Augmenting Leptin Circadian Rhythm Following a Weight Reduction in Diet-Induced Obese Rats: Short- and Long-Term Effects Anne Buison, Michael Pellizzon, Frank Ordiz Jr, and K.-L. Catherine Jen The current study sought to examine whether leptin injections following a weight reduction in diet-induced obese rats would reduce both the enhanced food intake and body weight (BW) regain observed during the refeeding phase. Female Wistar rats (n  100, 20 per group) were divided into 5 groups: (1) LEP rats were fed a high-fat (HF) diet (35% wt/wt) for 8 weeks to induce obesity and were then food-restricted (50% ad libitum) with a fortified high-fat diet for 2 weeks to induce a 20% BW loss. These rats were then refed the HF diet ad libtum for another 11 weeks. They were given leptin injections (200 g/kg BW, twice daily, intraperitoneally ) for 19 days concomitant with the onset of refeeding. (2) SAL rats were treated in the same manner as LEP rats except that they were given saline injections; (3) PF rats were treated like SAL rats except that they were pair-fed with the LEP rats; (4) HFC rats were fed HF diet ad libitum; and (5) LFC rats were fed a low-fat (LF) diet (AIN-93M) ad libitum. Ten rats from each group were killed after leptin treatment and at the end of the study. Food and caloric intakes were monitored, and body composition and plasma glucose, insulin, and leptin levels were assessed at death. Leptin injections after a weight reduction briefly reduced energy intake during the first week only. After 19 days of treatment and to the end of the study, LEP and SAL rats were similar in energy intake, BW (LEP: 393  11.2 g, SAL: 371  14.1; difference not significant [NS]) and total body fat percent (LEP: 19.3  1.5, SAL: 17.6  1.5; NS). Leptin treatment induced hyperinsulinemia and insulin resistance. All of the metabolic abnormalities observed at the end of treatment period disappeared at the end of the study (8 weeks post-leptin injection). We conclude that bolus leptin injections to manipulate leptin circadian rhythm in diet-induced obese rats after a weight reduction caused temporary insulin resistance and hyperinsulinemia, and were ineffective in influencing food intake, BW, and fat content. Leptin resistance was evident following 1 week of treatment in this study. Leptin treatment had no effect on body fat content both short-term and long-term. Exogenous leptin treatment may, in the long run, increase leptin resistance in diet-induced obese animals. Hence, long-term leptin treatment may not be beneficial to obese individuals consuming a HF diet. © 2004 Elsevier Inc. All rights reserved. O VERWEIGHT and obesity are problems of epidemic proportions. According to the World Health Organiza- tion, more than 300 million people are obese worldwide. 1 In the United States, the Centers for Disease Control and Prevention recently released a report estimating that more than 64.5% of all adults are overweight and that 31% are obese. 2 Because obesity is associated with increased incidence of many chronic diseases, 3 weight loss has become an important health goal for the American population. Among American men and women, the prevalences of attempting to lose and maintain weight were 28.8% and 35.1% among men and 43.6% and 34.4% among women, respectively. 4 However, long-term maintenance of weight loss is difficult. Studies have reported that more than 30% of weight lost is regained within 1 year, with more weight being regained in subsequent years. 5 Repetition of this pattern, called weight cycling or yo-yo dieting, is a very common phenomenon in the United States. In animal models, weight regain is typically enhanced in the initial weeks of refeeding following a weight reduction. 6,7 The rate of weight regain following weight reduction can be sepa- rated into 2 phases: (1) the critical period— defined as the first 2 to 3 weeks of initial weight regain, it is characterized by both an enhanced food intake and rate of weight gain compared to controls; and (2) the return phase— defined as the rate of weight regain which returns to that of the control, noncycling rats. With this in mind, we believed that if the rate of weight regain during the “critical” period is reduced, we may be able to alter both the final body weight and body composition of the rats. Exogenous leptin administration is known to reduce food intake, increase energy expenditure, and therefore induces a reduction in body weight (BW) in animals. 8 In addition, be- cause leptin also increases both fat lipolysis and oxidation while preserving lean body mass, 9 administering leptin during the critical period may also aid in attaining a more desirable body composition, as well as in altering final BW. While exogenous leptin treatment has been viewed as a possible remedy for obesity in some populations, there is evi- dence against this. It may actually accelerate the onset of leptin resistance. In our previous study, a leptin infusion with im- planted miniosmotic pumps for 2 weeks post-weight reduction reduced body fat content but had no effects on BW. 10 Martin et al 11 demonstrated in Long-Evans rats that leptin resistance was exhibited during the final 2 weeks of the study in a 28-day administration paradigm using a leptin infusion (mini-osmotic pumps). In addition, these investigators determined that there was a downregulation of both the leptin receptor mRNA and protein in the hypothalamus. Thus, administration of leptin via mini-osmotic pump may not be an ideal method for delivering leptin because it may accelerate the onset of leptin resistance by subjecting the animal to constant high levels of leptin. Also, it may mask the circadian rhythm of the endogenous leptin re- lease from adipose tissue. The purpose of this study was to determine whether leptin From the Department of Nutrition and Food Science, Wayne State University, Detroit, MI. Submitted September 8, 2003; accepted December 1, 2003. Supported in part by the a grant from American Institute for Cancer Research to K-L.C.J Address reprint requests to K. L. Catherine Jen, PhD, Department of Nutrition and Food Science, Wayne State University, 3009 Science Hall, Detroit, MI 48202. © 2004 Elsevier Inc. All rights reserved. 0026-0495/04/5306-0005$30.00/0 doi:10.1016/j.metabol.2003.12.022 782 Metabolism, Vol 53, No 6 (June), 2004: pp 782-789