Eating less protein may help curb gut bacteria’s growth

The microbes are limited by low nitrogen levels, a study in mice and other mammals suggests

Humans and other animals may have a way to control the growth of gut microbes: Eat less pr otei n.

That’s because protein contains nitrogen. And, it turns out, the amount of nitrogen in the diet of mice governed the growth of bacteria in the animals’large intestine, researchers report October 29 in Nature Microbiology. The finding may help researchers learn how to manipulate the types and amounts of people’s gut bacteria, which can contribute to health and disease.

Researchers know that something must limit bacterial growth. “If not, we’d be a few feet deep in E. coli in a couple of days,” says Thomas Schmidt, a microbiologist at the University of Michigan in Ann Arbor not involved in the study.

But so far, scientists have had limited success controlling which microbes inhabit the colon. That may be because researchers were looking at the wrong nutrients, Schmidt says. Most, including Schmidt, have usually considered carbon — found in fiber, starch and sugars, for example — to be the most important nutrient microbes eat, he says. The new study suggests that other nutrients such as nitrogen may be as important, or even more important, for controlling bacterial growth.

Microbial ecologist Aspen Reese of Duke University knew that in most ecosystems, nitrogen, an essential building block of many biological molecules, is a limited resource. “Nitrogen is pretty important and it’s pretty hard to come by,” she says. If the growth of organisms in other ecosystems is limited by the availability of nitrogen, perhaps bacteria in the intestines are also starving for nitrogen, she reasoned.

Reese, currently at Harvard University, and her colleagues started by measuring carbon and nitrogen concentrations in the feces of 30 mammalian species. Herbivores had the highest carbon levels and the lowest nitrogen levels in their feces, the team found. Carbon was also the major nutrient in the feces of carnivores and omnivores, but the meat eaters had more nitrogen than those animals that ate mostly plants.

Still, none of the animals’poop contained as much nitrogen as labgrown bacteria did, indicating that the microbes can use more nitrogen when they have it. Since gut bacteria incorporate far more carbon than nitrogen, it means the microbes aren’t getting much nitrogen, which could hold back their growth.

Reese’s team also lowered the amount of protein, thereby decreasing nitrogen, in the diets of lab mice. That reduced the numbers of bacteria in the rodents’feces, the researchers found.

Together, these results suggest that levels of nitrogen in the gut help determine bacterial growth. It’s unclear whether having more or less bacteria is good or bad, but when certain types of microbes take over, it’s generally bad for health.