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

We host around 38 trillion microbes, mostly bacteria, concentrated in the colon. Through co-evolution, we outsourced key jobs to them: digesting food, training the immune system, regulating metabolism (blood sugar, blood fats, visceral fat), influencing hormones (estrogen, testosterone, GLP‑1), and shaping mood, cognition, energy, and even erectile function. Damage to this ecosystem shows up not only as IBS, reflux, and IBD, but also as depression, diabetes, autoimmune disease, and hormone-related conditions.

Across a lifetime, we consume ~36,300 kg of food versus milligrams of pharmaceuticals. A landmark Nature study showed that switching between all-plant and all-animal diets changed human gut microbes measurably within 24 hours. High-fiber, plant-diverse diets raise beneficial short-chain fatty acids (SCFAs) that repair the gut barrier, calm chronic inflammation, improve insulin sensitivity and fat burning, and protect against heart disease, stroke, diabetes, and multiple cancers. Ultra-processed, low-fiber diets do the opposite.

About 60% of stool weight is microbial in origin. The Bristol Stool Scale (types 1–7) maps form to function: ideal is type 4 (smooth, sausage-like), with 3–5 still acceptable. Types 1–2 suggest constipation/slow transit; 6–7 suggest diarrhea/fast transit. ZOE’s “Blue Poo” research shows gut transit time (typically 24–48 hours) predicts microbiome patterns, cardiovascular risk, and visceral fat. Color also matters: bright red or tarry black can signal GI bleeding and colon cancer risk; white can indicate bile blockage; persistent yellow, fat malabsorption.

Most Westerners eat only 10–15 different plants per week; research (American/British Gut Projects) links better microbiome diversity to ~30+ plants weekly (fruits, vegetables, whole grains, legumes, nuts, seeds, herbs, spices). Different plants contain different fibers and polyphenols that feed different microbes; more diversity = more resilience. Large meta-analyses show higher fiber intake reduces risk of heart disease, stroke, type 2 diabetes, colorectal cancer, and lowers blood pressure, cholesterol, and weight. The key is to “start low and go slow” to let microbes adapt.

Weight-loss drugs like semaglutide (Ozempic/Wegovy) mimic the satiety hormone GLP‑1, but have digestive side effects, unknown long-term risks, and often require indefinite use—stopping tends to bring weight back. You can stimulate the same hormones naturally by eating high-prebiotic, high-fiber diets that induce fullness and reduce calorie intake automatically. Alcohol, even at low levels, damages microbes and the gut barrier: blood alcohol rises in lockstep with inflammatory bacterial endotoxin (LPS) in the bloodstream, likely explaining hangovers as microbiome injury, not just “dehydration.”