Do northern bobwhite quail modulate intestinal nutrient absorption in response to dietary change? A test of an adaptational hypothesis

We acclimated northern bobwhite quail (Colinus virginianus) to either chow (high carbohydrate/low protein) or crickets (low carbohydrate/high protein) and tested predictions of hypotheses based on the premise of the economical design of animals. The adaptive modulation hypothesis predicts chat D-glucose uptake would be higher and L-proline uptake lower in bobwhites acclimated to chow. The spare capacity hypothesis predicts that the capacity to absorb D-glucose actively will exceed the estimated nutrient load from daily food intake. There was no significant dietary effect on intestinal D-glucose (P = 0.8) and L-proline (P = 0.7) uptake rates measured in vitro using the everted sleeve technique. In chow eaters maximal mediated D-glucose uptake summed along the entire length of intestine (53 cm) was far too low (7.2 mmol/d) to explain observed rates of glucose absorption in vivo (> 35 mmol/d). Hence, both predictions were falsified. In vitro uptake may not be an appropriate measure of the intestine's absorptive capacity because it does not measure possibly important pathways of passive absorption. There is increasing evidence that substantial passive glucose absorption occurs in some birds. If passive absorption predominates the adaptive modulation hypothesis might not apply.