Ever higher: Constant rise of body weight set-point in growing Zucker rats

The mass of food hoarded by rats given access to food only 2 h per day is proportional to the rats' body weight deficit. The intersect of the regression line of hoarded food plotted against body weight gives an indication of the body weight set-point. In the present work, the hoarding behavior of six obese and six control rats was measured every day at various body weights from 8 to 24 weeks of age. Every other week the animals were anesthetized and their percentage of fat was measured in vivo with a total body electrical conductivity method (TOBEC). Lean mass and body length of the obese, and their controls, increased similarly in both groups over the period of the measurements. On the other hand, the body weights increased more in obese, from 174 +/- 5 to 729 +/- 18 g (n = 6), than in controls, from 157 +/- 5 g to 452 +/- 14 g (n = 6). The body weight set-point, calculated every other week for both groups of rats, increased progressively with age. At the age of 24 weeks, the mean set-point for body weight regulation was 758 +/- 13 g in obese and 467 +/- 12 g in controls. This result suggests that the obese fa/fa rat defends its fat content, or a variable correlated to the fat content.