Body weight regulation in obese and obese-reduced rats.

We previously developed a model of dietary obesity in adult male rats where duration of feeding a high fat diet (HFD; 60 percent of calories from fat) influences reversibility of the obesity following a reduction in dietary fat. In the model, rats fed the HFD for 4 months show an apparent complete reversal of obesity when switched to a low fat diet (LFD; 14 percent of calories from fat), but rats fed the HFD for 7 months show persistent obesity even when switched to the LFD. This model of dietary obesity is useful for studies of energy balance and energy requirements during obesity development and reversal. In the present study, rats fed the HFD for 4 or 7 months were switched to the LFD and, after weights stabilized, were subjected to food restriction followed by ad libitum refeeding. Food restriction (15 g/day of the LFD, or about 60 percent of usual control food intake) continued until body weights of the groups stabilized at new, reduced levels (about 2-2.5 months). This was followed by ad libitum refeeding of the LFD for 3--4.5 months. Rats fed the HFD for 4 months showed complete obesity reversal after 2 months of eating the LFD. They showed a similar response to food restriction to LFD controls, but surprisingly regained significantly more body weight, body fat and total body energy during refeeding than LFD controls. Rats fed the HFD for 7 months remained heavier and fatter than LFD controls after 2 months of eating the LFD. Despite losing more carcass energy than controls during food restriction, their body weights and body energy content stabilized at higher levels than LFD controls. During refeeding, they regained their obese state relative to controls. These results demonstrate that: (1) the duration of HFD feeding is an important factor in the reversibility of the obese state; (2) sustained HFD feeding produces an obese state that is defended more by a greater restoration of carcass energy during refeeding than by a preservation of carcass energy during food restriction; (3) sustained obesity appears to produce some reductions in energy requirements; (4) even a brief period of obesity may leave the rats prone to reattain an obese state when body weight is challenged.