Astronaut constipation gets a blood marker, and a Mars-length problem attached

A NASA-backed study published June 29 in Nature Communications put a measurable chemical signature under one of spaceflight's oldest nuisances, analyzing more than 400 blood samples from 52 astronauts who each spent more than two months aboard the International Space Station between 2006 and 2018 ^[2][3]. The result matters less for the constipation than for what the mechanism generates: metabolites that the authors say could climb to harmful levels on a Mars-duration flight ^[1].

The problem itself is not new. Difficulty with what the crews euphemistically call number two showed up as soon as people started living in orbit, and while it is an inconvenience for most astronauts, some need laxatives, which are routinely stocked on missions including Artemis II ^[11]. NASA lists constipation alongside bloating, indigestion and acid reflux as recurring digestive complaints ^[12]. Microgravity has long been the presumed culprit, but the hypothesis was hard to test, and the knock-on effects of chronic constipation harder still to characterize ^[13].

The blood work narrows it. The team screened for metabolites and flagged 40 compounds, with the most informative ones produced by gut bacteria breaking down protein in the intestines ^[4]. According to first author Giorgia La Barbera of the University of Copenhagen, the samples show gut bacteria fermenting protein more than usual within weeks of arrival in space, a shift that can persist until the crew is back on Earth ^[5]. Protein fermentation is what happens when gut microbes run short of their preferred substrate, complex carbohydrates such as fiber, and switch to protein instead ^[6].

The proposed cause is mechanical rather than exotic. Co-author Henrik Roager says the lack of gravity probably slows food's passage through the intestine, giving bacteria more time to strip out the fiber before turning to protein, and that the slower transit can help explain the constipation ^[7]. That causal step is stated as probable, not directly measured by the blood panel. Diet was checked as an alternative: altered caffeine, fish and fat intake may play a role, but accounts for less than a third of the observed metabolite changes, which the researchers say leaves microgravity as the primary driver ^[8]. Put the other way, more than two thirds of the shift is not explained by what the crews ate ^[14].

The downstream finding is the operationally awkward one. Co-author Lars Ove Dragsted says protein fermentation products are often associated with kidney damage and possible effects on mood and concentration ^[9], and in the gut they have been linked to inflammation and a weakened intestinal lining ^[10]. Over a station rotation, compounds such as ammonia and hydrogen sulfide likely pose limited risk; over a flight to Mars, the authors warn they could reach harmful levels ^[1].

Worth watching: whether the 40-compound panel migrates from research tool to routine crew monitoring, and whether food system design responds with more fermentable fiber rather than more laxatives ^[4][11]. A dietary lever that explains under a third of the effect is a weak lever for a multi-year mission ^[8].