Development of food motivated behavior in free feeding and food restricted zucker fatty (fa/fa) rats

The food motivated behavior of ad lib fed or calorically restricted male Zucker fatty and lean rats was compared at 12, 16, 20 and 30 weeks of age. The groups were fasted for 0, 12 and 24 hr and required to lever press for food pellets on VI 40 sec. Responding by ad lib fatties was elevated over lean controls during the rapid growth period (12 and 16 weeks of age) but was significantly reduced following the completion of growth (20 weeks). Elevated responding accompanied hyperphagia by the ad lib fatties during growth but did not accompany a second, adult onset period of hyperphagia in these fatties. In contrast, fatties calorically restricted from birth responded at elevated rates at all ages compared to lean controls. At 20 weeks restricted fatties, which were as obese on a percentage body composition basis as ad lib fatties, responded significantly more than the latter group at all fasting levels. Metabolic measurements revealed elevated fat cell size, LPL activity/cell and insulin levels in both fatty groups at 15 weeks, and elevated fat cell number in these groups at 33 weeks. The possibility is discussed that enhanced lipogenic factors present during the rapid growth period combine with the metabolic demands of growth to produce a “compensatory hyperphagia” in the fatty rat.