Regulation of Intestinal Nutrient Transport Is Impaired in Aged Mice

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Abstract
To determine the effect of age on the regulation of intestinal nutrient absorption, we fed young (7.6-mo-old) and aged (24.8-mo-old) C57BL mice diets designed to stimulate in vitro sugar or amino acid uptake in the isolated small intestine. In each age group, diet had no effect on feeding rates and body weights. d-Glucose and d-fructose uptakes by the small intestine each increased by about two times in young and 1.5 times in aged mice fed high carbohydrate diets as compared with those fed no carbohydrate. Adaptive increases in uptake by the aged group were not only reduced but also restricted to more proximal regions of the small intestine. In both age groups, diet-stimulated increases in d-glucose transport were accompanied by parallel increases in number of Na+-d-glucose cotransporters as estimated by specific phlorizin binding. Diet had no effect on transporter Kd for phlorizin, turnover rate of each transporter, mucosal mass or mucosal permeability. A high protein diet stimulated the uptake of l-aspartate and l-proline in young mice and of only l-aspartate in aged mice. Uptake of essential amino acids and of nonessential amino acids sharing transporters with essential ones were not regulated. Although aged mice possess adaptive mechanisms to diet that are similar to those in young mice, the effectiveness of these mechanisms may be impaired with age and may result in malabsorption symptoms so prevalent in the elderly.