Hibernating ground squirrels eat nothing for months, yet their bodies keep losing carbon with every breath. Where do their cells find the carbon they need? A study in the Proceedings of the National Academy of Sciences, led by Matthew Regan of Université de Montréal's Department of Biological Sciences, reveals an unexpected answer: the animals recycle their own metabolic waste, with their gut microbes acting as the processing plant.
The team injected squirrels with urea labelled with carbon-13, a rare isotope easy to trace through the body. Rather than being excreted in urine, the urea was rerouted to the intestines, where microbes equipped with enzymes vertebrates lack broke it down and converted its carbon into acetate, which the squirrels absorbed and used. When antibiotics eliminated the gut microbes, the recycling stopped. The process intensifies through hibernation and peaks just before the animals wake in spring, complementing the team's earlier discovery of an equivalent nitrogen-salvage pathway that helps preserve muscle.
The human implications are tantalising. Urea recycling is well documented in ruminants, and studies from the 1990s suggest humans possess the necessary microbes and cellular machinery, albeit far less efficiently than hibernators. Understanding how to enhance the pathway could inform strategies against sarcopenia, the muscle loss that begins around age 30 or 40, and against the accelerated wasting experienced by astronauts, who currently train for hours daily to limit it.