High-fat diet feeding reduces the diurnal variation of plasma leptin concentration in rats.

To investigate the response of plasma leptin and its diurnal variation to graded levels of dietary fat intake, adult (486.8+/-10.8 g), male rats (N = 52) were fed diets containing 12%, 28%, 44%, and 60% fat for 4 weeks. The body weight gain and abdominal fat pad weight were higher (P < .05) in groups fed diets containing 44% and 60% fat compared with the two diets containing less fat. There were no significant differences in terms of body weight or fat pad weight between animals fed the two diets with higher fat content or between animals fed the two lower-fat diets. Twenty-four-hour energy expenditure was not different among the dietary fat groups. After 3 days on the experimental diets, plasma leptin increased (P < .03) in all dietary groups. The increases in leptin in animals fed 12% and 28% fat diets occurred primarily in the morning. In contrast, in groups fed the two diets containing higher fat content, leptin levels increased mainly in the afternoon. As a result, the daily variation in leptin increased (P < .05) in the two groups fed lower-fat diets, but decreased (P < .04) in animals fed the two higher-fat diets. These data demonstrate that short-term high-fat diet feeding abolished the diurnal fluctuation of plasma leptin levels, which may prevent proper leptin function and eventually contribute to the development of obesity.