Over the years, consumers have become increasingly knowledgeable about the health benefits of fiber. Clinical studies have shown that a diet rich in fiber keeps the digestive system healthy, lowers the blood cholesterol level, and helps with weight management. As a result of growing consumer demand and the recognition that dietary fiber offers versatile functionality, the food industry has increased efforts to develop and place a greater number of “high fiber” food products and supplements on the market.
According to the U.S. Food and Drug Administration’s (FDA) Nutrition Facts Label final rule, dietary fiber is defined as “non-digestible soluble and insoluble carbohydrates (with 3 or more monomeric units), and lignin that are intrinsic and intact in plants; isolated or synthetic non-digestible carbohydrates (with 3 or more monomeric units) determined by FDA to have physiological effects that are beneficial to human health.” Apart from intact and intrinsic fibers, various synthetic or isolated nondigestible carbohydrates have been defined as “dietary fiber” by FDA. These include β-glucan soluble fiber, psyllium husk, cellulose, guar gum, pectin, locust bean gum, hydroxypropylmethylcellulose, arabinoxylan, alginate, inulin and inulin-type fructans, high amylose starch, galactooligosaccharide, polydextrose, and resistant maltodextrin/dextrin.
With continuous advances in food science, more and more carbohydrates have been added to the list of isolated, or synthetic, dietary fibers. For instance, cross-linked phosphorylated RS4, glucomannan, and acacia (gum arabic) have been defined as “dietary fiber” by FDA in 2019, 2020, and 2021, respectively. As stated by the agency, “Foods containing these fibers have been shown to be beneficial, and manufacturers do not need to demonstrate that they provide beneficial physiological effects to human health.”
Dietary fiber plays a role in promoting gut health by affecting fecal formation and the composition of the gut microbiota. Variations in the physicochemical properties (such as solubility, viscosity, and fermentability) of dietary fibers can lead to changes in functional properties and clinical utility. Such differences in physicochemical properties often arise from differences in the origin and processing of the fibers. Dietary fiber is also useful for lowering the risk of cardiovascular disease. According to Satija and Hu (2012) cereal fiber appears to be more beneficial in reducing the risk of cardiovascular disease (CVD) than fruit or plant fiber. This is consistent with the observation made by Sanchez-Muniz (2012), who indicated that dietary fiber consumption is advantageous for enhancing CVD risk factor profiles. All of these studies demonstrate the potential use of dietary fibers in functional food production.
Dietary fibers have shown great potential in food enrichment and fortification for a wide range of food groups, ranging from meat to bakery products. For instance, the mesocarp of the lontar palm (Borassus flabeliffer L.) fruit has been adopted to fortify ice cream as both natural antioxidants and dietary fiber (Idayati et al. 2018). Emulsion-based low-fat chicken meatballs have also been fortified with dietary fiber sources, including pearl millet flour, wheat flour, grape pomace powder, pomegranate pomace powder, carrot pomace powder, and beetroot pomace powder (Santhi et al. 2020). The fiber level of the chicken meatballs was found improved by the inclusion of dietary fibers from the fruit and vegetable sources, whereas the total ash content of the meatballs was enhanced with the inclusion of millets. Recently, wheat-rye bread has been fortified with dietary fiber by using the flour supplemented with β–glucan from oat fiber (Zakowska-Biemans et al. 2023). No significant impact on consumer acceptance or willingness to buy the wheat-rye breads was observed even after 20% of flour had been replaced with oat fiber. All of these examples have corroborated the practical feasibility of using dietary fibers in food product development.
Despite the potential mentioned above, when dietary fiber is applied to a food product, several factors need to be considered. One factor is the chemical structure of the fiber. Dietary fibers are generated from smaller units that are linked together via covalent bonds. For instance, starch and cellulose are polymers having glucose as the monomer, whereas alginates are copolymers consisting of homopolymeric blocks of (1à4)-linked β-D-mannuronate and α-L-guluronate residues. Variations in the chemical structure lead to changes in properties such as aqueous solubility and gel-forming capacity.
Apart from the chemical structure, the molecular weight of the fiber must be considered. The use of chitosan is a good example. Changes in the molecular weight of chitosan can lead to variations in the viscosity of the generated 2% (w/v) solutions. The situation is more complicated because dietary fibers are polymers. In addition to the molecular weight, polydispersity plays a role in altering polymer properties even though the polymers share the same empirical formula. This poses challenges to quality control of food products, especially when the food manufacturer is using different dietary fiber suppliers.
When dietary fiber is used in food enrichment or fortification, the health-promoting effects that can be achieved by the food product are determined by multiple factors, including the type and properties of fibers and the interactions of the fiber with the food matrix. All of these factors can affect how the fiber is acted upon after oral consumption and thus, the beneficial effects derived. Yet, it is still common for consumers in general to simply see whether a food product is rich in fibers rather than to examine the actual type of fiber that is used in that product.
In addition, knowing what carbohydrates can be defined as “dietary fiber” is crucial for food manufacturers. This is because when food manufacturers declare the amount of dietary fiber in a food product on the Nutrition and Supplement Facts label, they have to deduct the amount of carbohydrates that fail to meet the regulatory definition of dietary fiber from the total amount of fiber in the food product as determined by analytical methods. Unfortunately, right now there are no analytical techniques to distinguish nondigestible carbohydrates during quantification of the fiber content.
Finally, where fibers are concerned, the fact that there are two major types can be a challenge. One is soluble fibers, which can form a gel-like structure in the gastrointestinal tract, playing a role in lowering cholesterol and glucose levels. The other one is insoluble fibers, which can add bulk to the stool. Because the physiological functions of these two types of fibers are different, food manufacturers have to clearly distinguish the type of fibers that are added to a food item when it is marketed as a “high fiber” product.
Today, we rely on the food manufacturer to voluntarily disclose information about dietary fiber to the consumer. However, there may be an opportunity for providing better information to consumers by mandating and implementing regulatory measures that require clear and more detailed labels on food packages to clarify the quantity and identity of the type of dietary fiber added to a food product. Such a regulation is likely to result in more science-based applications of dietary fiber in food enrichment and fortification, which in turn would give consumers greater confidence that the “high-fiber” foods they buy contain the amount and type that is best for their health and well-being.