Dr. Will Bulsiewicz: The Gastroenterologist Who Made Fiber a Bestseller
Publishing wisdom says you cannot make a bestseller out of fiber. In May 2020 a gastroenterologist from Charleston put it in the title anyway — and five years later the least fashionable nutrient in the shop has a fandom.
Dr. Will Bulsiewicz, MD, MSCI — known online as @theguthealthmd — is a board-certified gastroenterologist and the author of Fiber Fueled (2020, an instant New York Times bestseller), The Fiber Fueled Cookbook (2022) and Plant Powered Plus (January 2026). He trained at Vanderbilt, Georgetown, Northwestern (chief medical resident) and UNC (chief gastroenterology fellow), holds an active South Carolina medical licence in good standing, serves as U.S. Medical Director of the personalised-nutrition company ZOE, and runs a telemedicine practice in Mount Pleasant, South Carolina. His central argument — that plant diversity and fiber are the best-evidenced everyday levers on the gut microbiome — sits inside mainstream nutrition science. He is also a co-founder of the prebiotic supplement company 38TERA and sells paid courses, commercial interests this profile names plainly.
Publishing wisdom says you cannot make a bestseller out of fiber. It is the least glamorous word in nutrition — the nutrient of bran flakes and doctor’s-office pamphlets, the thing your grandmother was told to eat. In May 2020, a gastroenterologist from Charleston put it in the title anyway, and Fiber Fueled became an instant New York Times bestseller. Five years on, the least fashionable nutrient in the shop has a fandom, and a considerable share of the credit belongs to the man the internet knows as the Gut Health MD.
The credentialsThe part that checks out completely
The résumé underneath the brand is the real thing, and it is worth laying out because this beat is full of people whose credentials dissolve on inspection. Dr. Will Bulsiewicz is a board-certified gastroenterologist: Vanderbilt undergraduate, Georgetown MD, chief medical resident at Northwestern — where he also took a master’s in clinical investigation — then a gastroenterology fellowship at the University of North Carolina, where he was chief fellow and took the programme’s award for excellence in clinical gastroenterology. His South Carolina medical licence is active and in good standing, with board certification in internal medicine and gastroenterology on the state’s own record. Nearly two decades after medical school, he still runs a practice — The Gut & Microbiome Center for Excellence in Mount Pleasant, South Carolina, now operating by telemedicine.
The thesisOne nutrient, argued like a career
Most health creators cover the whole waterfront. Bulsiewicz has essentially one argument, made at book length three times: that the strongest lever most people have on their gut microbiome is the diversity of plants they eat, and that fiber — the part of the plant that survives you and feeds your microbes — is where the evidence is deepest. Fiber Fueled made the case in 2020; The Fiber Fueled Cookbook turned it into recipes in 2022; and Plant Powered Plus, published by Avery this January, extends it. The argument’s spine — more plant variety, more fermentable fiber, a microbiome trained like a muscle rather than dosed like a prescription — sits comfortably inside the mainstream science, which is not something you can say about every bestseller in this aisle. It is the same ground we walk in our own fiber explainer, argued with considerably more evangelism.
Most health creators cover the waterfront. Bulsiewicz has one argument, made at book length three times — and it happens to be the one the evidence supports best.
The day jobZOE, and the science-adjacent life
Since the pandemic era he has also been the U.S. Medical Director of ZOE, the personalised-nutrition company built around the PREDICT studies — among the larger nutrition-science programmes running anywhere — which puts him at the consumer end of a genuine research operation. His newsletter reports more than 200,000 readers by its own count, his Instagram audience sits in the high six figures, and his podcast, The Gut Insiders, extends the same material in longer form. His stated mission, per his own site: to blend “cutting-edge science with clinical expertise and a whole-person approach” to health that, as he puts it, starts in the gut.
Worth naming plainlyThe commercial side
The business built on the audience is substantial, and a fair profile lists it rather than footnotes it. There are the three Avery books. The ZOE role is a paid position at a commercial company. He is a co-founder of 38TERA, which sells a prebiotic supplement he formulated — its marketing includes his line that you can think of it “like a multivitamin for your gut” — and which runs an affiliate programme. There are paid courses, including a Gut Health Masterclass, sold through his company; a telemedicine practice; and a speaking calendar. None of this is hidden, and none of it is unusual at his scale. But it does mean that a reader encountering his advice is often one click from something he sells, and the sensible posture is the one we apply to everyone on this beat: take the free, evidence-backed core — eat more plants, in more variety — and treat every purchasable extension of it as a separate decision.
What earns him the spot in this section is the ratio. Strip every product away and the central message survives intact, costs nothing, and matches the science: plant diversity and fiber are the best-evidenced dietary levers on the microbiome that exist. He is a real gastroenterologist with real training saying a true thing loudly — the commercial superstructure is built on top of the message rather than being the message. If your own digestion is doing something new, persistent or worrying, that is a conversation for your own doctor rather than any book, newsletter or supplement. This article is educational, not medical advice.