Dr Rangan Chatterjee speaks with US gastroenterologist Dr Will Bulsiewicz about gut health beyond probiotics, focusing on how diet and lifestyle may shape the microbiome, gut barrier and immune function. Bulsiewicz describes research he says shows measurable microbiome shifts can begin within 24 hours of dietary change, citing a 2014 Nature study that compared short plant-only and animal-only dietary patterns. He outlines a mechanistic model in which fibre feeds gut microbes, producing short-chain fatty acids such as butyrate, which he calls the most anti-inflammatory compound he has encountered and a key support for the gut barrier; he also argues these metabolites may influence systemic and brain inflammation.
The discussion covers lipopolysaccharide (LPS), intestinal permeability (“leaky gut” as a mechanism rather than a diagnosis), and Bulsiewicz’s claim that he identified evidence linking inflammation—and separately dysbiosis—to more than 130 conditions. He offers prevalence claims about fibre deficiency (UK 90%, US 95%) and ultra-processed food intake (UK ~55% of adult calories; US ~60%; children ~70%), arguing industrial additives may contribute to dysbiosis and immune overactivity.
Chatterjee and Bulsiewicz also address restricted diets, including keto and carnivore, with Bulsiewicz saying some approaches can help short-term symptoms while risking longer-term loss of microbial resilience if maintained without fibre. The conversation broadens into mind–gut links: Bulsiewicz recounts his own reconciliation with an estranged father and argues some patients require emotional and trauma-focused healing alongside physiological interventions. He also alleges his former mentor’s time-intensive IBS clinic model was undermined by hospital financial pressures.
Dr Will Bulsiewicz alleges hospital administrators forced out his irritable bowel syndrome (IBS) mentor because a time-intensive clinic model was not profitable. He recalls seeing two patients over a four-hour block with repeated room visits to build trust, and argues that health-system incentives can restrict relationship-based care.
Read this moment →Gastroenterologist Dr Will Bulsiewicz says the gut microbiome can show detectable changes as soon as the next day after changing what you eat, while stressing this does not mean a complete transformation in 24 hours. He points to a 2014 Nature study (lead author Lawrence David, Duke University) that tracked daily microbiome measurements during five-day animal-only and plant-only dietary patterns.
Read this moment →Dr Will Bulsiewicz says his book’s appendix lists more than 130 health conditions with published research linking them to inflammation. He says he only included conditions where he could cite a specific study, including inflammatory bowel disease and rheumatoid arthritis as well as psoriasis, eczema, asthma and low back pain.
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