Dietary Fiber – Food, Form, and Function
Many perceive a high fiber diet to be important only for senior citizens trying to maintain regularity and avoid associated digestive issues. While increasing dietary fiber certainly improves regularity and helps to prevent both diarrhea and constipation, mounting evidence links the consumption of dietary fiber to numerous other health benefits – regardless of age. However, as the discussion on dietary fiber grows, so too does confusion regarding the differences between soluble and insoluble fiber, fermentable and non-fermentable fiber, and even between prebiotics and probiotics.
Dietary fiber is the portion of the plant-based foods that is not digested by the enzymes in our body. In contrast, most types of starch are readily digested and are absorbed as sugar in the small intestine. Therefore, consuming foods high in dietary fiber can add to our sense of fullness and allow us to eat more without a rise in blood sugar.
We can broadly divide dietary fiber based on solubility, or the ability to dissolve rather than settle when mixed into water or a similar liquid. Soluble dietary fiber, like that found in passion fruit, avocados, and oranges, mixes with water to form a gel in the digestive tract. This gel interferes with the body’s ability to absorb cholesterol-containing material, thereby reducing overall cholesterol levels. Insoluble fiber is frequently found in the skins of fruits and grains, but is also common in many vegetables. Remaining largely intact during transit along the digestive tract, insoluble fiber adds bulk to stools, which prevents diarrhea and makes bowel movements easier. Foods like beans and lentils are actually rich in both soluble and insoluble fiber, and possess the benefits of both.
While all dietary fiber resists human digestion, a portion of dietary fiber is fermented by healthy bacteria in our colon. Fermentable fiber comes in many forms and is the nutritional foundation for the microbial ecosystem in our gut. Breakdown of fermentable fibers, including inulin and digestion resistant starch, confers numerous health benefits and serves as a source of energy for the body - both topics covered in previous posts. In contrast, non-fermentable fiber includes cellulose and lignin, substances integral to the cell walls found in plants, and passes through the gut largely untouched.
Finally, it may be helpful to differentiate between the terms ‘prebiotic’ and ‘probiotic’. Prebiotics are the fermentable parts of dietary fiber, as well as functional fibers, like fructooligosaccharides (FOS) and mannanoligosaccharides (MOS), that stimulate the growth of beneficial bacteria. These differ from probiotics, which are living, beneficial bacteria that ferment and consume prebiotics. In other words, prebiotics are food for probiotics.
Taken together, we can use these definitions to speak about different sources of fiber and the associated health benefits. For example, digestion resistant starch from potatoes is an insoluble, fermentable prebiotic that will help reduce both diarrhea and constipation, stimulate the growth of healthy bacteria in the colon, and serve as a valuable source of energy when fermented. Wheat bran, containing large amounts of cellulose, is also an insoluble but largely non-fermentable dietary fiber, and will help with bowel regularity but have little effect on the gut microbes.
By understanding how sources of dietary fiber differ, you can make better choices about your diet and the health benefits you hope to achieve.