A baby's first poop correlates with whether the newborn will develop allergies later on, because the metabolite content of this earliest excretion may play an important role in teaching an infant's immune system to accept stimuli ranging from peanuts to pollen.

Drawing on almost 1,000 infant stool samples, including 100 "firsts," from the Canadian Healthy Infant Longitudinal Development study, researchers found that babies were more likely to exhibit allergies by their first birthday when their first stool, also known as meconium, contained a less robust range of metabolites. The study was published Thursday in Cell Medicine.

Lead author Charisse Petersen began and completed most of this research as a postdoctoral researcher at the University of British Columbia. She says that meconium — a greenish-black, tarry substance that infants completely pass after the first few days of life — is kind of like a "time capsule," reflecting all of the exposures a baby received in utero from the mother, including what she ate and what she was exposed to, as well as other compounds, such as amniotic fluid and vernix, the white, waxy coating that develops in utero starting at about five months and protects the fetus's skin.  

The study authors identified 714 metabolites in the meconium, composed mostly of lipids, amino acids and xenobiotics, or plant compounds. These metabolites are crucial in the creation of a newborn's microbiome because they form the food for the first bacteria that will inhabit an infant's gut. More food means a better environment for bacterial growth. 

"What we've been learning is that more and more babies are not being colonized enough by these beneficial microbes early on," Petersen, who is now a research associate at British Columbia Children's Hospital, said in an interview with The Academic Times. "What this is leading to is more children developing allergic responses to signals that should be harmless to them, but their body's not treating it that way."

A prior study co-authored by Petersen showed that a diverse gut microbiome appears to reduce allergic reactions by the time a baby turns 1 year old. But less clear is the connection between the gut microbiome and a rich meconium metabolome, or the total number of metabolites present.

Though Petersen and her colleagues found a significant association between the number of meconium metabolites and subsequent allergies, microbial analysis of stool samples from babies at three months and at one year showed that low metabolomic diversity in meconium did not necessarily result in a less varied microbiome.

According to Petersen, many factors influence the initial microbiome, from how babies are born to how long they are breastfed to whether they've taken antibiotics. But a richly diverse meconium and a robust microbiome likely foster the best combination for preventing allergies. 

According to the National Institute of Allergy and Infectious Diseases, food allergies affect 5% of children and 4% of adults in the U.S. 

It is possible that a metabolically dense meconium could alone help newborn immune cells without needing bacteria, but that hypothesis would require further research and could be the subject of a follow-up study. 

Regardless of how meconium contents may shape it, the early microbiome is paramount to the prevention of allergies because it helps teach the infant immune system how to determine what signals are harmless and what ones are harmful. These friendly bacteria help expose the immune system to different molecules and tell it, "These are actually beneficial, too; don't cause an inflammatory response to these," according to Petersen. 

"It has to be able to protect the infants from pathogens, but it also can't constantly be causing inflammation, because that's not good either," she said. "Training our immune system to know that pathogens are bad, but not to overreact to things like peanuts or pollen and things like that, is something that having a diverse set of microbes early on is important for." 

To solidify the association between meconium metabolites and allergy onset, Petersen would like to see these findings replicated in an independent study as well as in experimental studies. With an observational study like this current one, Petersen says she can only draw associations between the presence of certain metabolites and a healthy immune response. 

Identifying the meconium metabolites that stave off allergies could encourage expectant mothers to introduce these metabolites into their diet, whether by taking vitamin supplements or eating a wide variety of foods.

"Our best chance at treating allergies right now is through prevention," Petersen said. "If we can identify kids or babies that are going to be at risk for developing allergies, the earlier we can do that, the more likely that we can restore their microbiota and help their immune system before allergies even occur, and that's what I'm most excited about."

The study, "A rich meconium metabolome in human infants is associated with early-life gut microbiota composition and reduced allergic sensitization," published April 29 in Cell Medicine, was authored by Charisse Petersen, Darlene L.Y. Dai, Hind Sbihi and Stuart E. Turvey, University of British Columbia and British Columbia Children's Hospital; Rozlyn C.T. Boutin and B. Brett Finlay, University of British Columbia; Malcolm R. Sears, McMaster University; Theo J. Moraes and Padmaja Subbarao, Hospital for Sick Children; Allan B. Becker and Meghan B. Azad, University of Manitoba; and Piush J. Mandhane, University of Alberta.