Phil P Harris en.wikipedia.org
(Or…Causation, Correlation, and Critters)
Michaeleen Doucleff recently posted about the gut microbiome in the NPR blog chronicling global health and development, “Goats and Soda: Stories of Life in a Changing World.” (If you haven’t visited “Goats and Soda” yet, check out the link below—G&S is one my favorites!) In How Modern Life Depletes Our Gut Microbes, Doucleff relates a New York University School of Medicine study characterizing the gut microbiome of the Yanomami tribe, a hunter-gatherer society that has lived for over 11,000 years in the Amazonian Mountains. Doucleff’s story begins in 2009, when Maria Gloria Dominguez-Bello, NYU colleagues, and a Venezuelan medical team first journeyed to the Venezuelan-Brazilian border to contact the Yanomami. Dominguez-Bello collected fecal samples from the tribe and used DNA sequencing to determine the Yanomami gut microbial profile. The Dominguez-Bello lab discovered that the Yanomami has roughly 50 percent more microbial diversity than the American gut. Or as Doucleff states, “Americans’ digestive tracts look like barren deserts compared with the lush, tropical rain forest found inside indigenous people.” As societies “Westernize”, communities lose microbial species. This loss of microbial diversity is typically accompanied by an increased incidence of autoimmune disorders and chronic diseases (think allergies and GI tract disorders). But are these facts related? Researchers don’t have a conclusive answer.
Indeed, microbiologists are still untangling the different factors that drive microbial diversity. Certainly dietary habits and antibiotic usage play an important role. Other researchers emphasize the hygiene hypothesis theory, noting that improved sanitation limit microbial diversity. Of course, Dominguez-Bello noted that these findings shouldn’t give license to romanticize the Yanomami lifestyle. The modern medical and hygiene advancements that may negatively impact microbial diversity have also enabled Western societies to enjoy a relatively healthy living and high life expectancy.
Dominguez-Bello’s study of modernization/urbanization and the gut microbiota is shared by her husband, Martin Blaser MD, the author of Missing Microbes: How the Overuse of Antibiotics Is Fueling Our Modern Plagues. A theme of the lively comments G&S section was the delineation between causation and correlation in microbiome research. Do our microbes drive specific autoimmune disorders (causation)? Or do the modern plagues arise from lifestyle/carcinogens/endocrine imbalance—resulting in subsequent microbial changes (correlation)?
Both the researcher and reader must distinguish between causation and correlation. And both Doucleff and Dominguez-Bello do an admirable job to avoid blurring of the two. When reading a journal/article, here are a few questions I ask to differentiate between cause and correlation:
- Is there a mechanism/possible mechanism to suggest causation?
- What are the limitations of the research?
- Do other studies support the conclusions of the authors?
- What further studies are necessary to demonstrate causation?
The best way to answer these questions is to be well-read in the field and/or have amazing research friends. As for now, I am excited to learn about the next developments in Dominguez-Bello’s project!
Sources + Additional Information