Invisible to the naked eye, trillions of microbes are in, on, and around your body. By the numbers, the amount of individual bacteria on you is nearly equal to the number of cells that make up your body. These bacteria populations– termed your ‘microbiome’– and the impacts they have on their human hosts fascinate microbiologists, and rightfully so. Studies correlating bacterial species A to human trait Z are frequent, but their popularity on social media tends to be more affected by the click-a-bility of the headline than the soundness of their science.
One such paper generated considerable buzz this past summer, stating that bacteria in an infant’s poop can predict the child’s cognitive development. If there’s one thing I love more than poop jokes, it’s vague assurances that one weird trick can make kids smarter. Internet audiences appear to feel the same. The original paper stands as the most downloaded paper ever from the journal Biological Psychiatry, and coverage from myriad sources drew sizable attention. The IFLScience article alone racked up over 7,100 shares, the title unabashedly stating “The Poop Of Babies Reveals How Smart They’ll Be.” How valid are these claims? How robust was the study? We felt that it was our duty to find answers.
Looking at the study itself, the researchers used well-established methods in their sample collection, bacterial identification, and statistical analysis. Eighty-nine fecal samples, each from a different 1-year-old, were collected, frozen, and checked for bacterial contents through DNA sequencing. The diversity of the fecal bacteria (meaning the number of species observed) between and within samples was calculated and compiled alongside other traits, such as family income, delivery method, maternal and paternal ethnicity, breastfeeding, antibiotic use during pregnancy, and more. The children were also examined over a period of three years with a set of cognitive tests and brain scans. The outcomes of these cognitive tests were then compared to the different variables collected by the researchers. The study found that multiple variables were associated with the bacterial content of the baby poop, and the bacterial content of the poop was associated with cognitive development. While correlations between traits like family income and cognitive development were observed, the researchers primarily reported on the link between bacterial diversity and cognitive abilities.
In a press release, Dr. Rebecca Knickmeyer, the lead professor associated with the study, stated “the big story here is that we’ve got one group of kids with a particular community of bacteria that’s performing better on these cognitive tests…This is the first time an association between microbial communities and cognitive development has been demonstrated in humans.” It’s particularly important to understand that this is only an association. However, the rest of the press release, and most the media coverage, were quick to suggest a stronger relationship. Were the media outlets accurately reporting what the science says?
Fortunately, Dr. Jonathan Eisen, an accomplished microbiology professor here at UC Davis, has been asking these kinds of questions of published microbiome studies for years. Dr. Eisen says that studies like these are often valid, but get oversold somewhere along the translation from the research bench to the general public. Regarding our questions on this study, he said, “many papers report correlations between microbiomes and some health or disease state. But then the press release or the scientist quoted claims a causal connection. It seems so simple, but so many people seem to butcher the ‘correlation does not equal causation’ concept.” When asked to put the study’s conclusions into his own words, Dr. Eisen responded, “the microbiome of baby’s poop is correlated to some aspects of their cognitive development and, as of yet, we have absolutely no idea why this is.”
This was not the first, and will certainly not be the last, study that finds interesting associations between aspects of the human microbiome and health. While these works build important foundations upon which further studies can elaborate, news media can be quick to overstate the connection. So, the next time you see a headline making a bold claim about your microbiome (or poop!):
- Ask yourself if they’re equating correlation and causation.
- Check if the claims are supported by the evidence.
- And, when in doubt, check if Dr. Eisen has given it his “Overselling the Microbiome Award.”
Lead Author: Eric Walters
Literature Review: Caryn Johansen
Expert Contact: Sam Tucci