Dietary conjugated linoleic acid reduces adiposity in lean but not obese Zucker rats.

Recent studies have demonstrated a reduction in body fat in growing animals fed conjugated linoleic acid (CLA). Two experiments were conducted to extend these observations to obese rats so that the mechanism of the actions of CLA might be more easily elucidated. In experiment 1, male lean and obese Zucker rats were fed diets containing either 0 or 0.5% CLA for 5 wk. There was no effect of diet on growth rate or food intake. Dietary CLA reduced retroperitoneal and inguinal fat pad weights in the lean rats but increased fat pad weights in the obese genotype (diet x genotype interaction; P < 0.05). Determination of fat pad cellularity indicated that these changes in fat pad weight were due to a reduction or increase in average fat cell size for the lean and obese Zucker rats, respectively. In experiment 2, we sought to reproduce these effects on fat pad size, as well as to determine the effect of dietary CLA on the catabolic response to bacterial endotoxin injection in obese Zucker rats. Growing female lean and obese Zucker rats were fed diets containing 0 or 0.5% CLA for 8 wk. On d 28, each rat was injected intraperitoneally with lipopolysaccharide from Escherichia coli serotype 055:B5 (1 mg/kg body weight) and body weight was determined over the next 96 h. There was a diet x genotype interaction (P < 0.05) for the body weight response to lipopolysaccharide 24 h postinjection. Lean rats fed CLA lost less weight than did lean controls, but obese rats fed CLA lost more weight than did obese controls. As in the first experiment, there was a diet x genotype (P < 0.05) for the effect of treatment on retroperitoneal fat pad weights determined at the end of the experiment. Lean rats fed CLA had smaller RP fat pads than did lean controls, but obese rats fed CLA once again had heavier RP fat pads than did obese controls. These results indicate that CLA reduces body fat and catabolic response to endotoxin injection in lean Zucker rats but not in the obese genotype. The observed interaction between diet and genotype warrants additional investigation into the specific mechanism(s) of the biological activities of CLA.