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.

We investigated the relationship between gut health, visceral fat dysfunction and metabolic disorders in diet-induced obesity. C57BL/6J mice were fed control or high saturated fat diet (HFD). Circulating glucose, insulin and inflammatory markers were measured. Proximal colon barrier function was assessed by measuring transepithelial resistance and mRNA expression of tight-junction proteins. Gut microbiota profile was determined by 16S rDNA pyrosequencing. Tumor necrosis factor (TNF)-α and interleukin (IL)-6 mRNA levels were measured in proximal colon, adipose tissue and liver using RT-qPCR. Adipose macrophage infiltration (F4/80+) was assessed using immunohistochemical staining. HFD mice had a higher insulin/glucose ratio (P = 0.020) and serum levels of serum amyloid A3 (131%; P = 0.008) but reduced circulating adiponectin (64%; P = 0.011). In proximal colon of HFD mice compared to mice fed the control diet, transepithelial resistance and mRNA expression of zona occludens 1 were reduced by 38% (P<0.001) and 40% (P = 0.025) respectively and TNF-α mRNA level was 6.6-fold higher (P = 0.037). HFD reduced Lactobacillus (75%; P<0.001) but increased Oscillibacter (279%; P = 0.004) in fecal microbiota. Correlations were found between abundances of Lactobacillus (r = 0.52; P = 0.013) and Oscillibacter (r = −0.55; P = 0.007) with transepithelial resistance of the proximal colon. HFD increased macrophage infiltration (58%; P = 0.020), TNF-α (2.5-fold, P<0.001) and IL-6 mRNA levels (2.5-fold; P = 0.008) in mesenteric fat. Increased macrophage infiltration in epididymal fat was also observed with HFD feeding (71%; P = 0.006) but neither TNF-α nor IL-6 was altered. Perirenal and subcutaneous adipose tissue showed no signs of inflammation in HFD mice. The current results implicate gut dysfunction, and attendant inflammation of contiguous adipose, as salient features of the metabolic dysregulation of diet-induced obesity.

Pan, D A.; Lillioja, S; Kriketos, A D.; Milner, M R.; Baur, L A.; Bogardus, C; Jenkins, A B.; and Storlien, L H., 1997, Skeletal muscle triglyceride levels are inversely related to insulin action, 983-988.Skeletal muscle triglyceride levels are inversely related to insulin action Skeletal muscle triglyceride levels are inversely related to insulin action Abstract Abstract In animal studies, increased amounts of triglyceride associated with skeletal muscle (mTG) correlate with reduced skeletal muscle and whole body insulin action. The aim of this study was to test this relationship in humans. Subjects were 38 nondiabetic male Pima Indians (mean age 28 ± 1 years). Insulin sensitivity at physiological (M) and supraphysiological (MZ) insulin levels was assessed by the euglycemic clamp. Lipid and carbohydrate oxidation were determined by indirect calorimetry before and during insulin administration. mTG was determined in vastus lateralis muscles obtained by percutaneous biopsy. Percentage of body fat (mean 29 ± 1%, range 14-44%) was measured by underwater weighing. In simple regressions, negative relationships were found between mTG (mean 5.4 ± 0.3 μmol/g, range 1.3-1.9 μmol/g) and log 10 M (r = −0.53, P ≤ 0.001), MZ (r = −0.44, P = 0.006), and nonoxidative glucose disposal (r = −0.48 and −0.47 at physiological and supraphysiological insulin levels, respectively, both P = 0.005) but not glucose or lipid oxidation. mTG was not related to any measure of adiposity. In multiple regressions, measures of insulin resistance (log 10 M, MZ, log 10 [fasting insulin]) were significantly related to mTG independent of all measures of obesity (percentage of body fat, BMI, waist-to-thigh ratio). In turn, all measures of obesity were related to the insulin resistance measures independent of mTG. The obesity measures and mTG accounted for similar proportions of the variance in insulin resistance in these relationships. The results suggest that in this human population, as in animal models, skeletal muscle insulin sensitivity is strongly influenced by local supplies of triglycerides, as well as by remote depots and circulating lipids. The mechanism(s) underlying the relationship between mTG and insulin action on skeletal muscle glycogen synthesis may be central to an understanding of insulin resistance.LFPIns, log fasting plasma insulin; M and MZ, h)w-dose (physiological) and high-dose (supraphysiological) in vivo insulin-mediated glucose disposal rates, respectively; mTG,

High levels of dietary fat may contribute to both insulin resistance and obesity in humans but evidence is limited. The euglycemic clamp technique combined with tracer administration was used to study insulin action in vivo in liver and individual peripheral tissues after fat feeding. Basal and nutrient-stimulated metabolic rate was assessed by open-circuit respirometry. Adult male rats were pair-fed isocaloric diets high in either carbohydrate (69% of calories; HiCHO) or fat (59% of calories; HiFAT) for 24 +/- 1 days. Feeding of the HiFAT diet resulted in a greater than 50% reduction in net whole-body glucose utilization at midphysiological insulin levels (90-100 mU/l) due to both reduced glucose disposal and, to a lesser extent, failure to suppress liver glucose output. Major suppressive effects of the HiFAT diet on glucose uptake were found in oxidative skeletal muscles (29-61%) and in brown adipose tissue (BAT; 78-90%), the latter accounting for over 20% of the whole-body effect. There was no difference in basal metabolic rate but thermogenesis in response to glucose ingestion was higher in the HiCHO group. In contrast to their reduced BAT weight, the HiFAT group accumulated more white adipose tissue, consistent with reduced energy expenditure. HiFAT feeding also resulted in major decreases in basal and insulin-stimulated conversion of glucose to lipid in liver (26-60%) and brown adipose tissue (88-90%) with relatively less effect in white adipose (0-43%). We conclude that high-fat feeding results in insulin resistance due mainly to effects in oxidative skeletal muscle and BAT.(ABSTRACT TRUNCATED AT 250 WORDS)

Lipids play varied and critical roles in metabolism, with function dramatically modulated by the individual fatty acid moities in complex lipid entities. In particular, the fatty acid composition of membrane lipids greatly influences membrane function. Here we consider the role of dietary fatty acid profile on membrane composition and, in turn, its impact on prevalent disease clusters of the metabolic syndrome and mental illness. Applying the classical physiological conformer-regulator paradigm to quantify the influence of dietary fats on membrane lipid composition (i.e. where the membrane variable is plotted against the same variable in the environment--in this case dietary fats), membrane lipid composition appears as a predominantly regulated parameter. Membranes remain relatively constant in their saturated (SFA) and monounsaturated (MUFA) fatty acid levels over a wide range of dietary variation for these fatty acids. Membrane composition was found to be more responsive to n-6 and n-3 polyunsaturated fatty acid (PUFA) levels in the diet and most sensitive to n-3 PUFA and to the n-3/n-6 ratio. These differential responses are probably due to the fact that both n-6 and n-3 PUFA classes cannot be synthesised de novo by higher animals. Diet-induced modifications in membrane lipid composition are associated with changes in the rates of membrane-linked cellular processes that are major contributors to energy metabolism. For example, in the intrinsic activity of fundamental processes such as the Na+/K+ pump and proton pump-leak cycle. Equally, dietary lipid profile impacts substantially on diseases of the metabolic syndrome with evidence accruing for changes in metabolic rate and neuropeptide regulation (thus influencing both sides of the energy balance equation), in second messenger generation and in gene expression influencing a range of glucose and lipid handling pathways. Finally, there is a growing literature relating changes in dietary fatty acid profile to many aspects of mental health. The understanding of dietary lipid profile and its influence on membrane function in relation to metabolic dysregulation has exciting potential for the prevention and treatment of a range of prevalent disease states.

Human physiology needs to be well adapted to cope with major discontinuities in both the supply of and demand for energy. This adaptability requires 'a clear capacity to utilize lipid and carbohydrate fuels and to transition between them' (Kelley et al. 2002b). Such capacities characterize the healthy state and can be termed 'metabolic flexibility'. However, increasing evidence points to metabolic inflexibility as a key dysfunction of the cluster of disease states encompassed by the term 'metabolic syndrome'. In obese and diabetic individuals this inflexibility is manifest in a range of metabolic pathways and tissues including: (1) failure of cephalic-phase insulin secretion (impaired early-phase prandial insulin secretion concomitant with failure to suppress hepatic glucose production and NEFA efflux from adipose tissue); (2) failure of skeletal muscle to appropriately move between use of lipid in the fasting state and use of carbohydrate in the insulin-stimulated prandial state; (3) impaired transition from fatty acid efflux to storage in response to a meal. Finally, it is increasingly clear that reduced capacity for fuel usage in, for example, skeletal muscle, as indicated by reduced mitochondrial size and density, is characteristic of the metabolic syndrome state and a fundamental component of metabolic inflexibility. Key questions that remain are how metabolic flexibility is lost in obese and diabetic individuals and by what means it may be regained. Metabolic flexibility: Metabolic syndrome: Lipid and carbohydrate utilizationMan is a meal-feeder with a diet that emphasizes carbohydrates and lipids in a more or less balanced manner. The necessity to handle this discontinuous nutrient supply of both macronutrients focuses attention on the brain, pancreas, liver, skeletal muscle and adipose tissue as organs of importance in proficient handling of incoming nutrients, storing efficiently at times of surplus and providing energy at times of need. The complexity of metabolism and the constraints of the present brief review both require a limited focus on examples of metabolic flexibility that are illustrative rather than exhaustive. Cephalic-phase insulin releaseIt may be Woody Allen's second favourite organ, but a good deal of metabolism is pretty sensible if put in the perspective of the brain's pre-eminence in the drive for a closely-regulated supply of glucose for energy. Efficient storage of incoming meal-associated nutrients, minimizing glucose levels after a meal (area under the curve), is a hallmark of healthy carbohydrate and lipid metabolism. This outcome involves coordinated regulation of the major organs of carbohydrate and lipid flux: pancreas; liver; skeletal muscle; adipose. The desired effect is rapid and appropriate insulin secretion from pancreatic b-cells, effective suppression of lipid mobilization and utilization, and activation of diverse metabolic pathways of tissue energy uptake and storage. An early metabolic event that initiates major elements of this coordinated pattern, and begins to o...

Skeletal muscle contains the majority of the body's glycogen stores and a similar amount of readily accessible energy as intramyocellular triglyceride (imTG). While a number of factors have been considered to contribute to the pathogenesis of insulin resistance (IR) in obesity and type 2 diabetes mellitus (DM), this review will focus on the potential role of skeletal muscle triglyceride content. In obesity and type 2 DM, there is an increased content of lipid within and around muscle fibers. Changes in muscle in fuel partitioning of lipid, between oxidation and storage of fat calories, almost certainly contribute to accumulation of imTG and to the pathogenesis of both obesity and type 2 DM. In metabolic health, skeletal muscle physiology is characterized by the capacity to utilize either lipid or carbohydrate fuels, and to effectively transition between these fuels. We will review recent findings that indicate that in type 2 DM and obesity, skeletal muscle manifests inflexibility in the transition between lipid and carbohydrate fuels. This inflexibility in fuel selection by skeletal muscle appears to be related to the accumulation of imTG and is an important aspect of IR of skeletal muscle in obesity and type 2 DM.

Some, but not all, fats are obesogenic. The aim of the present studies was to investigate the effects of changing type and amount of dietary fats on energy balance, fat deposition, leptin, and leptin-related neural peptides: leptin receptor, neuropeptide Y (NPY), agouti-related peptide (AgRP), and proopiomelanocortin (POMC), in C57Bl/6J mice. One week of feeding with a highly saturated fat diet resulted in ~50 and 20% reduction in hypothalamic arcuate NPY and AgRP mRNA levels, respectively, compared with a low-fat or an n-3 or n-6 polyunsaturated high-fat (PUFA) diet without change in energy intake, fat mass, plasma leptin levels, and leptin receptor or POMC mRNA. Similar neuropeptide results were seen at 7 wk, but by then epididymal fat mass and plasma leptin levels were significantly elevated in the saturated fat group compared with low-fat controls. In contrast, fat and leptin levels were reduced in the n-3 PUFA group compared with all other groups. At 7 wk, changing the saturated fat group to n-3 PUFA for 4 wk completely reversed the hyperleptinemia and increased adiposity and neuropeptide changes induced by saturated fat. Changing to a low-fat diet was much less effective. In summary, a highly saturated fat diet induces obesity without hyperphagia. A regulatory reduction in NPY and AgRP mRNA levels is unable to effectively counteract this obesogenic drive. Equally high fat diets emphasizing PUFAs may even protect against obesity.