A Guide to Recommending Fiber Supplements for Self-Care

Abstract

The health benefits of dietary fiber are extensive and wide-ranging, yet most Americans do not consume the recommended amount of fiber from their diet. Fiber supplements may be used as a means to augment a low-fiber diet. But which, if any, of the benefits of dietary fiber are achieved through supplementation? And are all fiber supplements equally effective? An evidence-based approach to recommending fiber supplements for self-care will be discussed.

“The health benefits associated with high intake of dietary fiber are extensive and wide-ranging.”

The health benefits associated with high intake of dietary fiber are extensive and wide-ranging. These benefits include a reduced risk of heart attack, stroke, cardiovascular disease, and cancer, as well as improved glycemic control, cholesterol levels, weight maintenance, and regularity (among others).^1-5 Despite these health benefits, most Americans are consuming well below the amount recommended.⁶ In fact, the 2015 Dietary Guidelines for Americans list dietary fiber as an underconsumed nutrient of public health concern.⁷ The Institute of Medicine recommends 14 g of dietary fiber per 1000 kcal consumed or approximately 25 g/d for women and 38 g/d for men.⁸ The average intake of fiber for American adults is just 17g/d.⁶

Fiber supplements may be used as a means to augment a low-fiber diet. But which, if any, of the benefits of dietary fiber are achieved through supplementation? And are all fiber supplements equally effective? An evidence-based approach to recommending fiber supplements for self-care will be discussed.

High-Fiber Diets and Fiber Supplementation

The Institute of Medicine distinguishes dietary fiber from functional fiber.⁸ Dietary fiber is the nondigestible carbohydrate and lignin that are intrinsic and intact in plants, whereas functional fibers are the isolated or synthetic nondigestible carbohydrates that have been shown to have beneficial physiological effects in humans. Not all known benefits of diets high in fiber can be achieved through supplementation. Likewise, not all fiber supplements are functional fibers, as many do not have clinically significant evidence of benefit to support their use.

Large epidemiologic studies associate high dietary fiber intake with a reduced risk of heart attack, stroke, cardiovascular disease and cancer.¹ Because of the nature of the methods of these studies, it is difficult to ascertain how much of this benefit is attributable to the fiber itself and what portion is due to the shift in eating patterns altogether. Fiber is found in food groups known to promote health, including fruits, vegetables, whole grains, and legumes. These intrinsic and intact sources of fiber, as opposed to isolated or synthetic supplements, have the added benefit of micronutrients and phytochemicals, which may work in concert with fiber to improve health. Additionally, diet patterns high in fiber are more likely to be lower in calories, sodium, added sugars and saturated and trans fats.

One of the largest, most recent analyses of health and fiber confirmed on a large scale that high intake of dietary fiber reduces mortality from cardiac disease, stroke, type 2 diabetes, and/or colon cancer by 16% to 24% compared with low-fiber intake.¹ Importantly, the analysis excluded clinical trials in which supplements were used. The authors noted that, for the studies included, limited data were available regarding the specific source or subcategory (soluble, insoluble, extracted) of the dietary fiber. Additionally, when translating the findings to dietary advice, the authors provide the following caveat: “the large body of literature that contributed to this article relate principally to fiber-rich foods as most of the studies were undertaken before synthetic and extracted fiber were widely used.” In other words, the mortality benefits associated with high dietary fiber as observed in the study were likely from intrinsic sources (intact foods) and may not translate to fiber supplements made from isolated or synthetic fiber sources. This caveat is important for health care providers to be aware of and include in their patient counseling regarding the benefits of fiber, the importance of source, and potential limitations of fiber supplements.

Clinically Significant Effects of Fiber Supplementation

The therapeutic effects of fiber supplements can be explained, in part, by the physiochemical characteristics of the fiber. There are 4 key characteristics: solubility, viscosity, gel formation, and degree of fermentation.⁹ Soluble fiber dissolves in water while insoluble fiber passes through the digestive system relatively intact. When hydrated, some soluble fibers thicken and form a gel while others dissolve in water but do not demonstrate an increase in viscosity or gel formation. Fibers experience varying degrees of degradation/fermentation by the gut bacteria. Once fermented, the fiber is no longer present in the stool and fermentation by-products such as short-chain fatty acids and gas increase. Beyond fiber type, the extent of fermentation is also dependent on gastrointestinal transit time as well as the host’s bacterial community.

High-viscosity, gel-forming fibers like beta-glucan (Quaker Oats), psyllium (Metamucil, Konsyl), and guar gum have been shown to improve glycemic control and lower cholesterol levels.^5,10-13 These agents significantly increase the viscosity of the intestinal chyme, slowing the interaction of the digestive enzymes with nutrients. This in turn slows the absorption of glucose, thereby reducing postprandial blood glucose excursions.^5,10 The fibers provide this beneficial response in a dose-dependent manner and the benefit is optimized when the fiber is dosed with meals.

Likewise, a high-viscosity gel significantly decreases bile reuptake in the bowel and increases bile loss in the stool. As a result, low-density lipoprotein (LDL)-cholesterol is cleared from the blood to synthesize more bile acids for digestion. This process improves serum cholesterol levels.^11-14 Importantly, viscous but non-gel-forming fibers like methylcellulose (Citrucel) do not significantly lower cholesterol.¹² Nonviscous fibers, including inulin (Fiber-Choice, Metamucil Clear, and Natural), wheat dextran (Benefiber), and insoluble fibers like wheat bran (All-Bran) do not improve glycemic control or lower cholesterol.¹³

Not all fibers induce laxation or improve regularity.¹⁵ Insoluble fibers like wheat bran (All-Bran) mechanically irritate gut mucosa. This irritation increases water and mucus secretion in the bowel leading to greater stool water content and decreased stool transit time. Because wheat bran is poorly fermented, the fiber’s effect is maintained throughout the entirety of the bowel.¹⁶ This effect is proportional to the size and shape of the fiber particle, where large rough particles have greater laxative effect. Likewise, psyllium (Metamucil, Konsyl) is a nonfermented, soluble, viscous gel-forming fiber that is also capable of increasing stool water content and improving symptoms of constipation.⁴ Fibers susceptible to fermentation including wheat dextrin (Benefiber), inulin (Fiber-Choice, Metamucil Clear, and Natural), guar gum (Sunfiber), and beta-glucan (Quaker Oats), do not provide these benefits.^13,15

Conclusion

When discussing fiber for self-care with patients, it is important to distinguish intrinsic, intact dietary fiber, which is found in fruits, vegetables, whole grains, legumes, and nuts, from isolated or synthetic fibers that are added to foods or found in supplements. Much of the data that associate fiber with a reduced risk of stroke, heart attack, cardiovascular disease, and cancer are based on high intakes of intrinsic, intact dietary fiber. Therefore, these benefits should not be attributed to the use of fiber supplements and patients seeking these benefits should be counseled on ways to increase fiber through food.

Fiber supplements have demonstrated health benefits, including lower LDL-cholesterol and improved regularity and glycemic control. High-viscosity, gel-forming fibers like beta-glucan (Quaker Oats), psyllium (Metamucil, Konsyl), and guar gum have been shown to improve glycemic control and lower cholesterol levels, whereas nonviscous and insoluble fibers do not confer these benefits. Soluble, viscous fibers that are not fermented by gut bacteria (Metamucil, Konsyl) can normalize stools and some insoluble fibers such as wheat bran (All-Bran) can achieve a laxative effect if sufficiently gritty (large, rough particles).