Famine, fiber, fatty acids, and failed colonic absorption: does fiber fermentation ameliorate diarrhea?

The salvage function of the colon for absorption of unabsorbed sodium and water from the jejunum and ileum depends upon the metabolic integrity of colonic epithelial cells maintained by luminal short-chain fatty acids. With the depletion of luminal short-chain fatty acids under conditions of starvation, metabolic compensation from vascular substrates is incomplete. Loss of luminal short-chain fatty acids diminishes cell membrane integrity and causes secretion by colonic epithelial cells, leading to starvation diarrhea. Because sodium absorption is dependent upon CO2 production from n-butyrate, no compensatory absorption occurs during starvation. Under conditions in which luminal short-chain fatty acids are depleted, dietary fiber is useful as a low osmolality food constituent and for renewal of short-chain fatty acid levels by bacterial fermentation. The "antisecretory" effect of dietary fiber depends on the degree of the preexisting depletion of short-chain fatty acids and the methodology used to assess absorptive function. Dietary fiber has not been found harmful in refeeding starvation victims for whom it is an essential food constituent.