte Na+/K+ -ATPase. Moreover, abnormal erythrocyte morphologies appeared after 3 days of exposure. The colonic BUN and ammonia levels increased significantly after the 12-h and 24-h exposure, respectively. The colonic level of amino acids, partly synthesized by gut microbiota using ammonia as the nitrogen source, was significantly higher on the 7th day. Furthermore, the level of fecal short-chain fatty acids was significantly higher after 3-day exposure and the level of branched-chain fatty acids increased on the 14th day. Overall, gut microbiota composition was continuously altered during exposure, facilitating the preferential proliferation of urea-nitrogen metabolism bacteria. Our findings suggest that short-term high RH exposure influences colonic urea-nitrogen metabolism by increasing the influx of colonic urea and altering gut microbiota, which might further impact the host health outcomes.