Dietary self-selection and meal patterns of obese and lean Zucker rats

Following two weeks of baseline measures on laboratory chow, young obese and lean male Zucker rats were given access to separate macronutrients for 18 weeks. First, the rats were given access to soybean meal, dextrinized starch, and lard for eight weeks. Daily caloric intake of obese rats was greater on laboratory chow than that of lean rats and remained so following the switch to the self-selection diet. While obese and lean rats consumed similar proportions of each macronutrient over the eight-week period, there were substantial changes in the pattern of intake across weeks. Obese rats decreased daily caloric intake primarily by decreasing lard consumption. In contrast, lean rats maintained daily caloric intake while increasing the proportion of calories from fat. The addition of a 25% sucrose solution (weeks 11-18) produced an elevated daily caloric intake for both obese and lean rats, achieved through an increased carbohydrate intake with a simultaneous decrease in fat intake. A diurnal pattern of intake was maintained for each macronutrient. Compared to controls, rats on the self-selection diet did not show differential growth. Measurement of daily meal patterns for soybean meal, sucrose, and lard during the last four days of the experiment showed that obese rats ate significantly larger meals comprised of all three food components than did lean rats. For both obese and lean rats, one-component meals were primarily sucrose with greater intake during the light period. Lean rats showed a greater tendency to indulge in these between-meal "snacks" than did obese rats. These data suggest that palatability and nutrient source as well as length of exposure are critical determiners of nutrient selection and total daily caloric intake.