Ontogenetic development of nutrient transporters in rat intestine.

We measured intestinal brush-border uptakes of three sugars and three amino acids, plus intestinal morphometric parameters, in rats from the day of birth until adulthood. Rates of body weight gain had pronounced peaks in the suckling phase and again during weaning, separated by a dip at the onset of weaning. These two peaks coincided with peaks or plateaus in intestinal growth and in glucose (Glc) and proline (Pro) uptake capacities, which may provide the basis for high rates of body growth. Pro uptake declined relative to Glc uptake upon weaning, reflecting decreasing protein needs for growth and decreasing protein intake relative to carbohydrate intake. Fructose (Frc) and lysine uptake increased steeply on weaning, whereas galactose uptake declined relative to that of Glc. Rats prevented from normal weaning by being maintained on dry milk were generally similar to normal rats weaned onto chow. Notably, their Frc uptake still rose steeply on weaning despite low dietary Frc levels, suggesting hard-wired regulation of Frc transporter development. Our in vitro uptakes are similar to modern in vivo values in the same strain of rats. Nutrient uptake capacities exceed normal dietary intakes by only a modest safety margin.