Tests of the adaptive modulation hypothesis for dietary control of intestinal nutrient transport.

According to the adaptive modulation hypothesis, an intestinal transporter should be repressed when its biosynthetic and other costs (of maintenance) exceed the benefits it provides. This leads to two contrasting predictions: transport of a sugar or amino acid worth calories should tend to be increased by its substrate, and transport of a vitamin should be modulated downwards by its substrate and upmodulated in deficiency. In a test of the first prediction, omnivorous desert iguanas eating alfalfa pellets (a high-carbohydrate diet) were compared with desert iguanas eating mealworms (a low-carbohydrate, higher-protein diet). In accord with the prediction, intact intestinal sleeves from the former group had higher rates of carrier-mediated D-glucose uptake/centimeter across the brush border than sleeves from the latter group. But in contrast to the first prediction, mealworm eaters had lower (not higher) proline uptake rates, and the ratio of glucose/proline uptake in the two groups did not differ. I review similar tests in 12 other species and show that overall the hypothesis has been quite robust with regard to the first prediction. Cases in which the hypothesis is rejected may reflect complications associated with changes in other dietary factors or phylogenetic constraints. In a test of the second prediction, uptake of the water-soluble vitamin choline was not increased in choline-deficient chicks, nor was it decreased in adults that have no dietary requirement for choline. I review similar tests for four other vitamins and five essential minerals. Dietary control of transport of the minerals and two of the vitamins seems to be in accord with the hypothesis. But transport rate for three vitamins (choline, pantothenic acid, ascorbic acid) seems not to be increased in deficiency. The best explanation seems to be that vitamin transport is modulated only if it is primarily by a carrier-mediated pathway.