The role of altered sympathetic nervous system activity in the pathogenesis of obesity

Associations between stress biology indicators and overweight across toddlerhood

Biological stress responses are proposed as a pathway through which stress exposure can "get under the skin" and lead to health problems, specifically obesity. Yet, it is not clear when such associations may emerge or whether they are bidirectional. Cortisol and salivary alpha amylase (sAA) were considered indicators of the biological stress response. We tested the longitudinal association between cortisol and sAA and weight in 215 low-income children at ages 21, 27, and 33 months (52% male; 46% non-Hispanic white). sAA and cortisol intercept and slope (representing morning level and rate of change across the day) were calculated for each age point using random effect models. Children were weighed and length measured and categorized as overweight versus normal weight (overweight defined as weight-for-length z-score ≥85th percentile for age and sex). Cross-lagged models stratified by sex and controlling for birthweight z-score tested the concurrent and cross-lagged associations between each of 4 indices of stress biology individually (cortisol and sAA intercept and slope) and overweight. Overweight status was correlated across time. Cortisol and sAA were correlated across occasions of measurement, though somewhat less strongly in boys. There were no concurrent associations between stress indicators and overweight. sAA at 27 months predicted greater risk of overweight at 33 months in girls, such that both lower sAA intercept and more rapidly increasing sAA at 27 months predicted greater risk of overweight at 33 months (β=-0.64, p<0.05 and β=1.09, p<0.05, respectively). For boys only, overweight at 21 months predicted lower sAA intercept at 27 months (β=-0.35, p<0.05). Findings suggest that longitudinal associations of stress biology and weight status may be present only on a limited basis very early in the lifespan.

Defective adaptive thermogenesis contributes to metabolic syndrome and liver steatosis in obese mice

Fatty liver diseases are complications of the metabolic syndrome associated with obesity, insulin resistance and low grade inflammation. Our aim was to uncover mechanisms contributing to hepatic complications in this setting. We used foz/foz mice prone to obesity, insulin resistance and progressive fibrosing non-alcoholic steatohepatitis (NASH). Foz/foz mice are hyperphagic but wild-type (WT)-matched calorie intake failed to protect against obesity, adipose inflammation and glucose intolerance. Obese foz/foz mice had similar physical activity level but reduced energy expenditure. Thermogenic adaptation to high-fat diet (HFD) or to cold exposure was severely impaired in foz/foz mice compared with HFD-fed WT littermates due to lower sympathetic tone in their brown adipose tissue (BAT). Intermittent cold exposure (ICE) restored BAT function and thereby improved glucose tolerance, decreased fat mass and liver steatosis. We conclude that failure of BAT adaptation drives the metabolic complications of obesity in foz/foz mice, including development of liver steatosis. Induction of endogenous BAT function had a significant therapeutic impact on obesity, glucose tolerance and liver complications and is a potential new avenue for therapy of non-alcoholic fatty liver disease (NAFLD).

Effects of high-fat diet on salivary α-amylase, serum parameters and food consumption in rats

Salivary α-amylase, a major protein in saliva, has been described as a marker of sympathetic nervous system activity, hence for metabolic energy balance. In this context, its expression in overweight and obesity is of interest. Rats fed with a diet enriched with sunflower oil differentially gained weight yielding two subgroups according to their susceptibility (OP) or resistance (OR) to obesity. Elevated plasmatic levels of leptin in the OP subgroup and altered plasmatic lipid profiles (lower triglycerides and higher total cholesterol/high-density lipoprotein (HDL) ratio compared to controls) in the OR subgroup were observed. Animals from the OP subgroup presented higher α-amylase expression and activity even prior to the dietary treatment, suggesting that this salivary protein may constitute a putative indicator of susceptibility for fat tissue accumulation. After 18 weeks of high-fat diet consumption, salivary α-amylase levels did not significantly change in the OP subgroup, but increased 3-fold in the OR subgroup. The increase in α-amylase levels for the latter might represent an adaptation to lower starch intake. These results suggest that salivary α-amylase secretion might be useful to predict susceptibility for weight gain induced by high-fat diet consumption.

Effects of Chronic Social Stress on Obesity

The prevalence of overweight and obesity has markedly increased during the past few decades. Stress has been suggested as one environmental factor that may contribute to the development of obesity. In this review, we discuss the role that exposure to chronic stress may play in the development of obesity, with particular attention to the effects of chronic psychosocial stress. Of particular importance is the effect that social stress has on dietary preference, food consumption, and regional distribution of adipose tissue. We present evidence from human and animal studies that links sympathetic nervous system and hypothalamic-pituitary-adrenal axis hyperactivity with visceral obesity, and that stress tends to alter the pattern of food consumption, and promotes craving of nutrient-dense "comfort foods." Lastly, we discuss the visible burrow system, a model of chronic social stress used in our laboratory to assess the effects of social subordination on behavioral and metabolic profile.

Central control of thermogenesis

In mammals and birds, conservation of body heat at around 37 °C is vital to life. Thermogenesis is the production of this heat which can be obligatory, as in basal metabolic rate, or it can be facultative such as the response to cold. A complex regulatory system has evolved which senses environmental or core temperature and integrates this information in hypothalamic regions such as the preoptic area and dorsomedial hypothalamus. These areas then send the appropriate signals to generate and conserve heat (or dissipate it). In this review, the importance of the sympathetic nervous system is discussed in relation to its role in basal metabolic rate and adaptive thermogenesis with a particular emphasis to human obesity. The efferent sympathetic pathway does not uniformly act on all tissues; different tissues can receive different levels of sympathetic drive at the same time. This is an important concept in the discussion of the pharmacotherapy of obesity. Despite decades of work the medicine chest contains only one pill for the long term treatment of obesity, orlistat, a lipase inhibitor that prevents the absorption of lipid from the gut and is itself not systemically absorbed. The central controlling system for thermogenesis has many potential intervention points. Several drugs, previously marketed, awaiting approval or in the earlier stages of development may have a thermogenic effect via activation of the sympathetic nervous system at some point in the thermoregulatory circuit and are discussed in this review. If the balance is weighted to the "wrong" side there is the burden of increased cardiovascular risk while a shift to the "right" side, if possible, will afford a thermogenic benefit that is conducive to weight loss maintenance. This article is part of a Special Issue entitled 'Central Control Food Intake'

The effects of capsaicin and capsiate on energy balance: critical review and meta-analyses of studies in humans

Consumption of spicy foods containing capsaicin, the major pungent principle in hot peppers, reportedly promotes negative energy balance. However, many individuals abstain from spicy foods due to the sensory burn and pain elicited by the capsaicin molecule. A potential alternative for nonusers of spicy foods who wish to exploit this energy balance property is consumption of nonpungent peppers rich in capsiate, a recently identified nonpungent capsaicin analog contained in CH-19 Sweet peppers. Capsiate activates transient receptor potential vanilloid subtype 1 (TRPV1) receptors in the gut but not in the oral cavity. This paper critically evaluates current knowledge on the thermogenic and appetitive effects of capsaicin and capsiate from foods and in supplemental form. Meta-analyses were performed on thermogenic outcomes, with a systematic review conducted for both thermogenic and appetitive outcomes. Evidence indicates that capsaicin and capsiate both augment energy expenditure and enhance fat oxidation, especially at high doses. Furthermore, the balance of the literature suggests that capsaicin and capsiate suppress orexigenic sensations. The magnitude of these effects is small. Purposeful inclusion of these compounds in the diet may aid weight management, albeit modestly.

Impaired sympathoadrenal axis function contributes to enhanced insulin secretion in prediabetic obese rats

The involvement of sympathoadrenal axis activity in obesity onset was investigated using the experimental model of treating neonatal rats with monosodium L-glutamate. To access general sympathetic nervous system activity, we recorded the firing rates of sympathetic superior cervical ganglion nerves in animals. Catecholamine content and secretion from isolated adrenal medulla were measured. Intravenous glucose tolerance test was performed, and isolated pancreatic islets were stimulated with glucose and adrenergic agonists. The nerve firing rate of obese rats was decreased compared to the rate for lean rats. Basal catecholamine secretion decreased whereas catecholamine secretion induced by carbachol, elevated extracellular potassium, and caffeine in the isolated adrenal medulla were all increased in obese rats compared to control. Both glucose intolerance and hyperinsulinaemia were observed in obese rats. Adrenaline strongly inhibited glucose-induced insulin secretion in obese animals. These findings suggest that low sympathoadrenal activity contributes to impaired glycaemic control in prediabetic obese rats.

Effect of epinephrine and insulin resistance on human monocytes obtained from lean and obese healthy participants: a pilot study

We assessed the effect of epinephrine on human monocytes. Monocytes were isolated from 16 healthy obese and 10 lean healthy subjects. Insulin sensitivity was assessed by euglycemic hyperinsulinemic clamp. Obese subjects were subdivided into 2 sub-groups, insulin sensitive (IS) and insulin resistant (IR). Monocyte properties [attachment to laminin 1, migration through laminin 1, oxidized-low density lipoprotein (oxLDL) phagocytosis] were assessed pre- and post-stimulation in vitro with epinephrine. Experiments were repeated after incubation with a Na(+)/H( +) exchanger-1 inhibitor (NHE-1) (cariporide). Epinephrine increased monocyte attachment to laminin in lean and obese IR subjects through involvement of NHE-1, PKC, NO synthase, NADPH oxidase and actin polymerization. In contrast, epinephrine did not affect monocyte migration. Epinephrine increased oxLDL phagocytosis in all groups studied. Incubation with cariporide attenuated oxLDL phagocytosis. Epinephrine induces monocyte dysfunction which may be atherogenic.

Examining the pathways linking chronic sleep restriction to obesity

A growing number of studies have identified chronic sleep restriction as a potential risk factor for obesity. This could have important implications for how obesity is prevented and managed, but current understanding of the processes linking chronic sleep restriction to obesity is incomplete. In this paper, we examined some of the pathways that could underlie the relationship between chronic sleep restriction and obesity. This involved exploring some of the potential environmental, health, behavioral, and sociodemographic determinants of chronic sleep restriction, which require further investigation in this context. Three pathways that could potentially link chronic sleep restriction to obesity were then examined: (1) altered neuroendocrine and metabolic function, (2) impaired glucose regulation, and (3) waking behavior. The selected pathways linking chronic sleep restriction to obesity reviewed in this paper are presented in a schematic representation; this may be used to guide future research in this area. This area of research is important because it may lead to more effective interventions and strategies to combat the present obesity epidemic.

Sympathetic nervous system behavior in human obesity

The sympathetic nervous system (SNS) plays an essential role in the regulation of metabolic and cardiovascular homeostasis. Low SNS activity has been suggested to be a risk factor for weight gain and obesity development. In contrast, SNS activation is characteristic of a number of metabolic and cardiovascular diseases that occur more frequently in obese individuals. Until recently, the relation between obesity and SNS behavior has been controversial because previous approaches for assessing SNS activity in humans have produced inconsistent findings. Beginning in the early 1990s, many studies using state of the art neurochemical and neurophysiological techniques have provided important insight. The purpose of the present review is to provide an overview of our current understanding of the region specific alterations in SNS behavior in human obesity. We will discuss findings from our own laboratory which implicate visceral fat as an important depot linking obesity with skeletal muscle SNS activation. The influence of weight change on SNS behavior and the potential mechanisms and consequences of region specific SNS activation in obesity will also be considered.

Body size, body composition, and metabolic profile explain higher energy expenditure in overweight children

Lower relative rates of energy expenditure (EE), increased energetic efficiency, and altered fuel utilization purportedly associated with obesity have not been demonstrated indisputably in overweight children. We hypothesized that differences in energy metabolism between nonoverweight and overweight children are attributable to differences in body size and composition, circulating thyroid hormones, sympathetic nervous system, and adrenomedullary activity. A total of 836 Hispanic children, 5-19 y old, participated in 24-h calorimetry, anthropometric, and dual-energy X-ray absorptiometry measurements. Biochemistries were determined by standard techniques. Absolute total EE (TEE) and its components (sleep EE, basal EE, sedentary EE, cycling EE, walking EE, activity EE, nonexercising activity thermogenesis) were higher in overweight children (P = 0.001). Net mechanical energetic efficiency of cycling was lower in overweight children (P = 0.001). Adjusting for body size and composition accounted for differences in TEE, its components, and energetic efficiency. Net carbohydrate and fat utilization did not differ between groups. TEE was independently influenced by sex, Tanner stage, fat free mass, fat mass (FM), fasting serum nonesterified fatty acids (NEFA), leptin, free thyroxine, triiodothyronine, and 24-h urinary norepinephrine and epinephrine. Fat utilization was independently associated with age2, sex, FM, fasting serum NEFA, triacylglycerol, adiponectin, leptin, total thyroxine, and free triiodothyronine. Higher EE in overweight children was largely explained by differences in body size and composition, with minor contributions of thyroid and sympathoadrenal systems. Alterations in EE, energetic efficiency, and substrate utilization were not evident in the overweight children.

Nonexercise activity thermogenesis--liberating the life-force

Obesity occurs when energy intake exceeds energy expenditure over a protracted period of time. The energy expenditure associated with everyday activity is called NEAT (Nonexercise activity thermogenesis). NEAT varies between two people of similar size by 2000 kcal day(-1) because of people's different occupations and leisure-time activities. Data support the central hypothesis that NEAT is pivotal in the regulation of human energy expenditure and body weight regulation and that NEAT is important for understanding the cause and effective treatment for obesity.

NEAT--non-exercise activity thermogenesis--egocentric & geocentric environmental factors vs. biological regulation

Non-exercise activity thermogenesis (NEAT) is the energy expenditure of all physical activities other than volitional sporting-like exercise. NEAT includes all those activities that render us vibrant, unique and independent beings such as going to work, playing guitar, toe-tapping and dancing. The factors that account for the 2000 kcal day(-1) variability of NEAT can be categorized as environmental or biological. The environmental determinants of NEAT can be view using one of two models. In the egocentric model we consider a single person as the focus, e.g. 'my job'. In the geocentric model we consider the 'environment' as the focus, e.g. well-lit and safe walk ways. These models provide us with a theoretical framework to understand NEAT and how best to intervene to promote NEAT. As well as environmental effectors of NEAT, there are also biological regulatory mechanisms that enable us to account for three-quarters of the biological variance in susceptibility and resistance to fat gain with human over-feeding. NEAT is likely to be regulated through a central mechanism that integrates NEAT with energy intake and energy stores so that NEAT is activated with over-feeding and suppressed with under-feeding. In conclusion, NEAT is likely to serve as a crucial thermoregulatory switch between energy storage and dissipation that is biologically regulated and influenced, and perhaps over-ridden, by environment. Deciphering the role of NEAT may lead to a better understanding of the pathogenesis, prevention and treatment of obesity.

The global epidemic of obesity: are we becoming more sympathetic?

The relationship between obesity and sympathetic nervous system (SNS) behavior has been controversial. Existing paradigms have been opposed in their views of the directional changes in SNS activity in obesity. In addition, limitations of previous approaches to assess SNS activity have produced inconsistent findings. However, the use of state-of-the-art neurochemical and neurophysiologic techniques has improved our current understanding of this issue. There is regional heterogeneity in the SNS activation associated with obesity, with the renal and skeletal muscle circulations, but not the heart, being targets for SNS activation. Abdominal visceral fat appears to be an important depot linking obesity and skeletal muscle SNS activation. The impact of this depot on SNS activity to the kidney or other regions is unknown. Future studies are needed to address this issue as well as the mechanisms and consequences of SNS activation in visceral obesity.

Adrenal medullary function and expression of catecholamine-synthesizing enzymes in mice with hypothalamic obesity

The mechanisms underlying the onset of obesity are complex and not completely understood. An imbalance of autonomic nervous system has been proposed to be a major cause of great fat deposits accumulation in hypothalamic obesity models. In this work we therefore investigated the adrenal chromaffin cells in monosodium glutamate (MSG)-treated obese female mice. Newborn mice were injected daily with MSG (4 mg/g body weight) or saline (controls) during the first five days of life and studied at 90 days of age. The adrenal catecholamine content was 56.0% lower in the obese group when compared to lean controls (P < 0.0001). Using isolated adrenal medulla we observed no difference in basal catecholamine secretion percentile between obese and lean animals. However, the percentile of catecholamine secretion stimulated by high K+ concentration was lower in the obese group. There was a decrease in the tyrosine hydroxylase enzyme expression (57.3%, P < 0.004) in adrenal glands of obese mice. Interestingly, the expression of dopamine beta-hydroxylase was also reduced (47.0%, P < 0.005). Phenylethanolamine N-methyltransferase expression was not affected. Our results show that in the MSG model, obesity status is associated with a defective adrenal chromaffin cell function. We conclude that in MSG obesity the low total catecholamine content is directly related to a decrease of key catecholamine-synthesizing enzymes, which by its turn may lead to a defective catecholamine secretion.

Effect of hyperthyroidism on spontaneous physical activity and energy expenditure in rats

Thyroid hormone excess is associated with increased energy expenditure. The contributions of increases in spontaneous physical activity and nonexercise activity thermogenesis (NEAT) to this effect have not been defined. To address the hypothesis that hyperthyroidism is associated with increased spontaneous physical activity and NEAT, we rendered rats hyperthyroid by using continuous infusion of high-dose triiodothyronine for 14 days and measured the effects on physical activity and NEAT. On day 14, in the hyperthyroid group the mean +/- SD triiodothyronine concentration was 755 +/- 137 (range 574-919) ng/dl and in the control group 59 +/- 0.5 (58-59) ng/dl. Over the 14-day treatment period, mean spontaneous physical activity increased in the hyperthyroid rats from 24 +/- 7 to 36 +/- 6 activity units (AU)/min; P < 0.001 but did not increase in the controls (23 +/- 7 vs. 22 +/- 4 AU/min). Also, over the 14-day period, daily NEAT increased in the hyperthyroid rats from 8.1 +/- 2.8 to 19.7 +/- 5.0 kcal/day (P < 0.001) but did not increase in the controls (8.7 +/- 3.5 cf 9.4 +/- 1.7 kcal/day; not significant). In conclusion, hyperthyroidism is associated with increased spontaneous physical activity and NEAT.

Human uncoupling proteins and obesity

Uncoupling protein (UCP) 2 and UCP3 are newly discovered proteins that can uncouple ATP production from mitochondrial respiration, thereby dissipating energy as heat and affecting energy metabolism efficiency. In contrast to UCP1, which is only present in brown adipose tissue, UCP2 has a wide tissue distribution, whereas UCP3 is expressed predominantly in skeletal muscle. Some evidence of a role for UCPs in modulating metabolic rate was provided by linkage and association studies. Furthermore, UCP3 gene expression was found to correlate negatively with body mass index and positively with sleeping metabolic rate in Pima Indians. Treatment with thyroid hormone increases expression of the UCP2 and UCP3 genes. Other regulators of UCP2 and UCP3 gene expression are beta3-adrenergic agonists and glucocorticoids. Surprisingly, fasting has a stimulatory effect on UCP2 and UCP3 mRNA levels, possibly explained by the effects of free fatty acid on UCP2 and UCP3 gene expression.

Organ-tissue mass measurement allows modeling of REE and metabolically active tissue mass

Investigators have expressed interest in the associations between resting energy expenditure (REE) and body mass for over a century. Traditionally, descriptive models using regression analysis are applied, linking REE with metabolically active compartments such as body cell mass (BCM) and fat-free body mass (FFM). Recently developed whole body magnetic resonance imaging (MRI) and echocardiography methods now allow estimation of all major organs and tissue volumes in vivo. Because measured values are available for REE, BCM, and FFM content of individual organs and tissues, it should now be possible to develop energy expenditure-body composition estimation models based on MRI-measured organ-tissue volumes. Specifically, the present investigation tested the hypothesis that in vivo estimation of whole body REE, BCM, and FFM is possible using MRI- and echocardiography-derived organ volumes combined with previously reported organ-tissue metabolic rates and chemical composition. Thirteen subjects (5 females, 8 males) had REE, BCM, and FFM measured by indirect calorimetry, whole body 40K counting, and dual-energy X-ray absorptiometry, respectively. Models developed from estimated and measured variables were highly correlated, with no significant differences between those estimated and measured [e.g., calculated vs. measured REE: r = 0.92, P < 0. 001; (mean +/- SD) 6,962 +/- 1,455 and 7,045 +/- 1,450 kJ/day, respectively (P = not significant)]. Strong associations were observed between REE, individual or combined organ weights, BCM, and FFM that provide new insights into earlier observed metabolic phenomona. The present approach, the first to establish an energy expenditure-body composition link with a mechanistic model in vivo, has the potential to greatly expand our knowledge of energy expenditure-body size relationships in humans.

Exercise prescription for weight management

The debate surrounding the level of intensity of exercise that is best for health improvement has potentially clouded the issue of optimal exercise prescription for weight management. Low-intensity activity is potentially superior to moderate to high intensity for improving metabolic risk factors, and accumulated small bouts of physical activity are as effective to this end as single longer bouts, as long as the overall volume of energy expenditure is equivalent. What should not be forgotten however, is that for weight-loss it is the total volume of energy expended that will dictate the size of the energy deficit imposed, not the composition of the exercise per se. Exercise prescription for weight management is a conundrum. Whilst it is the total volume of energy expended that will dictate the magnitude of weight lost, not the composition of the exercise per se, it is the nature of the exercise prescription that will dictate the long-term success of an exercise programme. It is how well the exercise prescription is individualized that influences tolerance of and interest in the programme and, thus, the adherence to it in the long term.