After more than thirty-five years practicing internal medicine, I’ve come to appreciate something that received remarkably little attention during my medical education. A good bowel movement is as much a problem of physics as it is of physiology.
We spend years learning about gastrointestinal hormones, digestive enzymes, microbiology, inflammation, and intestinal motility. We memorize the causes of constipation and diarrhea. We prescribe medications, recommend fiber, and order colonoscopies.
But here’s a question I don’t remember anyone asking in medical school: What actually makes a good poop?
Not how often someone goes. Not what medications they take. Not whether their colonoscopy was normal. What makes stool physically capable of moving comfortably through the human colon and emerging in a form that makes its owner happy?
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It sounds like a ridiculous question. I increasingly think it’s an important one.
We measure what is easy to measure
When patients complain about their bowel habits, physicians generally begin with familiar questions. How often do you go? Are you constipated? Do you have diarrhea? Are you eating enough fiber? Drinking enough water? All reasonable questions.
But frequency alone tells us remarkably little about the experience. A patient may have a bowel movement every day and still struggle with hard, fragmented stool and incomplete evacuation. Another may go every other day, comfortably and without difficulty. Both might fall within the range of normal bowel frequency. Only one is satisfied.
We’ve become quite good at counting bowel movements. We’re less sophisticated about understanding their physical quality. And that quality matters enormously to the person sitting on the toilet.
Your colon is not simply a plumbing system
I like to think of the colon as a remarkably sophisticated materials-processing machine. It receives liquid and partially digested material, absorbs water, allows microorganisms to perform their work, and moves the remaining contents toward the exit.
What eventually emerges is not merely waste. It is a physical structure. It has water content, viscosity, cohesiveness, deformability, and frictional properties. Each influences how comfortably stool travels through the bowel and how easily it can be evacuated.
Too little water, and stool may become hard and difficult to move. Too much water relative to its solid structure, and the result may be loose, poorly formed stool. But water alone doesn’t explain everything.
Consider the difference between wet sand, modeling clay, and gelatin. All contain water. Yet each behaves differently when compressed, stretched, or pushed through an opening. Why should we assume that stool is any less sensitive to its physical structure?
Fiber is more complicated than grams
For decades, our standard advice has been remarkably consistent. Eat more fiber. Drink more water. Exercise.
Sometimes that works beautifully. Sometimes it doesn’t.
Patients regularly tell me they’ve increased their fiber intake only to become more bloated, uncomfortable, or dissatisfied with their bowel movements. The usual response is to recommend even more fiber, often without considering the type of fiber already being consumed.
But fiber is not a single substance. Different fibers possess different physical properties. Some dissolve in water. Some form viscous gels. Others contribute bulk with relatively little viscosity. Their fermentability, particle size, and interactions with water also differ.
Psyllium, for example, has clinical evidence supporting its usefulness in constipation and in improving stool consistency in some patients with loose stools. But that doesn’t mean every fiber behaves like psyllium, or that simply increasing the number of grams produces a predictable result. The physical characteristics of the material matter.
In engineering, nobody would expect two materials with different compositions and structures to behave identically simply because they weigh the same. Yet discussions of dietary fiber sometimes come remarkably close to making that assumption.
The overlooked concept of structure
Consider what we actually want from a bowel movement. Ideally, stool should be soft enough to pass comfortably but cohesive enough to maintain its form. It should deform under pressure without becoming either a collection of hard pellets or an uncontrolled liquid.
These are problems of materials science. Water retention, gel formation, viscosity, and the mechanical behavior of fiber-containing mixtures may all influence that outcome.
The clinical challenge is determining which properties matter most for a particular patient. And that’s where our knowledge remains incomplete.
We have meaningful clinical evidence for certain fibers and bowel disorders, but we have far less direct evidence connecting specific measurable physical properties of individual fiber preparations to patient-important outcomes. That’s not a reason to dismiss the idea. It’s a reason to investigate it.
A little humility after thirty-five years
One of the benefits of practicing medicine for several decades is recognizing how often conventional wisdom is incomplete. I’ve spent much of my professional life in preventive medicine and lipidology, fields where strongly held assumptions have repeatedly been challenged by better evidence. Gastrointestinal health deserves the same intellectual curiosity.
I’m not suggesting we’ve discovered some universal secret to the perfect bowel movement. There are many causes of abnormal bowel habits, including medications, pelvic floor dysfunction, inflammatory disease, altered motility, and disorders of gut-brain interaction. A physics-based perspective won’t replace a careful history, appropriate testing, or established treatment. But it may help us ask better questions.
Instead of simply asking whether patients are getting enough fiber, perhaps we should ask what that fiber actually does when it encounters water. Instead of treating all loose stools or all constipation as equivalent, perhaps we should pay closer attention to the physical characteristics of what patients produce. And instead of assuming a daily bowel movement represents success, perhaps we should ask the patient whether the experience was actually satisfactory.
After thirty-five years, I’m increasingly convinced that one of medicine’s most neglected subjects has been hiding in plain sight. We have studied the colon extensively.
Perhaps it’s time to give the poop itself a little more respect.
Larry Kaskel is an internist and “lipidologist in recovery” who has been practicing medicine for more than thirty-five years. He runs a concierge practice in the Chicago area and serves on the teaching faculty at the Northwestern University Feinberg School of Medicine. He is also affiliated with Northwestern Lake Forest Hospital.
Before podcasts were even a thing, Dr. Kaskel hosted Lipid Luminations on ReachMD, building a library of more than four hundred shows featuring thought leaders in cardiology, lipidology, and preventive medicine.
He is the author of Dr. Kaskel’s Living in Wellness, Volume One: Let Food Be Thy Medicine, a book that blends evidence-based medicine with practical strategies for a longer healthspan. His current work focuses on rethinking the cholesterol hypothesis and exploring the infectious roots of atherosclerosis. More information is available at larrykaskel.com.
