Dietary modulation of intestinal fructose transport and GLUT5 mRNA expression in hypothyroid rat pups

Background: Intestinal fructose transport rates or GLUTS mRNA levels typica lly show a two- to threefold increase after weaning in rats allowed to wean normally but can he enhanced precociously by high-fructose diets during ea rly weaning. Developmental increases in serum thyroxine levels coincide wit h the onset of weaning and have been linked to changes in intestinal sucras e and lactase activities. Methods: Rat pups were made hypothyroid by giving the dam 0.01% propylthiou racil as drinking water from day 18 of gestation. The hypothyroid pups and age-matched euthyroid control pups were then fed high-fructose or high-gluc ose solutions by gavage, twice a day starting at 17 days of age for 3 days, and then killed at 20 days of age. Results: Serum thyroxine levels were five times lower in the hypothyroid pu ps. Rates of intestinal fructose uptake in the proximal and middle small in testine were 2.0 to 2.5 times higher in the hypothyroid and euthyroid pups fed high-fructose solution than in littermates fed high-glucose solution or those allowed to wean normally with the darn. Intestinal glucose uptake al so increased in hypothyroid but not in euthyroid pups fed high-fructose or high-glucose solutions. GLUTS mRNA levels increased in euthyroid and hypoth yroid pups fed high fructose and paralleled the increase in fructose uptake . Conclusion: During weaning, dietary fructose can precociously enhance intes tinal fructose uptake and GLUTS mRNA expression, independent of development al increases in serum thyroxine levels. Modest changes in glucose transport rates indicate that nonspecific mechanisms may provide a minor contributio n to diet-induced changes in nutrient absorption in hypothyroid pups.