A baby's very first poo, known as the meconium, is not something most people want to look at too closely.
This dark green, tar-like substance contains swallowed amniotic fluid, skin cells, and fine hairs from a life in the womb. At a microscopic level, the meconium also holds the starting material for our developing microbiome and our immune system.
If a rich and balanced mix of molecules isn't found in this initial stool, researchers now think it could stunt the colonization of good bacteria in our gut, putting us at risk of developing allergies, like asthma or eczema, later in life.
Researchers analyzed 100 meconium samples taken as part of the CHILD Cohort Study, a much larger, ongoing longitudinal research project about child health. They then compared those representative samples to allergy tests performed on the infants at one year of age.
The results showed that infants who had developed allergic sensitization by that point had less diverse metabolites in their first poo.
"We found that the reduced diversity was mainly detected in the number of different amino acids, vitamins, and plant compounds," explains Charisse Petersen, who studies the early microbiome at The University of British Columbia in Canada.
These clusters of molecules are known to feed the growing microbiome, which begins to form when a child enters the world and microbes begin colonizing their gut. The presence of these new invaders also allows the immune system to learn what actually poses a danger.
"The new microbes are tolerated and allowed to colonize us, and they, in turn, teach our immune cells not to overreact to benign signals," says Petersen.
"Unfortunately, we think that some babies are not being colonized with enough of these beneficial bacteria to train their immune cells."
The sample size is small, and further research will need to verify the results among a more diverse cohort. If the authors are right, however, it means we could feasibly start using a child's first poo to predict their risk of developing allergies.
The meconium begins forming in a fetus at just 16 weeks, which means its make-up is influenced by whatever the fetus encounters from that point on.
As such, the authors of the study decided to collect a large quantity of data from expectant parents, to see if they could figure out what might be influencing the diversity of metabolites in their child's first poo.
The team considered everything from exposure to antibiotics during pregnancy, a family history of allergies, the way the child was born (vaginally or via C-section), and a history of maternal smoking, but ultimately, none of these factors could explain their results.
If we can further identify key metabolites that are missing, we could even start providing supplements to promote a healthy immune response in newborn kids.
Some researchers have even suggested using poop transplants to rebalance the guts of babies born via C-section, although that currently remains a radical idea.
"I would like to see more mechanistic studies that try to understand precisely which metabolites are the most helpful for supporting the infant microbiota and immune system," says Petersen.
"In a perfect world, we would eventually make sure that these were included in a healthy diet or vitamin supplement during pregnancy."
Further studies have already been started to examine how diet during pregnancy might play a role.
"As a new mother myself, I was initially hesitant to put any more pressure on pregnant women or moms (we worry about so much as it is!)," says Petersen.
"But when it comes down to increasing the number of metabolites in a newborn's meconium, many pregnant women are likely already doing this without realizing it."
To improve a child's gut health, Petersen recommends avoiding unnecessary antibiotic prescriptions, allowing kids to play outdoors, reducing disinfectant use, and promoting vaginal births and breastfeeding where possible.
The study was published in Cell Reports Medicine.