Digestion and Metabolism

During the transit of digested food material through the gastrointestinal tract many changes occur in the intestinal lumen that could influence nutrient bioavailability. Digestion of food constituents is an important aspect of nutrient bioavailabil-ity. The secretion of acid into the stomach following food ingestion activates certain digestive enzymes, as well as creating an acidic environment that influences mineral solubility and extraction from food. In this regard, the choice of a mineral to use for supplementation purposes might be influenced by certain physiological conditions, such as achlorhydria. Aging is associated with a decrease in gastric acid secretion in many elderly persons leading to either hypochlorhydria or achlorhydria, characterized by either low or complete absence of acid secretion, respectively, leading to a neutral or slightly alkaline gastric pH. This raised pH condition in the stomach can have detrimental effects on micronutrient bioavailability. For example, elderly persons with achlorhydria are at risk of vitamin B12 deficiency because of an inability to remove properly protein-bound vitamin B12 from food when it enters the stomach. In these subjects, the capacity to absorb vitamin B12 is normal, however, because they can readily absorb crystalline (nonprotein-bound) vitamin B12 from a supplemental vitamin B12 dose. Lowering the gastric pH towards normal by administering hydrochloric acid restores vitamin B12 absorption from food.

In achlorhydric elderly persons the absorption of calcium carbonate from a dietary supplement after an overnight fast is very poor, presumably because calcium carbonate is a relatively insoluble calcium salt and needs gastric acid to be solubilized. Again, these subjects have normal calcium absorption if the calcium is delivered in a more soluble form, such as calcium citrate. In addition to elderly persons who develop achlorhydria, a large number of people regularly use gastric acid-lowering medications, such as the gastric proton pump inhibitor omeprazole, for antiulcer therapy. These medications can reduce zinc absorption and presumably may affect the absorption of other divalent minerals. Acidification of the gastric contents and solu-bilization of minerals in the gastric juice is important because many mineral nutrients are preferentially absorbed in the duodenum (upper small intestine) and need to be available in free or low-molecular-weight complexes when they leave the stomach to facilitate contact with intestinal nutrient transporters on the apical (luminal) surface of the absorptive enterocytes.

The lower gastrointestinal tract can also be a potentially important site affecting the bioavailability of bioactive substances found in food. In this regard, intestinal bacteria found in the large intestine can influence bioavailability. Bacteria are instrumental in the metabolic conversion of certain phytonutrients into forms that are more readily absorbed. In addition, bacteria likely play an important role in the enhancing effects of prebiotics on mineral bioavailability. Consumption of nonabsorbable carbohydrates, such as inulin, can have a positive effect on mineral absorption. A possible mechanism of this effect is that the nonabsorbable carbohydrates pass the small intestine and enter into the large intestine where they serve as a food substrate for bacteria. The metabolism of these prebiotic substances by intestinal bacteria lowers the pH of the lumen of the large intestine and may thereby serve to solubilize some insoluble mineral complexes that have passed through the small intestine. The freed mineral would then be available for absorption from the large intestine, thereby increasing overall mineral bioavailability. Other scenarios are possible as well, such as the release of short-chain fatty acids by the bacteria, that may facilitate mineral absorption.

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