OBJECTIVE: To investigate the development of high fat diet-induced obesity and leptin resistance. DESIGN: Two experiments were carried out in this study. Firstly, we fed the mice with a high-or low-fat diet for up to 19 weeks to examine a progressive development of high fat diet-induced obesity. Secondly, we examined peripheral and central exogenous leptin sensitivity in mice fed high-or low-fat diets for 1, 8 or 19 weeks. SUBJECTS: A total of 168 C57BLa6J mice (3 weeks old) were used in this study. MEASUREMENTS: In the ®rst experiment, we measured the body weight, energy intake, adipose tissue mass, tibia bone length, and plasma leptin in mice fed either a high-or low-fat diet for 1, 8, 15 and 19 weeks. In the second experiment, body weight change and cumulative energy intake were measured at 6 h intervals for 72 h after leptin injection in mice fed a high-or low-fat diet for 1, 8 or 19 weeks. RESULTS: The results from the ®rst experiment suggested that the development of high fat diet-induced obesity in mice could be divided into early, middle and late stages. Compared with the mice fed a low-fat diet, the mice fed a high-fat diet showed a gradually increased body weight ( 5.2%), fat storage (epididymal plus perirenal; 6.7%) and plasma leptin ( 18%) at 1 week; 11.4%, 68.1%, and 223%, respectively, at 8 weeks; and 30.5%, 141%, and 458%, respectively, at 19 weeks. Energy intake of high fat diet-fed mice was equal to that of low fat diet-fed controls for the ®rst 3 weeks; it fell below control levels over the next 5 week period, but began to increase gradually after 8 weeks of high-fat diet feeding and then increased dramatically from 15 weeks to be 14% higher than that of controls after 19 weeks. The results from our second experiment showed that: (1) after 1 week of feeding, the mice fed a highfat diet were sensitive to a 2 mgag (body weight) intraperitoneal (i.p.) injection of leptin, with no differences in body weight change or cumulative energy intake post-injection; (2) after 8 weeks of feeding, the mice fed a high-fat diet were insensitive to 2 mgag (body weight) i.p. leptin, but were sensitive to a 0.1 mg intracerebroventricular (i.c.v.) injection of leptin; (3) after 19 weeks of feeding, the mice fed a high-fat diet were insensitive to 0.1 mg i.c.v. leptin, but were sensitive to a high dose of 2 mg i.c.v. leptin. CONCLUSIONS: The present study demonstrated that the development of high fat diet-induced obesity (19 weeks) in C57 B1a6J mice could be divided into three stages: (1) an early stage in response to high-fat diet that mice were sensitive to exogenous leptin; (2) a reduced food intake stage when mice had an increase in leptin production and still retained central leptin sensitivity; and (3) an increased food intake stage, accompanied by a reduction of central leptin sensitivity.

Critical insights into the etiology of insulin resistance have been gained by the use of animal models where insulin action has been modulated by strictly controlled dietary interventions not possible in human studies. Overall, the literature has moved from a focus on macronutrient proportions to understanding the unique effects of individual subtypes of fats, carbohydrates and proteins. Substantial evidence has now accumulated for a major role of dietary fat subtypes in insulin action. Intake of saturated fats is strongly linked to development of obesity and insulin resistance, while that of polyunsaturated fats (PUFAs) is not. This is consistent with observations that saturated fats are poorly oxidized for energy and thus readily stored, are poorly mobilized by lipolytic stimuli, impair membrane function, and increase the expression of genes associated with adipocyte profileration (making their own home). PUFAs have contrasting effects in each instance. It is therefore not surprising that increased PUFA intake in animal models is associated with improved insulin action and reduced adiposity. Less information is available for carbohydrate subtypes. Early work clearly demonstrated that diets high in simple sugars (in particular fructose) led to insulin resistance. However, again attention has rightly shifted to the very interesting issue of subtypes of complex carbohydrates. While no differences in insulin action have yet been shown, differences in substrate flux suggest there could be long-term beneficial effects on the fat balance of diets enhanced in slowly digested/resistant starches. A new area of major interest is in protein subtypes. Recent results have shown that rats fed high-fat diets where the protein component was from casein or soy were insulin-resistant, but when the protein source was from cod they were not. These are exciting times in our growing understanding of dietary factors and insulin action. While it has been clear for some time that 'oils ain't oils', the same is now proving true for carbohydrates and proteins.