Calcium absorption in the human distal colon, effect of short chain fatty acids from fiber fermentation

grantor: University of TorontoIncreased intake of dietary fiber is recommended by health agencies but there is a concern that it may reduce the bioavailability of minerals such as calcium (Ca) due to its ability to bind with minerals. However, it has been hypothesized that Ca is released and short chain fatty acids (SCFA) are produced upon fiber fermentation. The released Ca is absorbed in the distal colon and the absorption is enhanced by SCFA. To determine the amount of Ca potentially absorbed in the colon, Ca released in basal diets or single dairy foods with and without pectin, psyllium or cellulose were tested using an all in vitro method. Compared with the basal diet, pectin did not affect the total Ca release from food matrix but rather shifted the release of Ca from the small intestinal to the colonic conditions. Psyllium and cellulose caused an insignificant release of Ca in the colon. To determine the Ca actually absorbed in the colon with and without SCFA, four rectal infusion studies were done in humans. The serum Ca response was increased particularly in the presence of SCFA in a 3:1 acetate (Ac):propionate (Pr) ratio, indicating colonic absorption of Ca. Ca disappearance in the colon as a measure of Ca absorption showed that Pr was more effective than Ac in enhancing Ca absorption. Ca absorption increased linearly with Ca concentration with and without Ac, or Pr, suggesting a non-saturable diffusion process. Ac and Pr absorption were enhanced by the presence of Ca. A human feeding study using two stable isotopes of Ca e.g. $\sp{42}$Ca and $\sp{44}$Ca showed that Ca absorption occurs in the colon. The presence of pectin caused a reduction in the total Ca absorption but shifted the amount of Ca absorbed from the small intestine to the colon. In summary, the in vitro, rectal infusion and oral feeding studies have shown that Ca is released after fiber fermentation and can be absorbed in the human distal colon. The colonic Ca absorption is enhanced by Ac or Pr, through a non-saturable diffusion process with Pr being more effective than Ac.Ph.D