By Joseph E. Scherger, MD, MPH

Core Faculty, Eisenhower Health Family Medicine Residency Program, Eisenhower Health Center, La Quinta, CA; Clinical Professor, Keck School of Medicine, University of Southern California, Los Angeles

Dr. Scherger reports no financial relationships relevant to this field of study.

SYNOPSIS: Dietary fiber is crucial to maintaining a healthy gut microbiome. The microbiome helps determine our mental and physical health in ways that continue to be discovered.

SOURCE: O’Grady J, et al. Review article: Dietary fibre in the era of microbiome science. Aliment Pharmacol Ther 2019;49:506-515.

Advancing science tells us the microbiome is crucial for human health, both in body and mind. Denis Burkitt, an Irish physician and surgeon (1911-1993), famously said, “If you pass small stools you have to have large hospitals.”1 Until recently, this expression was considered facetious; now, it seems prescient.

Traditionally, dietary fiber has been divided into soluble and insoluble. O’Grady et al reviewed the literature on the interplay of dietary fiber with the human microbiome and resultant metabolic effects. They described three more appropriate biologic effects of fiber: solubility, viscosity, and fermentation. Solubility refers to whether the fiber dissolves in water. Viscosity refers to the consistency of fiber and its effects on digestion, absorption, and satiety. For example, the common fiber supplement psyllium delays the degradation and absorption of nutrients and can reduce total glucose and cholesterol absorption by up to 12%.2 Fermentation of fiber by the gut microbiota yields short-chain fatty acids that provide energy and exert an immunoregulatory and gut-brain signaling role.3

O’Grady et al listed the following dietary fiber subtypes, their sources, and their metabolic effects: cellulose, hemicellulose, lignin, gums, pectin, beta-glucan, inulin, psyllium, oligosaccharides, and resistant starch. All fiber subtypes come from plants. The metabolic effects include increasing stool bulk; stimulating peristalsis; lowering glucose, cholesterol, and triglyceride levels; slowing digestion and absorption; and providing the benefits of fermentation by the microbiota.

Current recommendations from the Academy of Nutrition and Dietetics (formerly known as the American Dietetic Association) call for 14 g of fiber for every 1,000 kcal consumed, or about 25 g for women and 38 g for men daily.4 Current fiber consumption in the United States is estimated at only 12-18 g/day.5 Ancestral humans consumed an estimated 100 g of fiber per day.6 No wonder Burkitt referred to the United States as a “constipated nation.”1


The evolutionary biologist Daniel Lieberman considers modern man in an industrialized culture in a state of disevolution.7 Only recently have we begun to understand the health costs associated with consuming highly processed foods. The good news is that natural plant foods are available to everyone in the United States and elsewhere in supermarkets. Eating an adequate amount of fiber requires education and choices. The gut microbiome plays a major role in determining our mood and physical health. The benefits of a healthy microbiome go far beyond lowering the rate of colon cancer. Primary care physicians should play a leading role in advising patients to eat a diverse number of plants with healthy fiber, also referred to as prebiotics.


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  2. Anderson JW, et al. Effects of psyllium on glucose and serum lipid responses in men with type 2 diabetes and hypercholesterolemia. Am J Clin Nutr 1999;70:466-473.
  3. Shanahan F, et al. Feeding the microbiota: Transducer of nutrient signals for the host. Gut 2017;66:1709-1717.
  4. Slavin JL. Position of the American Dietetic Association: Health implications of dietary fiber. J Am Diet Assoc 2008;108:1716-1731.
  5. Deehan EC, Walter J. The fiber gap and the disappearing gut microbiome: Implications for human nutrition. Trends Endocrinol Metab 2016;27:239-242.
  6. Eaton SB. The ancestral human diet: What was it and should it be a paradigm for contemporary nutrition? Proc Nutr Soc 2006;65:1-6.
  7. Lieberman DE. The Story of the Human Body: Evolution, Health and Disease. New York: Random House; 2014.