Variation In The Gut Highlights Human Individuality
By Deborah Borfitz
May 16, 2023 | Anatomical investigations performed on 45 individuals who donated their bodies to science have revealed that the human gut is stunningly diverse based on the size of the organs making up the gastrointestinal (GI) system. Whether those differences are based on what people eat, where they live, or their ancestry is anyone’s guess, according to Erin McKenney, assistant professor of applied ecology at North Carolina State University.
For the study, which published recently in the open-access journal PeerJ (DOI: 10.7717/peerj.15148), investigators took post-mortem measurements of human digestive organs as well as those of 10 rats, 10 pigs, and 10 bullfrogs. Imagine how much more diversity might be uncovered in a larger cohort or other species beyond humans, McKenney says, noting that doctors today are trained on a standardized gut that may bear little resemblance to real-world varieties.
In addition to finding pronounced differences between individuals, the study also found that women tend to have longer small intestines than men, which may help them extract nutrients from food more efficiently. Women’s heavier investment in digestion and absorption of lipids supports the so-called “female buffering hypothesis” speculating that women are better able to survive during periods of stress.
All the morphological variation reopens an area of study that hasn’t been charted in over 100 years, says McKenney. The last paper making such a comparison of human guts was published in 1885.
At the time, physicians would demonstrate or refine their surgical techniques on cadavers, she says. A “gentleman scientist movement” was also underway endeavoring to catalogue the world of nature, including the one inside each of us.
What they didn’t do, which was done here, is arrive at a “quantitative sense” of human individuality that could help future physicians and investigators better appreciate the value of individualized medicine, McKenney says. The cecum (at the nexus of the small and large intestine) may be only a few centimeters long in one person but the size of a coin purse in another, she cites as an example.
Precision medicine is an emerging practice whereby biological samples (e.g., blood or stool) from individuals can get analyzed and sequenced to look for markers of diseases, or the potential for them, and guide clinical decision-making, says study co-author Roxanne Larsen, associate professor of veterinary and biomedical sciences at the University of Minnesota. An exploration of the GI tract—what it looks like, the microbes it harbors, and its bidirectional communication with the brain—could offer important new clues since the gut is the primary interface with ingested materials as well as other organs throughout the body.
It's not easy to get personalized information about gut development and morphology that correlate to lifestyle characteristics in the context of a clinical trial, says McKenney. People generally have little interest in participating in highly controlled dietary studies, and even financial rewards wouldn’t entice them.
Examining the guts of animals wouldn’t shed much light on the human condition, she adds. “One of the other findings in our paper is that there was generally less variation in the nonhuman species and part of that is likely due to those animals being reared in captivity on standardized diets and in highly controlled environments.” Given that there is inherently more diversity in human lifestyles than there are animals available to dissect, it is “extra special” that she and her team were able to discover significant differences across non-human species in the study setting.
The value of anatomical education and research, enabled largely by anatomical gifts or donation programs in place at many medical schools, cannot be understated, says Larsen. The 45 donors in the latest study had gifted their bodies to such a program at Duke University School of Medicine.
At the very least, it is hoped that the published paper will raise awareness among medical students that a single diagram of the gut doesn’t describe the anatomical variation they might encounter in the real world, McKenney says. Instead, they can “start to ask questions to enhance or refine their education.”
A better appreciation of the diversity of humans should contribute to our collective humanity and “inspire a bit of humility,” she adds. People might start talking to their doctor differently and looking for more individualized medical treatments or just stand amazed that discoveries about the human gut are still being made in 2023.
All the variation should ultimately delight rather than overwhelm people, says Larsen, since “we continue to function relatively normally.” But, scientifically speaking, the differences could potentially help explain the origin and/or course of many GI disorders.