In the 1950s, fiber was described as any nondigestible portion of a plant cell wall (1). Fast forward more than half a century, and not much has changed. In 2002, the Institute of Medicine (IOM) stated that total fiber is the sum of dietary fiber plus functional fiber (2). Dietary fiber consists of nondigestible carbohydrates and lignin that are intrinsic and intact in plants, whereas functional fiber consists of isolated, nondigestible carbohydrates that have beneficial physiologic effects in humans. Similar fiber definitions have been described by governments and organizations worldwide.

A globally accepted fiber definition was proposed by the Codex Alimentarius Commission (part of the Food and Agriculture Organization and the World Health Organization) in mid-2009. This definition has not yet been approved by the US Food and Drug Administration (FDA), however. Despite nuanced differences, all definitions agree that fiber is mostly carbohydrate that is not completely digested or absorbed in the small intestine but that may be fermented in the large intestine.

No matter which definition one chooses to accept, many different types of fiber exist, and each is unique. In the United States, fiber must be included on the Nutrition Facts panel on food packaging, but soluble and insoluble fiber can also be specifically identified (3). These fiber values are measured by methods accepted by the Association of Official Analytical Chemists. Fiber is undeniably a complex substance, so to characterize a fiber by solubility alone would be remiss. In fact, in 2001 the IOM Fiber Panel recommended that this practice be abandoned because the solubility of fiber was not a predictor of physiologic effects. Characteristics such as viscosity and fermentability may be more important in predicting the health benefits of fiber in humans.

Viscosity is similar to solubility and is often (but not always) associated with the fiber’s water-holding properties (4). Determining the viscosity of a liquid product is relatively simple, but the methods used to determine the viscosity of fiber as part of a food or diet are difficult, and the results are inconsistent among methods. For example, a particular fiber may be extremely viscous in water, but when it is baked into bread with other ingredients, the same fiber may behave quite differently. Animal studies attempted to determine intestinal content viscosity after an animal ate various fibers (5). However, it is unreasonable to extrapolate the results of viscosity at a set point in the digestive process because viscosity likely changes at different points in the digestive tract and at different times throughout the digestive process.

The fermentability of fiber is also important yet difficult to assess. Because fiber is not digested in the small intestine, it arrives in the large intestine intact and available for fermentation by the resident microflora (6). The fermentation process yields short-chain fatty acids (SCFAs), which are available for uptake by colonocytes. Fiber fermentation is believed to play a key role in colonic health. Neither in vitro nor in vivo assessments clarify
the way in which a specific fiber would be fermented in a specific individual, however. In vitro methods attempt to determine fermentability by inoculating various fibers with human fecal samples, but this closed, static system does not represent the dynamic and changing environment of the human colon (7). In vivo measurements of fiber fermentation are impossible to extrapolate to the in vivo situation because the large intestine of every individual is colonized with different types and amounts of microflora.

The dilemma is that viscosity and fermentability are two important characteristics of fiber, but no “gold standards” exist for measuring either property. Ultimately, this limitation makes discussions of fiber challenging and should be considered when one interprets research on fiber and health.

Most commonly consumed foods are low in dietary fiber (Table 3.1). In general, standard food portions only contain approximately 1 to 3 g of fiber per serving. Higher fiber contents are found in drier foods such as whole grain cereals, legumes, and dried fruits. Other fiber sources include over-the-counter laxatives containing fiber, fiber supplements, and fiber-fortified foods.

According to the FDA, the official method for reporting the calorie content of fiber is to assume that soluble fibers provide 4 kcal/g; this assumption is surprising to some people because 4 kcal/g is the same number of calories as fully digestible carbohydrate. Also notable, insoluble fiber is reported to provide 0 kcal/g. Some insoluble fibers, however, are fermented in the large intestine and produce SCFAs. SCFAs are absorbed in the colon; therefore, the concept that insoluble fiber contributes 0 kcal/g is not always true. Assigning a caloric value to fiber is difficult, however, because each type of fiber is not fermented to the same extent across individuals. The best estimate of calories supplied by fiber fermentation is probably between 1.5 and 2.5 kcal/g of fiber (8), compared with 4 kcal/g for digestible carbohydrates.

The Nutrition Facts panel recommends 25 g of dietary fiber for a 2000-kcal diet. The IOM recommends an adequate intake (AI) level of 14 g of fiber per 1000 kcal of energy consumed for all people who are more than 1 year old. Based on average energy intakes across the United States, this equates to approximately 25 g/day for women and 38 g/day for men ages 19 to 50 years. The recommendation for adults older than 51 years of age is 21 g/day for women and 30 g/day for men. The recommended amount of fiber decreases for older adults because average energy intakes tend to decrease with age.

No data suggest that pregnant or lactating women would benefit from increased fiber intake. Because energy intakes increase for these two groups, however, the recommended AIs are 28 g/day for pregnant women and 29 g/day for lactating women.

In addition, given that fiber recommendations are linked to calorie recommendations, 1- to 3-year-old children have an AI for fiber of 14 g/day. This value is unrealistically high. The fiber guide of “age plus 5” is more useful. This means that a 2-year-old child would be expected to consume approximately 7 g of fiber daily (9).

Residents of the United States typically consume less than half of the recommended amounts of fiber each day (approximately 15 g/day) (8). Flours, grains, and potatoes are the most popular sources of fiber in the US diet, whereas fruits, legumes, and nuts are the sources consumed in the least quantities (10). Many food manufacturers add fiber to foods that would not normally contain fiber (this is called functional fiber). Whether functional fiber actually increases the amount of fiber consumed or whether these products merely become a substitute for other fiber-containing foods in the diet is unclear, however (10). The American Dietetic Association (ADA) suggests that the addition of functional fiber to foods is likely less beneficial to health than is the consumption of whole foods that are naturally high in fiber (9).

Meeting recommended fiber intake levels without drastically altering food choices is possible. In fact, the 2005 dietary reference intake book provides specific examples of omnivorous diets that provide adequate fiber (and other nutrients) within reasonable calorie limits (8). The US Department of Agriculture Nutrient Data Laboratory website provides a comprehensive listing of the fiber content of commonly consumed foods (11).

In general, an average meal empties from the stomach in approximately 2 to 5 hours, clears through the small intestine in approximately 3 to 6 hours, and then resides in the colon for anywhere from 12 to 42 hours (12). Fiber may speed up or delay the process at any point throughout the digestive tract. The role of fiber in the digestive tract is specific to each fiber’s unique physical and chemical properties. For example, certain viscous fibers (e.g., β-glucan) may absorb large quantities of water and form gels, which can increase gastric distension and slow gastric emptying time (13). Other fibers (e.g., wheat bran and resistant starch), however, may not influence gastric distension or emptying time (14). Regardless of the time it takes fiber to empty from the stomach, most fiber remains intact and is resistant to degradation in the stomach.

In the small intestine, certain fibers may slow the digestion and absorption of all nutrients, including digestible carbohydrates, protein, and fat (15, 16). The delayed or reduced absorption of carbohydrate explains the potential for certain fibers to blunt the glycemic response. Although many studies provided evidence that fiber-containing foods can reduce glucose or insulin levels compared with fiber-free foods (17), other studies implied that these relationships are more complex than previously believed. Several randomized controlled trials suggested that the glycemic response to fiber-containing foods likely depends on fiber viscosity, fiber dose, and food matrix (18).

The function of fiber in the large intestine depends on two key factors: the fermentability of the specific fiber and the microflora residing in an individual’s large intestine. Fibers such as pectin and fructooligosaccharides are extensively fermented, whereas cellulose and wheat bran are slowly fermented or are not fermented at all (19). The degree of fermentation affects fecal bulk, such that less fermentable fibers may increase fecal bulk and contribute to a laxative effect. Fermentable fibers also have the potential to create fecal bulk, but this effect does not stem from the fiber itself. Instead, fermentable fibers may lead to an increase in bacterial mass, which can attract water and increase stool size.

Summarizing the conclusions of fiber and health research is difficult because it is often impossible to determine whether health outcomes are a result of consuming fiber itself or a result of changes in nutrient density and nutrient intake that occur when fiber is present in a food. Specifically, high-fiber diets often increase the intake of biologically active compounds such as phytochemicals and antioxidants that are not present in lower-fiber diets. That said, many epidemiologic and intervention studies do suggest that regular fiber intake is associated with various beneficial health outcomes. These benefits, however, largely depend on the type of fiber consumed, as well as on the individual person.