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.