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The downsides of a high-fiber diet

MDlinx Jan 24, 2024

Recommended fiber intakes are more complex than at first blush, and they vary worldwide and by age. Overall, 25 g–30 g or more of daily fiber is recommended for adults. Originally, these levels were based on providing for normal bowel movements and heart health, but emerging evidence shows that adequate fiber levels also benefit gut microbiota. 

In the US, Canada, and Europe, grain-based foods make up most of the dietary fiber consumed, followed by vegetables, fruits, and potatoes. Legumes, nuts, and seeds contribute little, according to researchers writing in the BMJ.

McKeown NM, Fahey GC Jr, Slavin J, et al. Fibre intake for optimal health: how can healthcare professionals support people to reach dietary recommendations? BMJ. 2022 Jul 20;378:e054370.



Although eating adequate amounts of fiber offers plenty of benefits, there’s also a flipside, where too much fiber can be detrimental to your patient’s health.


Worldwide deficiencies


Globally, most people fall short in terms of fiber intake. The dietary guidelines in most countries stress food vs nutrients, with most people not eating enough fruits, vegetables, legumes, or whole grains.

“People are unlikely to understand the type or quantity of individual foods or food combinations needed to achieve recommended dietary fibre intakes,” wrote the BMJ authors. “Coupled with this, some consumers do not recognise the contribution of whole grains to fibre intake, and others do not know how to identify whole grain, compared with refined grain products.” The authors also note that those who adopt a low-carb diet, typically for weight-loss reasons, are also likely to fall short of meeting fiber recommendations. 


Benefits of fiber


All physicians are aware of the two types of fiber: soluble and insoluble. Soluble fibers mix with water and slow digestion, and they are linked to a lower risk of heart disease, improved glycemic control, and lower cholesterol levels. They are found in some grains, legumes, seeds, and certain vegetables. Insoluble fibers add bulk to the stool and are found in whole grains, beans, and root vegetables. Interactions with the gut microbiome also mediate the benefits of different dietary fiber.

Gearhart-Serna L. Health benefits of dietary fibers vary. NIH Research Matters. May 24, 2022.



Fiber-enriched diets may benefit patients with neurodegenerative diseases, although the mechanism underlying this association needs to be elucidated. These diets may also benefit children with autism, as noted by the authors of a review in Nutrients. The authors reference data indicating that children with autism exhibit improved behavior when fed fiber.

Ioniță-Mîndrican CB, Ziani K, Mititelu M, et al. Therapeutic benefits and dietary restrictions of fiber intake: A state of the art review. Nutrients. 2022 Jun 26;14(13):2641.




Risks of fiber overconsumption


Despite the health benefits of fiber, the Nutrients authors explain that there are necessary limits to its intake. Excess consumption of natural dietary fiber can result in bloating, flatulence, diarrhea, and intestinal obstruction in people who don’t chase fiber intake with adequate fluids.

Fiber intake must be limited in patients with Crohn’s disease or intestinal obstruction, with such consumption monitored by a medical professional. Additionally, patients who experience malabsorption should approach the consumption of fiber with care. Patients who have undergone bowel surgery should also be wary of fiber intake. In all patient populations, increased fiber intake should be paired with higher fluid intake to minimize the chance of dehydration. 

Some individuals are predisposed to form phytobezoars, which are dense masses of natural dietary fiber, including seeds, leaves, and so forth, that collect in the stomach or small intestine and lead to obstruction.

In patients with a predisposition to phytobezoars, improper chewing can exacerbate the problem. Aging can play a role and result in the loss of intestinal elasticity, with constipation further promoting the formation of phytobezoars.

Fiber content can lower intestinal transit times, thus decreasing the bioavailability of drugs and decrease their pharmacodynamic effects. As well, excessive fiber intake can hinder the absorption of essential minerals from foods, such as calcium, iron, or zinc.


What this means for you

Most people benefit from high-fiber diets. Increases in fiber intake, however, must be paired with concomitant increases in water intake to decrease the risk of dehydration and bowel obstruction. Certain patient populations, such as those with inflammatory bowel disease, a history of small-bowel obstruction or malabsorption, or a predisposition to bezoar formation, should limit fiber consumption.


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