We call it roughage. Bulk. Filler. We process it out of our food, and our bodies barely digest it. It just passes through supposedly...but it does a lot along the way.
In this episode, we follow fiber from the roller mills of the 1870s, which stripped the bran and germ from our flour and sent them to livestock, to Denis Burkitt, the surgeon who argued that what we’d thrown away was keeping us well.
We meet the trillions of microbes that depend on fiber, and what happens when they go hungry. And we look at a country where 95 percent of people fall short, and how to start making room for the filler again.
References & further reading
The mill and the missing fiber
- Wheat Foods Council — A kernel of wheat: the endosperm, bran, and germ, what each contains, and why white flour is milled from the endosperm alone.
- Wikipedia — Roller-milled white enriched flour: how roller mills separated bran and germ from the kernel, and the 1941 FDA standard for "enriched" flour.
- Wikipedia — Wheat middlings (millfeed): the bran and germ left over from white flour milling, and their use as livestock feed.
- Kansas State University — Wheat co-products (bran, midds, shorts) and their nutritional properties in swine diets.
The Fibre Man
- Cummings, John H. & Engineer, Amanda (2018) — Denis Burkitt and the origins of the dietary fibre hypothesis. Nutrition Research Reviews 31(1). Traces Burkitt's work and the physicians he built on, and notes that animal nutritionists recognized fiber's role in digestion more than 200 years ago.
- Center for Nutrition Studies — Dr. Denis Burkitt: the origins of the dietary fiber hypothesis and his 1971 colorectal cancer research.
- Burkitt, Denis P. (1971) — Epidemiology of cancer of the colon and rectum. Cancer 28(1). The paper that brought the fiber hypothesis into mainstream medicine.
- Kellock, Brian (1985) — The Fibre Man: The Life-Story of Dr. Denis Burkitt. Lion Publishing. Biography of Burkitt's career in Uganda and beyond.
The evidence and the fiber gap
- Reynolds, Andrew et al. (2019) — Carbohydrate quality and human health: a series of systematic reviews and meta-analyses. The Lancet 393(10170). WHO-commissioned analysis linking high fiber intake to 15–30% lower mortality and disease risk.
- Quagliani, Diane & Felt-Gunderson, Patricia (2017) — Closing America's fiber intake gap. American Journal of Lifestyle Medicine 11(1). Source for average US intake (~16 g/day) and the 95% shortfall.
- American Institute for Cancer Research — Why fiber matters, why it's found only in plant foods, and practical ways to eat more.
- Fortune (2026) — The fibermaxxing boom and why whole-food fiber differs from supplements.
The animals inside us
- Desai, Mahesh S. et al. (2016) — A dietary fiber-deprived gut microbiota degrades the colonic mucus barrier and enhances pathogen susceptibility. Cell 167(5).
- Sonnenburg, Erica D. et al. (2016) — Diet-induced extinctions in the gut microbiota compound over generations. Nature 529.
- Stanford Medicine (2017) — Hadza hunter-gatherers, their ~100 g/day fiber intake, and the diversity of their gut microbes.
- David, Lawrence A. et al. (2014) — Diet rapidly and reproducibly alters the human gut microbiome. Nature 505. Evidence that the gut community begins shifting within days of a diet change.
The cost and the guidelines
- American Cancer Society (2026) — Colorectal Cancer Statistics, 2026: rising incidence in younger adults and its rank as the leading cancer killer under 50.
- Colorectal Cancer Alliance (2026) — New data showing colorectal cancer is the deadliest cancer for adults under 50.
- Harvard T.H. Chan School of Public Health — Analysis of the 2025–2030 Dietary Guidelines, including fiber and whole grains.
- U.S. News & World Report (2026) — The inverted food pyramid and concerns about reduced whole-grain servings and fiber intake.
Take action
- NutritionFacts.org — Free, evidence-based videos and guides on fiber and whole-food plant-based eating.
- Physicians Committee for Responsible Medicine — Plant-based nutrition resources, including free starter guides.
- Forks Over Knives — Recipes and meal plans built around beans, whole grains, and vegetables.
- American Cancer Society — Colorectal cancer screening guidance; screening is recommended starting at age 45.
- animalinthemachine.com — Show notes and episode archive.
