Meal pattern analysis of diet-induced obesity in susceptible and resistant rats

Objective: To characterize the meal patterns of free feeding Sprague-Dawley rats that become obese or resist obesity when chronically fed a high-fat diet. Research Methods and Procedures: Male Sprague-Dawley rats (N = 120) were weaned onto a high-fat diet, and body weight was monitored for 19 weeks. Rats from the upper [diet-induced obese (DIO)] and lower [diet-resistant (DR)] deciles for body-weight gain were selected for study. A cohort of chow-fed (CF) rats weight-matched to the DR group was also studied. Food intake was continuously monitored for 7 consecutive days using a BioDAQ food intake monitoring system. Results: DIO rats were obese, hyperphagic, hyperleptinemic, hyperinsulinemic, hyperglycemic, and hypertriglyceridemic relative to the DR and CF rats. The hyperphagia of DIOs was caused by an increase in meal size, not number. CF rats ate more calories than DR rats; however, this was because of an increase in meal number, not size. When expressed as a function of lean mass, CF and DR rats consumed the same amount of calories. The intermeal intervals of DIO and DR rats were similar; both were longer than CF rats. The nocturnal satiety ratio of DIO rats was significantly lower than DR and CF rats. The proportion of calories eaten during the nocturnal period did not differ among groups. Discussion: The hyperphagia of a Sprague-Dawley rat model of chronic diet-induced obesity is caused by an increase in meal size, not number. These results are an important step toward understanding the mechanisms underlying differences in feeding behavior of DIO and DR rats.