Diet cycling and age alter weight gain and insulin levels in rats

For assessment of the effect of diet cycling on body weight gain patterns, 3-mo-old male Sprague-Dawley rats were either cycled from chow to a high-energy condensed milk (CM) diet, back to chow, and then back to CM diet at 3-mo intervals (cyclers) or were fed chow to 9 mo of age and then CM diet for 3 mo (noncyclers). Nine of 21 cyclers developed diet-induced obesity (DIO), gaining 36, 59, and 281% more weight than chow-fed controls (CF) at each cycle, respectively. Twelve cycled rats were diet-resistant (DR) with comparable weight gain to CF rats after the first CM diet and chow cycles. However, they gained 202% more than CF rats and 50% more, with 29% heavier retroperitoneal fat pads, than noncycled DR rats after their second CM diet cycle begun at 9 mo of age. Enhanced weight gain in DR cyclers was probably due to enhanced food efficiency in the last few weeks of CM diet intake. Plasma insulin levels were 70% higher in cycled vs. noncycled DIO and DR rats, and both were higher than CF rats. Unlike 6-mo-old DR rats in a prior study, 12-mo-old noncycled DR rats after 3 mo on CM diet here had 45-172% higher alpha 2-adrenoceptors binding in 6 of 17 brain areas than noncycled DIO and/or CF rats. Thus age, diet cycling, and brain alpha 2-adrenoceptors interact to affect long-term changes in weight gain and metabolism.