What can your poop actually tell you about your health?

Short answer

Less than the internet claims. Stool frequency is a poor health signal (normal spans three times a day to three times a week, and 98% of people with IBD or IBS fall in that same range), and there is no reference "healthy" microbiome to compare yours to. What does matter: blood, black or pale stool, unexplained changes, and night-time pooping all warrant a doctor's call; fiber (Americans average 17 g/day against a 20 g target) and a footstool that raises your knees, which tripled the sense of complete emptying in a 52-person crossover trial, are the two changes with real support.

Americans do not like talking about poop, which is a shame, because it is one of the few health readouts you get for free, twice-ish a day, without a lab order. The question is how much it actually tells you. My co-host Emily Oster framed it as: is poop a window to your soul?

Partly. Just not in the ways the supplement industry would like you to believe.

What poop is, and what "normal" looks like

Stool is water, in amounts that depend on how much your colon reabsorbed. It's sloughed epithelial cells from the lining of the colon. It's fiber you couldn't digest. And it's bacterial biomass.

The brown comes from stercobilin, a bilirubin metabolite, which means the color of your poop originates in the heme of your broken-down red blood cells. The smell is partly hydrogen sulfide, the rotten-egg gas, from bacterial metabolism. Part of it is a bacterial product called skatole, which gives human stool its particular signature. Skatole is also a popular perfume additive, because in very small amounts it reads as a floral musk. The dose makes the poison. It is synthesized in a lab, not harvested.

Now, frequency. In NHANES data, the typical range runs from three times a day to three times a week, and the modal American poops once daily. That range sounds useful. It isn't. Roughly 98% of people with IBD or IBS fall inside it too. Being in range tells you your habit is common, not that your gut is fine. There is as much variation between healthy people as there is between health and disease.

Consistency is the more informative variable, and nobody talks about it. The Bristol stool scale runs from type one, separate hard lumps like rabbit pellets, to type seven, watery with no solid pieces. Both ends are abnormal. You want three and four: sausage-shaped with surface cracks, or thinner, snake-like, smooth and soft.

That consistency is mostly a story about water. Your colon pulls water out of stool as it passes, and how much it pulls depends on how long the stool sits there. Longer transit, drier and harder output.

Normal mouth-to-toilet transit time is 10 to 75 hours. That is an enormous range, and you can measure your own. Eat corn on the cob and watch for the kernels, since most people can't digest them. Or dye a muffin with royal blue food coloring and wait for the blue, or greenish, arrival on the other side. The blue poop challenge is an Instagram thing, and it's genuinely fun.

What it is not is a performance metric. Transit time inside that 10-to-75-hour window hasn't been linked to bad outcomes, and speeding things up with laxatives is not a health goal. Too fast means malabsorption, and digesting your food is rather the point. Hold onto transit time, though. It comes back when we get to the microbiome.

The things that actually warrant a phone call

Blood in the stool always deserves a call to your doctor. It's also very common in endurance athletes: something like 40% of serious runners see blood at some point, because during a hard sustained effort your body pulls blood flow away from the gut and digestion falls apart. It comes out bright red and looks exactly as alarming as you'd expect. Tell your doctor anyway, then decide together what it means.

Black stool is abnormal unless you're taking a lot of iron. Pale stool is the one that worries me most. It suggests obstruction of the common bile duct, which at best is a gallstone and at worst is a cancer sitting in the way. Get it evaluated.

Unexplained weight loss and unexplained changes in your pattern also count. So does pooping in the middle of the night. Your bowels run partly on the brain-gut axis, which is why your social environment cues you to go. You don't feel the urge all day, you get home to your own bathroom, and suddenly you do. Coffee does it too, and decaf does it as well, which suggests a good chunk of that is learned. Your brain knows you shouldn't be pooping while asleep. If you regularly are, that's worth a call.

Fiber, posture, and the two things that work

I'll assert this without much evidence: fiber may be the nutrient Americans fall short on more than any other. The recommended intake is roughly 20 to 25 grams a day, and mean US intake per NHANES is 17 grams. And about 20 grams a day is what tracks with normal bowel habits. Fiber is hard to add back to food, which is roughly what makes an ultra-processed food ultra-processed: strip the fiber out and everything gets easier to eat.

Fiber sorts along two axes, soluble versus insoluble and fermentable versus not, and knowing which one you want explains why your doctor recommends fiber for both constipation and diarrhea without specifying.

• Insoluble, non-fermentable fiber, cellulose and wheat bran, is nature's broom. It transits through and cleans you out without generating gas. That's your constipation answer.
• Soluble, non-fermentable fiber, psyllium being the prototype, gels and slows things down and bulks up loose stool. Also low gas.
• Fermentable fibers, resistant starch and the FODMAP roster, are good fiber sources, but your gut bacteria ferment them. That's where the gas comes from.

Beans, chia seeds, and oatmeal are all solid sources, and most of the good options are whole foods, fruits, and vegetables.

You also need adequate hydration for fiber to work. As stool moves through the colon, water gets pulled out, and if you're short on water your body prioritizes it elsewhere and leaves you with type-one pellets. But more is not better. Drinking well past what you need won't improve anything.

On the health claims beyond regularity, I'm more enthusiastic than Emily is. There are strong observational data, including a very large Lancet meta-analysis, linking higher fiber intake to lower cardiovascular and stroke risk, plus lower colon cancer risk. Emily's objection is the right one: people who eat lots of fiber do lots of other healthy things, and stacking more observational data with identical biases doesn't fix confounding, it just makes your correlations more precise. Randomized data haven't supported the colon cancer claims. In fairness, the trial you'd need would run twenty years.

Then there's the squatty potty, which is a plastic stool that raises your knees and approximates a squat, the posture humans used before toilets. There is randomized crossover data: a 52-person trial of a defecation posture modification device found the sense of complete emptying improved severalfold and straining dropped substantially. You obviously can't blind someone to whether their knees are up. But it's cheap and the effect sizes are big, so Emily's position is just get one, and I don't have a good counterargument.

The phone is the flip side. A 2025 study associated prolonged smartphone use on the toilet with hemorrhoids, and the mechanism is loitering. A quick scroll is fine. Sitting there for twenty minutes hiding from your family is how you get hemorrhoids. For what it's worth, all mammals poop in about 12 seconds.

The microbiome sells a lot of product and answers very few questions

Your colon holds an ecosystem: thousands of bacterial species interacting, digesting food for you, releasing compounds you absorb. These are symbiotes, and without them you'd be very sick. Not ten pounds of them, though. It's about 0.2 kilograms, an apple's worth, which is still a staggering number of organisms given how small bacteria are.

Most microbiome studies work by sequencing DNA from a stool sample and inferring which bacteria were present from the unique short sequences each one carries. Nearly everyone shares a core of about 14 genera. That massively undersells the individuality: in a study of 4,000 people, 75% carried at least one bacterial type found in nobody else in the dataset. You probably have a bacterium in your colon that, at least in your town, no one else does.

Here's the pairing that defines the field. Microbiome signatures are stable enough to re-identify more than 80% of people a year later. And yet within one person, any individual bacterium's concentration can swing tenfold from one day to the next. Fingerprints are unique too. That doesn't mean the individual whorls cause disease.

Which brings up the fact that surprises people most: there is no reference microbiome. There is no normal. So when someone tells you to optimize yours, ask them toward what. Whose gut are we aspiring to? And it's plausible that your optimal microbiome and mine are different anyway.

When researchers do repeat testing on the same person to find out what drives change, the primary driver turns out to be transit time. That makes biological sense. The longer stool sits, the more oxygen aerobic bacteria consume, and the anaerobes take over as oxygen falls. There is associational data linking a higher proportion of anaerobes to worse health outcomes. That might argue for shorter transit. Or it might mean a diet that produces short transit is healthy on its own and the bacteria are just downstream scenery.

That ambiguity runs through the whole literature. Serious researchers won't tell you a given bacterium is good or bad. They use sophisticated statistics to find patterns, and they have found patterns associated with inflammatory bowel disease, colon cancer, obesity, and metabolic disease. A common thread is that colonic inflammation lowers oxygen tension, which raises anaerobes. So here's one story: you have IBD for some other reason, it inflames your colon, the inflammation changes your bacteria, we read that off your stool, and the bacteria did nothing except enjoy the low-oxygen real estate. Here's an equally consistent story: you got colonized with more anaerobes because you ate poorly, they release substances that irritate your colon, and that gives you IBD. Observational data cannot separate those. The only way to establish causality is to do something dramatic to a person's microbiome and watch their health.

What happens when you actually change the microbiome

The best case for the microbiome mattering is Clostridium difficile. C. diff colonizes your gut when antibiotics wipe out your resident bacteria. It's frequently hospital-acquired and can become a brutal recurrent diarrheal illness. Transplanting healthy stool into the colon of someone with recurrent C. diff resolves it roughly 80% to 90% of the time, far better than standard antibiotics, and it's endorsed by every major society. When the ecosystem is genuinely destroyed, restoring it works.

The same approach has been tried in adjacent conditions, with promising signals in ulcerative colitis and more mixed results in IBS. But the leap people make from there, that if replacing an entire wrecked ecosystem cures a serious disease then swallowing capsules of bacteria will make a healthy person healthier, is not a small leap. Those aren't the same activity at all. This is the recurring wellness move: start with real biologic plausibility, skip every intermediate step, sell the product. The intermediate steps are where the effect disappears.

Probiotics have been definitively established as useful for exactly three things: preventing C. diff during a course of antibiotics, necrotizing enterocolitis in premature infants, and pouchitis in people who've had a colectomy. That's the list, against a multi-billion-dollar industry. The American Gastroenterological Association specifically advises against probiotics for inflammatory bowel disease, irritable bowel syndrome, and pediatric gastroenteritis, which is to say against nearly everything they're actually purchased for.

There are mechanistic reasons to expect this. Looking directly at the gut lining rather than at stool DNA, researchers found that in a substantial fraction of people the probiotic simply never colonizes. Resident bacteria out-compete it or digest it. And probiotics are an assortment of more or less random organisms. Even if you were genuinely deficient in something specific, there's no reason the blend at CVS addresses it. Telling someone to take a probiotic is like telling them to take an antibiotic. There are a lot of antibiotics. Plenty of influencers will name you a precise strain, and sell it to you, and there is no robust data behind any of them.

Stool testing services complete the loop. Send in a sample, get a report, buy the recommended supplements. When researchers sent the same standardized stool to multiple direct-to-consumer companies, the results came back different. Different counts, different organisms, one company calling a sample healthy and another calling it unhealthy. Companies disagreed with themselves. Four hundred dollars for the test, plus the supplements.

I don't think it's fraud. PCR testing is routine and cheap, it's just hard to do correctly. Consider that when you mail poop, the bacteria aren't dead. They keep growing, and the composition shifts with the temperature it shipped at and how long it sat before analysis. Most of it ships at room temperature. There is no valid way to do this for clinical purposes right now.

The mouse study that launched a thousand supplements

The origin of "obesity is a microbiome disease" is a 2013 study in Science, and it's genuinely elegant science. Researchers found four female twin pairs discordant for obesity: one twin over BMI 30, the other under, separated by at least 5.5 BMI points for years. Only one of the four pairs was identical, which tells you something about how much of obesity is genetic. They transplanted each twin's fecal microbiota into germ-free mice, raised without any bacteria at all, and let them eat freely.

The mice that got the obese twin's stool gained more total and fat mass and developed metabolic complications like elevated blood sugar. The mice that got the lean twin's stool grew up like normal mice.

Then they housed them together and the obese mice lost weight. The reason is that mice eat each other's poop. The lean microbiome apparently dominates.

Sample size four, in animals, with an exposure route that has no human analog. Human replication has not gone well, and randomized trials of probiotics for obesity show essentially no effect. Whatever is driving human obesity, this isn't it.

Bottom line

Pay attention to your poop, but for practical reasons rather than mystical ones. A comfortable daily poop that you then don't think about for the rest of the day is a real quality-of-life win, and the levers that get you there are unglamorous: eat more fiber, drink enough water to make it work, get your knees up, put the phone down. Blood, black stool, pale stool, unexplained weight loss, and night-time pooping go to your doctor.

Past that point, there is no further optimization to do. The microbiome is real, it matters, and destroying it would be a disaster. None of that means anyone currently knows how to improve yours, or what improved would even mean.

Here's our discussion from the episode:

I covered this in depth on Wellness, Actually, listen below.

Wellness, Actually Podcast

"What's the deal with poop?" — Listen to the full episode, including the week's health news and listener Q&A.

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