Researchers used stable isotope tracking in breath to unravel how animal’s waste is recycled by gut microbes. The study found nitrogen recycling via gut symbionts increases in ground squirrels over the hibernation season. Unique application of breath and tissue markers shows how hibernating squirrels rely on gut bacteria to recycle carbon and nitrogen as well as to maintain muscle mass. These findings may one day help treat people with muscle atrophy, which can occur due to malnutrition, old age or even spending time in outer space.