Long-Term Effects of Moderate Fructose Feeding on Glucose Tolerance Parameters in Rats

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Abstract
The effects on glucose tolerance of prolonged fructose feeding, at a level approximating that currently in the American diet, were examined in weanling, male Wistar rats. Two groups of rats were fed ad libitum diets containing either 54% cooked cornstarch (w/w) [CS] or 39% cooked cornstarch plus 15% d-fructose (CSF) for 3, 5, 7, 9 and 15 months. All rats were given an oral glucose tolerance test (250 mg glucose/100 g body weight) after each designated feeding period (hereafter referred to as age groups). Serum insulin and glucose were determined from blood obtained after fasting and ½, 1, 2 and 3 hours following the glucose load. Neither body weight nor relative food intake (g/day/100 g body weight) differed significantly with diet. Fasting serum insulin increased linearly (r = 0.97) with age in both dietary groups, but was significantly higher (P < 0.01) in CSF- than in CS-fed rats. Fasting serum glucose levels were also higher (P < 0.05) in CSF- than in CS-fed rats. The levels decreased with age (r = -0.61) in CS-fed rats, but increased linearly with age (r = 0.96) in CSF-fed rats. Serum insulin response to the oral glucose load was higher (P < 0.03) in all CSF-fed than in CS-fed rats. The serum glucose response curve following the oral load was significantly higher (P < 0.025) in CSF-fed than in CS-fed rats at 7 months but not at other ages. Liver phosphoenolpyruvate carboxykinase (PEPCK, EC 4.1.1.32) activity, measured only in the 3-month group, was higher (P < 0.05) in the CSF-fed rats, indicating higher gluconeogenic activity.