The No.1 Poo & Gut Scientist: If Your Poo Looks Like This Go To A Doctor! Dr Will Bulsiewicz

Summary

In this interview, gastroenterologist Dr. Will Bulsiewicz argues that the gut microbiome—the community of mostly bacteria living in the colon—helps shape digestion and may also influence immune function, metabolism, mood, and other aspects of health. He presents diet and lifestyle as ways to support that microbial community, with particular emphasis on eating a varied, fiber-rich range of plant foods.

The conversation also explains how stool appearance and bowel transit time can offer clues about gut function. Dr. Bulsiewicz describes the Bristol Stool Scale and says that visible blood, black tarry stool, or unusually pale stool can warrant medical attention. These observations are clues, not a diagnosis.

Main Ideas

The Gut Microbiome and Health

• Dr. Bulsiewicz describes humans as “superorganisms”: our bodies coexist with vast populations of microbes, concentrated in the colon.
• He says gut microbes help break down food and train the immune system. He also discusses research linking the microbiome with blood sugar and fat responses, inflammation, mood, cognition, hormones, and other health conditions.
• The guest emphasizes that individual microbiomes vary substantially—even identical twins may share only a portion of their microbial communities.
• He says diet can change the microbiome relatively quickly: research discussed in the interview found measurable shifts within about a day of changing what people ate.
• These connections are not all equally established. The conversation refers to human studies, observational associations, and animal experiments; some proposed effects and treatments remain under investigation.

Fiber, Plant Variety, and Microbial By-products

• Fiber is important, Dr. Bulsiewicz says, because gut microbes ferment certain fibers and resistant starches into short-chain fatty acids, including butyrate, acetate, and propionate.
• He describes these compounds as signaling molecules that may support the gut barrier and influence immune and metabolic processes.
• He distinguishes:
  • Prebiotics: Foods or substances that beneficially nourish gut microbes.
  • Probiotics: Live microorganisms shown to benefit people.
  • Postbiotics: Compounds produced by microbes.
• He cautions that a postbiotic supplement should not be assumed to have the same effects as eating fiber-rich foods.
• He recommends aiming for a broad variety of plants. A figure discussed from the American Gut Project is 30 different plant foods per week—including fruits, vegetables, whole grains, nuts, seeds, and legumes. He frames this as a goal to work toward, not a strict cutoff.
• For people who currently eat little fiber, he recommends increasing it gradually. Gut microbes need time to adapt, and increasing fiber too quickly can cause discomfort.

Fermented Foods and Food Preparation

• Fermented foods such as sauerkraut, kimchi, and traditionally fermented pickles are presented as a way to introduce live microbes and increase dietary variety.
• Dr. Bulsiewicz explains fermentation as a controlled process in which microorganisms transform food. For example, cabbage can be chopped, packed into a jar with salt brine, and left to ferment.
• He describes fermented foods as potentially containing microbes, microbial food sources, and compounds produced during fermentation.
• The interview cites a Stanford trial in which adding fermented foods was associated with increased gut-microbiome diversity over eight weeks.

Practical Eating Framework: “F Goals”

Dr. Bulsiewicz offers the following categories as a way to remember foods to include:

• F — Fruit
• F — Fermented foods
• G — Greens
• G — Whole grains
• O — Omega-3-rich seeds and nuts, such as chia, flax, hemp, and walnuts
• A — Aromatics, such as onions, garlic, and shallots
• L — Legumes, including beans, peas, and lentils
• S — Mushrooms, seaweed, and sprouts

His overall approach is to add more nourishing foods rather than focus mainly on restriction, and to choose changes that are realistic and sustainable.

Stool, Transit Time, and What to Look For

Dr. Bulsiewicz says that stool can give a rough indication of gut transit and bowel function. He discusses a “blue poo” test in which people eat a blue-dyed muffin and record how long it takes for the dye to appear in their stool.

• He describes roughly 24–48 hours as a common transit-time range.
• In the studies he discusses, under 14 hours is described as very fast and over 58 hours as very slow.
• Slow transit is associated with harder, drier stool; fast transit with looser stool. He links adequate fiber intake with more normalized bowel movements, while noting that symptoms may have causes beyond fiber intake.

Bristol Stool Scale Overview

• Types 1–2: Hard pellets or lumpy stool; associated with constipation.
• Type 3: Formed stool with cracks; can move toward the healthier middle of the scale.
• Type 4: Smooth, soft, sausage-shaped stool; presented as the ideal.
• Type 5: Soft pieces; can occur with high fiber intake and is not necessarily a problem.
• Types 6–7: Mushy stool or liquid diarrhea.

The guest says types 3–5 are generally within the preferred range, with type 4 as the target. He links types 1 and 7 to low fiber intake in some studies, but stool patterns should be considered in context.

Stool Color and Possible Warning Signs

• Brown: Usually due to bile, which is involved in fat digestion.
• White or very pale: May occur if bile is not reaching the intestine; Dr. Bulsiewicz says this can indicate a blockage and should be discussed with a doctor.
• Yellow or oily: May suggest difficulty digesting fat, which can occur with pancreatic problems.
• Green: Can result from foods such as green smoothies, but can also occur with certain infections.
• Blue: May follow blue food dye or eating many blueberries.
• Red: Can follow eating beets, but visible blood should not automatically be dismissed as hemorrhoids. Dr. Bulsiewicz recommends medical evaluation, particularly because blood can sometimes be associated with polyps or cancer.
• Black: Can result from medicines such as bismuth (Pepto-Bismol), but black, tarry, foul-smelling stool can indicate bleeding higher in the digestive tract and warrants prompt medical attention.

Weight, Metabolism, and Medication

• Dr. Bulsiewicz argues that food quality and the microbiome affect how the body handles energy, so weight is not explained by calorie intake alone. He acknowledges that calorie reduction can cause weight loss but says restriction may be difficult to sustain and can lead to weight regain.
• He says fiber-rich foods may promote fullness through gut hormones such as GLP-1 and peptide YY.
• He discusses GLP-1 medicines such as semaglutide (Ozempic/Wegovy), noting that they can be useful for people who need them but may cause digestive side effects. He says long-term effects are still being studied and weight may return after stopping.
• His preference is to address underlying diet and lifestyle factors where possible, rather than treating medication and lifestyle change as interchangeable.

Fecal Microbiota Transplants

• Fecal transplants have an established role in treating certain C. difficile infections, according to the guest.
• Their use for conditions such as ulcerative colitis is not yet reliably established. He describes early research as promising but limited, and says identifying suitable donors and proving effectiveness require further study.
• He also discusses animal studies in which transferring microbiomes appeared to transfer traits related to weight, diabetes, inflammation, or mood. He cautions that such results do not mean these conditions can currently be treated in humans by simply transferring stool.

Lifestyle, Stress, Alcohol, and Relationships

• Dr. Bulsiewicz says stress and early-life trauma can affect gut symptoms and may be relevant for some people whose symptoms persist despite diet and exercise changes.
• He argues that alcohol can damage the gut barrier and microbiome, particularly when consumed to the point of a hangover. This is presented as his interpretation of the research he discusses.
• He also emphasizes sleep, exercise, and human connection as parts of a gut-supportive lifestyle.
• The guest discusses possible links between gut microbes, sex hormones, libido, and sexual attraction. Some of these ideas—especially the proposed role of microbes in human attraction—are presented as hypotheses or emerging areas of research rather than settled conclusions.

Practical Takeaways Given in the Interview

• Gradually increase the variety of plant foods in your diet; work toward a broad range rather than treating 30 plants per week as a pass/fail threshold.
• Include fiber-rich foods such as legumes, whole grains, fruits, vegetables, seeds, and nuts.
• Consider fermented foods such as traditionally fermented sauerkraut, kimchi, or pickles.
• Increase fiber slowly, especially if you currently eat little of it or experience digestive symptoms.
• Pay attention to stool frequency, form, and color—but use these as clues, not as a substitute for medical assessment.
• Seek medical advice for visible blood, black tarry stool, or unusually pale stool; do not assume blood is caused by hemorrhoids.
• Support general well-being with exercise, adequate sleep, and meaningful social connection.

Speakers and Sources Featured

Speakers

• Dr. Will Bulsiewicz — Gastroenterologist and gut-health author; the main guest.
• Steven Bartlett — Host and interviewer. He also delivers the episode’s promotional reads.

Researchers, Studies, and Organizations Mentioned

• Tim Spector — Associated in the discussion with the American Gut Project and British Gut Project.
• Warren David, identified in the subtitles as a Duke University researcher — Associated with a Nature study comparing plant-based and animal-based diets.
• Andrew Reynolds — Researcher whose review of fiber studies is discussed.
• Justin Sonnenburg — Stanford researcher associated with mouse research on diet, microbial diversity, and changes across generations.
• Zoe research — Including the Predict study, the “Blue Poop” work, the Blue Poop Challenge, and the UK “Big Poo Review.”
• Stanford fermented-food trial — Discussed as finding increased microbiome diversity after fermented foods were added to participants’ diets.
• Bristol Stool Scale research — Described as originating from a study of people in Bristol.
• Fecal-transplant research — Including work on C. difficile, ulcerative colitis, and animal studies.

The interview also contains promotional segments for the channel subscription, Conversation Cards, Zoe, and Huel.