First, fiber has always been recognized as a plant-based nutrient. To make a simple comparison, plants have fiber in the same way that animals have muscles and bones. Fiber allows plants to maintain their shape and structure. And even though fungi (including mushrooms) are classified by scientists as belonging to their own separate category of living things, we would want to include them alongside of plants in this fiber discussion since they often contain chitooligosaccharides (CHOS) which most researchers consider a type of fiber. Although not widely popular in the U.S., there are other non-plant foods that contain CHOS and would be considered by many researchers as non-plant sources of fiber. Insects would be the largest single group here, followed by crustaceans like shrimp, crab, and lobster. While these last three crustaceans can be popular foods in the U.S., their CHOS content is not usually consumed because it is found in their outer shells which typically aren’t consumed. At WHFoods, the bottom line here is simple: for most U.S. eaters, plant foods (and fungi like mushrooms) are going to be your exclusive whole, natural food sources of fiber.
Second, fiber does not digest in the same way that most other nutrients digest. Most nutrients undergo full digestion as they get chewed, chemically transformed in our stomachs, and then combined with enzymes and digestive fluids in our small intestine. This combination of chewing plus digestive fluids plus digestive enzymes is usually sufficient to allow our bodies to digest and absorb nutrients from food. In the case of fiber, however, the above processes above do not substantially alter the fiber’s basic nature. Fiber passes all the way through our stomach and small intestine and then proceeds on to our large intestine, still largely recognizable as dietary fiber. Our chewing and digestive fluids and enzymes are enough to dramatically change it. It is only in the last portion of our digestive tract—our large intestine—that fiber can undergo a major transformation; if this transformation occurs, it is not brought about by human enzymes or human digestive fluids but rather by trillions of bacteria living in our large intestine.
Current Issues in Fiber Research
Given these two basic distinctions that are fully embraced by all researchers, how has fiber become such a complicated nutrient to define? First, most scientists like to define nutrients according to their chemical structure. Their confidence in the nourishment provided to us by a vitamin like vitamin C is closely related to their ability to define vitamin C in chemical terms (i.e., a 6-carbon molecule with the chemical formula C6H8O6). The fact that vitamin C functions as an antioxidant is not regarded as a defining characteristic of vitamin C, since many nutrients function as antioxidants. So scientists tend to rely on definitions involving chemical structure. But researchers also know that a simple chemical formula is not possible when it comes to fiber. From a chemistry standpoint, fiber is structurally diverse, and this diversity has made fiber more difficult than other nutrients for researchers to define and universally adopt.
Second, while some of the health benefits of fiber are unrelated to its bacterial breakdown in the large intestine, many important benefits from this nutrient depend on its transformation by bacteria. As you might imagine, with literally trillions of bacteria in our large intestine and as many as 1,000 different species, interactions between any substance and bacteria can be difficult to predict. So it can be difficult to conclusively determine if a substance should be classified as a fiber based on any function that it might serve following transformation by bacteria.
Most Common Classification Systems for Fiber
Soluble versus Insoluble Fiber. This distinction between soluble and insoluble fiber is probably the most familiar fiber classification system, and it also has the advantage of being the simplest. Fiber can be analyzed to determine how easily it dissolves in water, and fibers with a strong tendency to dissolved get classified as “soluble” while ones that don’t get classified as “insoluble.” In addition, as a very general rule, insoluble fibers tend to provide more benefits in the area of preventing constipation due to increased stool bulk and speeding up the rate of food passage through our digestive tract, while soluble fibers tend to provide better support for blood sugar balance, cardiovascular health, and satisfaction of appetite.
However, there are also some important disadvantages to this simple classification system. First, it is possible for fibers to have soluble and insoluble components. In this case, the distinction tends to be less helpful. Second, the vast majority of foods providing fiber not only contain both soluble and insoluble types, but they often contain both in nutritionally significant amounts. So this distinction isn’t always helpful in making food choices. Third, these two categories are often insufficient to account for important health benefits. For example, within the soluble fiber group can be found both viscous and non-viscous soluble fiber. Viscous soluble fibers are gel-forming and much more closely associated with cardiovascular and blood-sugar regulating benefits than non-viscous soluble fibers. So as you can see, even though this distinction between soluble and insoluble makes sense and has value, it has seemed less helpful over time as researchers learn more and more above fiber and try to determine the best way to obtain optimal benefits from fiber-containing foods.