AUTHOR: Dr. Jason Bush
A common way to introduce people to the concept of prebiotics is to explain that ‘prebiotics are food for probiotics’. Indeed, probiotics (aka. Healthy bacteria) obtain energy by digesting prebiotics. Prebiotic digestion triggers a cascade of events: One type of bacteria will start by metabolizing the raw prebiotic, and the break-down products of this process become food for other types of bacteria. In turn, these other bacteria produce metabolites that reward the human inhabited by the microbial ecosystem.
Among the best characterized beneficial metabolites are short chain fatty acids (SCFA), which include acetic acid, propionic acid, butyric acid, and valeric acid. Butyric acid, often simply called butyrate, is particularly important because it constitutes the primary energy source for the cells lining the colon. Butyrate is produced by bacteria belonging to the group Firmicutes (pronounced FIRM-eh-CUTE-eez) who feed on the acetic acid and lactic acid produced by digestion resistant starch-fermenting Bifidobacteria.
SCFAs, including butyrate, also appear to provide a crucial link between fiber consumption and the sense of fullness we feel following a fiber-rich meal. Our text book understanding suggests that fiber contributes to a sense of fullness because it resists digestion, and remains largely intact as it passes through the stomach and small intestine. Here, the bulkiness of the fiber exerts pressure, which is sensed by touch receptors lining the digestive tract and this message is communicated to the satiety center in the brain.
While this mechanism explains part of the fiber-fullness connection, a more complicated physiological system appears to be at work. As mentioned above, SCFAs produced from the fermentation of prebiotic fiber constitute an important source of energy for the body, normally accounting for approximately 10% of the body’s energy needs. However, unlike sugar produced from starch digestion, we are not aware of any complex hormonal system that responds to SCFAs the way that insulin secreted by the pancreas controls blood sugar levels.
So how does the body know that you are getting energy from fiber – and to tell you to stop eating?
SCFAs produced by fiber fermentation stimulate cells in the small intestine to produce their own sugar. This creates a small but detectable rise in blood sugar levels that are sensed by cells lining portal vein – the return system that takes blood rich in nutrients from the absorptive areas of the intestines to the liver for processing. Neurons touching the portal vein sense this rise in blood glucose and send a signal to the satiety center in the brain, thus reducing the urge to eat. Remarkably, the body has co-opted the system it normally uses to sense fullness from a sugar-rich meal to sense fullness from a fiber-rich meal.
While increasing your fiber intake may help provide a sense of fullness and even help with weight loss, beneficial SCFAs are only produced when prebiotics are fermented, meaning that not every type of fiber will provide this dual-impact on hunger reduction. Ultimately, the only fibers that are both insoluble and fermentable are digestion resistant starches, and it is this class of prebiotic that provides bulk and acts as a source of hunger-fighting SCFAs.