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

Intestinal fructose transport rates or GLUT5 mRNA levels typically show a two- to threefold increase after weaning in rats allowed to wean normally but can be enhanced precociously by high-fructose diets during early weaning. Developmental increases in serum thyroxine levels coincide with the onset of weaning and have been linked to changes in intestinal sucrase and lactase activities.

Rat pups were made hypothyroid by giving the dam 0.01% propylthiouracil as drinking water from day 18 of gestation. The hypothyroid pups and age-matched euthyroid control pups were then fed high-fructose or high-glucose solutions by gavage, twice a day starting at 17 days of age for 3 days, and then killed at 20 days of age.

Serum thyroxine levels were five times lower in the hypothyroid pups. Rates of intestinal fructose uptake in the proximal and middle small intestine 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 dam. Intestinal glucose uptake also increased in hypothyroid but not in euthyroid pups fed high-fructose or high-glucose solutions. GLUT5 mRNA levels increased in euthyroid and hypothyroid pups fed high fructose and paralleled the increase in fructose uptake.

During weaning, dietary fructose can precociously enhance intestinal fructose uptake and GLUT5 mRNA expression, independent of developmental increases in serum thyroxine levels. Modest changes in glucose transport rates indicate that nonspecific mechanisms may provide a minor contribution to diet-induced changes in nutrient absorption in hypothyroid pups.