Myths and Truths About Fiber
In fact, many studies have demonstrated that excess intake of fiber may actually be harmful, particularly for gut health.
The majority of the research supporting the benefits of dietary fiber come from epidemiological studies that link the consumption of fiber-rich fruits and vegetables with a lowered risk of certain diseases such as obesity, heart disease and cancer, particularly colon cancer. (1) Yet when tested in the lab, controlled intervention trials that simply add fiber supplements to an otherwise consistent diet have not shown these protective effects. (2) (3) (4)
The Institute of Medicine recommends a daily fiber intake of 38 grams for men and 25 grams for women (5), which may come from dietary fibers, both soluble or insoluble, or the addition of “functional fibers” to the diet. The IOM defines functional fibers as non-digestible carbohydrates that have been isolated or extracted from a natural plant or animal source, or they may be manufactured or synthesized. Examples of functional fibers are psyllium husks, chitin from crustacean shells, fructooligosaccharides, polydextrose, and resistant dextrins. (6)
These functional fibers are often added to processed foods as a way to bulk up the fiber content for consumers looking to meet the IOM intake guidelines. A recent report by NPR commented that despite the lack of significant evidence linking fiber intake to health outcomes such as reduced heart disease or cancer, many consumers are buying foods that are fortified with synthetic fiber additives under the guise of health promotion. (7) Three grams of added fiber is enough to allow these food products to claim to be a good source of fiber, and the food industry has used these fiber guidelines as a way to increase their sales of grain-based products in particular. (8)
Tan and Seow-Choen, in their 2007 editorial on fiber and colorectal disease, call insoluble fiber “the ultimate junk food”, as “it is neither digestible nor absorbable and therefore devoid of nutrition.” (9) Excess insoluble fiber can bind to minerals such as zinc, magnesium, calcium, and iron, preventing the absorption of these vital nutrients. (10) Large excesses of certain soluble fibers like pectin and guar may also inhibit pancreatic enzyme activity and protein digestion in the gut, leading to an anti-nutritive effect. (11)
The addition of insoluble and soluble fibers to processed foods may actually cause these foods to be even less nutritious than if they were not enriched with any fiber at all.
A high-fiber diet has also been described as a preventative strategy for the development of diverticulosis, a disease that is markedly more common in Western countries. However, when researchers tested the theory that a high-fiber diet prevented diverticulosis, they not only found that a high intake of fiber did not reduce the prevalence of diverticulosis, but that a high-fiber diet and greater number of bowel movements were independently associated with a higher prevalence of diverticula. (12) Interestingly, this study found no association between the presence of diverticulosis and red meat intake, fat intake, or physical activity, which are other factors commonly attributed to diverticulosis.
The researchers hypothesized that one possible effect of a high-fiber diet in the development of diverticulosis could be the quantitative and qualitative changes in gut bacteria due to the excessive fiber intake. Both insoluble and soluble fibers are shown to alter gut bacteria in as little as two weeks. (13) It is possible that the high levels of excess fiber and overgrowth of intestinal bacteria may have contributed to the development of diverticular pouches in the colon.
This hypothesis brings up another side to the fiber debate: the effect of dietary fiber on beneficial gut bacteria, as well as the bacterial fermentation of undigested soluble fiber into short-chain fatty acids such as butyrate. When we eat the soluble fibers found in whole plant foods, the bacteria in our gut ferment these fibers into short-chain fatty acids like butyrate, proprionate, and acetate, and greater amounts of fiber consumed will lead to greater short-chain fatty acid production. (14) In this case, naturally occurring soluble fibers are very important for feeding the friendly bacteria that live in our guts.
One of the risks of long term very low-carbohydrate (VLC) diets, in my view, is the potentially harmful effect they can have on beneficial gut flora. VLC diets starve both bad and good gut bacteria, which means these diets can have therapeutic effects on gut infections in the short term, but may actually contribute to insufficiency of beneficial strains of gut bacteria over the long term. Providing adequate levels of carbohydrate and soluble fiber to feed friendly bacteria is important for optimizing digestive health and maintaining the integrity of the gut lining through the production of short-chain fatty acids.
Stephan Guyenet has written an excellent blog post describing the benefits of butyrate and other short-chain fatty acids on the maintenance of healthy gut integrity. (15) Butyrate has anti-inflammatory effects, increases insulin sensitivity, and may delay the development of neurodegenerative diseases. It may also be helpful in the treatment of diseases of the colon such as Crohn’s, IBS or ulcerative colitis. (16)
Stephan believes that butyrate may play a significant role in healthy metabolic function, stress resistance, and the immune response. He also asserts that the epidemiologically observed benefits of a diet high in naturally occurring fiber are likely due to the higher butyrate production from these diets. In this case, a higher fiber diet could be protective and beneficial for health, particularly if the fiber is soluble.
So what does this mean for our own consumption of fiber?
Ideally, dietary fiber should be coming from whole food plant sources. Many foods in the Paleo diet are great sources of both soluble and insoluble fiber, such as yams and sweet potatoes, green leafy vegetables, carrots and other root vegetables, fruits with an edible peel (like apples and pears), berries, seeds, and nuts. Interestingly, butyrate itself is also found in high-fat dairy products such as butter and cheese, and can also be provided by the bacteria found in fermented foods. (17)
Although I recommend that most people get fiber from whole foods, there are some people that may benefit from soluble fiber supplementation – including those that aren’t able to eat fruit or starch due to blood sugar issues or weight regulation, and those with severely compromised gut flora or gut dysbiosis. In these cases I’ve found soluble fiber and/or prebiotic supplements to be helpful.
For healthy people, including a variety of fibrous whole plant foods, fermented foods, and high-fat dairy as tolerated should eliminate the need to supplement with extra fiber, especially those insoluble fibers that are from sources high in anti-nutrients. A Paleo diet with some level of attention paid to the quality and quantity of vegetables, fruits, and starchy tubers can provide adequate levels of soluble fiber to feed the friendly bacteria in the gut that convert these fibers into beneficial short-chain fats like butyrate.
