Early growth in infants may be tied to the composition of bacteria living in their guts.

Traditionally, doctors measure an infant’s growth in pounds and inches—or kilos and centimeters. But how about in terms of…gut bacteria? While it’s not a foolproof way to predict how big an infant will grow, the microbiota in a newborn baby’s gut appears to impact his or her growth rate.

Researchers from the Norwegian Institute of Public Health report that the composition of gut microbiotathe body’s microbial ecosystem, which is composed of good bacteria like probiotics and other less-than-great bacteria that can cause infectionsis directly linked to growth in the early stages of infancy.

“We were interested in exploring associations between the gut microbiota and growth, since prior research in animals and humans have indicated that gut microbiota can play a role in the development of obesity,” said study author Merete Eggesbø, M.D., in an interview with Healthline.

It seems odd that, in addition to the important factors of diet, care, and genetics, the bacterial ecosystem in the digestive tract can affect growth so strongly. But the composition of gut microbiota can influence how many calories a diet yields, in addition to the deposition of fat in our fat cells, Eggesbø says.

As it turns out, there are “fat” and “skinny” microbes. “Lean animals have put on weight when the microbes from obese animals were transferred to them,” Eggesbø said.

Depending on the type of bacteria in the baby’s gut, researchers observed either slower-than-normal or expected growth. In the study, Bacteroides species detected in 1-month-old males were significantly associated with reduced growth, while the presence of E. coli between four days and one month after birth was associated with expected growth in both boys and girls. E. coli, the same species that can cause intestinal infections, is actually quite common in the guts of humans, even very young ones.

However, Eggesbø is quick to point out that gut micobiota is not a perfect indicator of growth. Instead, she hopes that “in the future we will be able to manipulate gut microbiota and thereby both prevent and cure disease,” although that is far down the road and will require years of additional research.

There are many common misconceptions about microbes, especially that all are bad for us. In a world of antibacterial everything, the idea that bacteria can be a good thing may seem a bit far-fetched.

“In fact, we depend on them for many of our normal functions,” Eggesbø said. “Optimal gut microbiota will help us digest our food, synthesize nutrients we are not able to make ourselves, fend off invasive bacterium, and [help] our immune system to develop and mature.”

In the study, Eggesbø’s team tested the composition of gut microbiota in infants when they were 4, 10, 30, and 120 days old. While there are about 500 species of microbes in the early infant gut, researchers had 22 probes that mapped only broad bacterial groups. So unfortunately, the findings only hint at “the possibility that components of the gut microbiota and its functioning may play a role in early growth and obesity,” Eggesbø said.

Researchers must now try to understand how the various species of bacteria work together. Just as your body is composed of many different organs with differing functions, gut microbiota is an ecosystem in which everything works in concert. “Overall functionality, and not the ‘name of the microbes’ is what matters,” Eggesbø said.

Once researchers develop a more complex and complete picture of how different gut microbes collaborate, it’s possible that they will be able to manipulate the gut ecosystem to help infants who are not thriving reach their growth targets. “If in the future we find the optimal composition, which would be subject- and diet-dependent, it could have huge potential,” Eggesbø said.