The onset of developmentally induced changes in rat intestinal nutrient absorption is well known: brushborder glucose and fructose transporters appear during prenatal and postweaning periods, respectively. The onset of diet-induced regulation, however, is unknown. To test the hypothesis that intestinal glucose and fructose transport is regulated by diet during weaning and postweaning, we fed rats experimental diets containing high (65%) glucose, high fructose, high sucrose, or no carbohydrate. In 16-d-old rats, 6 d of dietary fructose but not glucose modestly increased fructose absorption in everted sleeves of small intestine (SI) over control (mother-fed with access to chow) rats (p = 0.02). In 21-d-old (age when sucrase is present) rats, dietary fructose and sucrose each dramatically enhanced (p = 0.004) fructose absorption over control rats and rats fed high glucose or carbohydrate-free diets. In 35-(postweaning) and 60-d-old rats, dietary fructose and sucrose, but not glucose, stimulated fructose absorption (p < 0.005) over rats fed a carbohydrate-free diet. In all age groups, intestinal glucose absorption was independent of diet (p 2 0.12), and experimental rats grew at the same rate as control rats. Absorption of fructose or glucose was 2-3 times greater in the proximal and middle than in the distal SI. Intestinal fructose, but not glucose, absorption can be induced by diet even during early weaning, and dietary fructose followed by sucrose is the most potent inducer. Thus, mechanisms of diet regulation can change ontogenetically, and early introduction of certain diets can induce appearance of certain nutrient transporters. (Pediatr Res 37: 777-782, 1995) Abbreviations ANOVA, analysis of variance HF, high fructose HG, high glucose HS, high sucrose MMC, mother-fedlchow NC, no carbohydrate SI, small intestine Advances in infant nutritional support have increased premature infant survival, especially in the low birth weight neonates (1). Premature neonates are typically fed simple sugars and even infant formula not only to prime their gut but also to provide an enteral source of nutrition. Approximately 40% of the energy intake of all infants is provided by carbohydrates (2), mainly glucose, galactose, and even fructose, and the neonatal small intestine encounters them mainly as breakdown products of carbohydrate digestion. Intestinal glucose transport appears prenatally, at around the 24th wk of gestation in humans (3) and just before birth in rats (4, 5). The onset of intestinal fructose transport in rats, on the other hand, is postnatal (5). There is an enhancement of fructose transport at the end of weaning in rats, and this happens despite the absence
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