The metabolic syndrome as a vicious cycle: does obesity beget obesity?

ENDOCRINE OBESITY: Pituitary dysfunction in obesity

Obesity, the growing pandemic of the 21st century, is associated with multiple organ dysfunction, either by a direct increase in fatty organ content or by indirect modifications related to general metabolic changes driven by a specific increase in biologic products. The pituitary gland is not protected against such a situation. Different hypothalamic-pituitary axes experience functional modifications initially oriented to an adaptive situation that, with years of obesity, turn to maladaptive dynamics that contribute to perpetuating obesity and specific symptoms of their hormonal nature. This paper reviews the recent knowledge on obesity-related pituitary dysfunction and its pathogenic mechanisms and discusses potential therapeutic actions aimed at contributing to ameliorating the complex treatment of severe cases of obesity.

Interaction between CETP Taq1B polymorphism and HEI, DQI and DPI on metabolic biomarkers in patients with type 2 diabetes

BACKGROUND

Type 2 diabetes mellitus (T2DM) is a multidimensional consequence of environmental and genetic factors. Cholesteryl ester transfer protein (CETP) Taq1B polymorphism has been reported as a main predictor of dyslipidaemia, comprising an important complication in persons with T2DM. However, diet could affect T2DM patients metabolic health.

METHODS

We investigated the combination of gene-diet effects on some metabolic biomarkers. In our cross-sectional study, blood samples of 220 patients were collected. Dietary indices (healthy eating index, dietary quality index and dietary phytochemical index) were obtained from a validated semi-quantitative food frequency questionnaire. CETP Taq1B polymorphism was genotyped by a polymerase chain reaction-restriction fragment polymorphism method. Data were analysed by analysis of covariance.

RESULTS

The interaction between the CETP Taq1B polymorphism and dietary indices on low density lipoprotein/high density lipoprotein was significant (p < 0.001 both crude and adjusted models). In addition, the interaction between polymorphism and dietary quality index on total antioxidant capacity (p = 0.004 crude model, p = 0.005 after adjusting) and pentraxin 3 (p = 0.01 both crude and adjusted models) was significant. Also, the interaction between polymorphism and healthy eating index on waist circumference (p = 0.005 both crude and adjusted models) and dietary phytochemical index on interleukin-18 (p = 0.03 crude model) was significant.

CONCLUSIONS

Our results indicated the effect of CETP Taq1B polymorphism on some inflammatory and anthropometrics markers (total antioxidant capacity, pentraxin 3, interleukin-18, low density lipoprotein/high density lipoprotein and waist circumference) with high and low adherence to dietary incides.

Early overnutrition sensitizes the growth hormone axis to the impact of diet-induced obesity via sex-divergent mechanisms

In addition to its essential role in the physiological control of longitudinal growth, growth-hormone (GH) is endowed with relevant metabolic functions, including anabolic actions in muscle, lipolysis in adipose-tissue and glycemic modulation. Adult obesity is known to negatively impact GH-axis, thereby promoting a vicious circle that may contribute to the exacerbation of the metabolic complications of overweight. Yet, to what extent early-overnutrition sensitizes the somatotropic-axis to the deleterious effects of obesity remains largely unexplored. Using a rat-model of sequential exposure to obesogenic insults, namely postnatal-overfeeding during lactation and high-fat diet (HFD) after weaning, we evaluated in both sexes the individual and combined impact of these nutritional challenges upon key elements of the somatotropic-axis. While feeding HFD per se had a modest impact on the adult GH-axis, early overnutrition had durable effects on key elements of the somatotropic-system, which were sexually different, with a significant inhibition of pituitary gene expression of GH-releasing hormone-receptor (GHRH-R) and somatostatin receptor-5 (SST5) in males, but an increase in pituitary GHRH-R, SST2, SST5, GH secretagogue-receptor (GHS-R) and ghrelin expression in females. Notably, early-overnutrition sensitized the GH-axis to the deleterious impact of HFD, with a significant suppression of pituitary GH expression in both sexes and lowering of circulating GH levels in females. Yet, despite their similar metabolic perturbations, males and females displayed rather distinct alterations of key somatotropic-regulators/ mediators. Our data document a synergistic effect of postnatal-overnutrition on the detrimental impact of HFD-induced obesity on key elements of the adult GH-axis, which is conducted via mechanisms that are sexually-divergent.

Neuroendocrine Regulation of Growth Hormone Secretion

This article reviews the main findings that emerged in the intervening years since the previous volume on hormonal control of growth in the section on the endocrine system of the Handbook of Physiology concerning the intra- and extrahypothalamic neuronal networks connecting growth hormone releasing hormone (GHRH) and somatostatin hypophysiotropic neurons and the integration between regulators of food intake/metabolism and GH release. Among these findings, the discovery of ghrelin still raises many unanswered questions. One important event was the application of deconvolution analysis to the pulsatile patterns of GH secretion in different mammalian species, including Man, according to gender, hormonal environment and ageing. Concerning this last phenomenon, a great body of evidence now supports the role of an attenuation of the GHRH/GH/Insulin-like growth factor-1 (IGF-1) axis in the control of mammalian aging.

Adiposity, muscle, and physical activity: predictors of perturbations in heart rate variability

OBJECTIVES

This study examines cross-sectional associations of indices of adiposity, lean body mass, and physical activity, with heart rate variability (HRV), a marker for parasympathetic cardiac vagal control.

METHODS

The study population consists of 360 officers from the Buffalo New York Police Department. Indices of adiposity include body mass index, waist circumference, and a fat-mass index taken from dual-energy X-ray absorptiometry (DEXA) measurements. Lean body mass indices were derived from DEXA measurements of trunk mass and extremity lean mass. Physical activity was measured using a 7-day self-report questionnaire. HRV was obtained from 5-min electrocardiogram measurements by means of parametric spectral analysis resulting in estimates for high-frequency (HF) and low-frequency (LF) HRV.

RESULTS

Both HF and LF HRV were significantly associated with markers for adiposity, two components of lean mass and physical activity with all associations being in the expected direction except that for trunk lean mass. This unexpected result is explained by the possibility that trunk mass is a marker for visceral adiposity rather than lean mass. Body mass index did not explain any additional variance in HRV above and beyond waist circumference and the DEXA indices.

CONCLUSIONS

Higher levels of physical activity, lower levels of markers for central adiposity and higher lean mass in the extremities predict higher levels of HRV in this population of police officers. This association between modifiable risk factors and markers for autonomic function suggest possible interventions that may improve health and performance.

Increased adiposity and insulin correlates with the progressive suppression of pulsatile GH secretion during weight gain

Pathological changes associated with obesity are thought to contribute to GH deficiency. However, recent observations suggest that impaired GH secretion relative to excess calorie consumption contributes to progressive weight gain and thus may contribute to the development of obesity. To clarify this association between adiposity and GH secretion, we investigated the relationship between pulsatile GH secretion and body weight; epididymal fat mass; and circulating levels of leptin, insulin, non-esterified free fatty acids (NEFAs), and glucose. Data were obtained from male mice maintained on a standard or high-fat diet. We confirm the suppression of pulsatile GH secretion following dietary-induced weight gain. Correlation analyses reveal an inverse relationship between measures of pulsatile GH secretion, body weight, and epididymal fat mass. Moreover, we demonstrate an inverse relationship between measures of pulsatile GH secretion and circulating levels of leptin and insulin. The secretion of GH did not change relative to circulating levels of NEFAs or glucose. We conclude that impaired pulsatile GH secretion in the mouse occurs alongside progressive weight gain and thus precedes the development of obesity. Moreover, data illustrate key interactions between GH secretion and circulating levels of insulin and reflect the potential physiological role of GH in modulation of insulin-induced lipogenesis throughout positive energy balance.

Functional variability in corticosteroid receptors is a major component of strain differences in fat deposition and metabolic consequences of enriched diets in rat

We aimed to distinguish mineralocorticoid (MR) from glucocorticoid receptor (GR) actions in the nutritional differences between the Fischer 344 (F344) and LOU/C (LOU) rat strains. The decrease of urinary Na+/K+ ratio induced via MR activation by aldosterone and decrease of circulating lymphocyte counts exerted via GR activation by dexamethasone revealed a higher efficiency of corticosteroid receptor in LOU than in F344 rats. Afterward, we submitted F344 and LOU male rats to adrenalectomy and to substitution treatments with agonists of MR or GR under 3 successive diets--standard, free choice between chow and pork lard, and an imposed high-fat/high-sugar diet--to explore the involvement of the interactions between activation of corticosteroid receptors and diet on food intake, body composition, and metabolic blood parameters in these rats. Lastly, we measured energy expenditure and substrate oxidization in various experimental conditions in LOU and F344 rats by indirect calorimetry. In LOU rats, we showed greater basal and MR-induced energy expenditure, diet-induced thermogenesis, and lipid oxidization. We showed that the F344 rat strain constitutes a relevant model of the unfavorable effects exerted by glucocorticoids via GR on food preference for high-calorie diets, abdominal fat deposition, diabetes, and other deleterious consequences of visceral obesity. Contrary to F344 rats, the LOU rats did not exhibit the expected visceral fat deposition linked to GR activation. This strain is therefore a relevant model of resistance to diet-induced obesity and to the deleterious effects exerted by glucocorticoids on metabolism.

Prevalence of the metabolic syndrome in moderately-severely obese subjects with and without growth hormone deficiency

BACKGROUND AND AIM

There is a considerable heterogeneity in metabolic phenotype among equally obese subjects. Impaired GH secretion is frequent in obese patients, with GH secretion reduced up to levels that are comparable to those found in adult patients with organic GH deficiency (GHD). Low GH status exerts detrimental effects onmetabolic abnormalities in organic GHD patients. The aim of this observational, retrospective study was to investigate the prevalence of the metabolic syndrome (MetS) in moderately-severely obese subjects who met criteria for GDH (GHD) and in those with normal GH status (GH sufficient: GHS).

METHODS AND RESULTS

One-hundred and ninety-five moderately-severely obese individuals partecipated, 149 women and 46 males [bodymass index (BMI) 43.0+/-4.4 kg/m2 aged 34.3+/-11.8 yr] . Main outcome measures were: GH peak after GHRH plus arginine test, IGF-I, MetS parameters according to National Cholesterol Education Program criteria. Fifty-five subjects (27.3%) were GHD (49 females and 6 males). The prevalence of MetS parameters was 70.9% in GHD subgroup vs 52.9% in GHS (chi2=5.281; p=0.02) and the likelihood of MetS was highest in GHD subgroup (odds ratio: 2.174; 95% confidence interval 1.113 to 4.248). At the multiple regression analysis either GH peak or IGF-I were the major determinants of waist circumference (beta=-0.380, t=-6.110 and beta=-0.326, t=-4.704, respectively; p<0.001), while age and IGFI were the major determinants of MetS (beta=0.255, t= 3.342, and beta=-0.282, t=-3.270; p=0.02, respectively).

CONCLUSIONS

Among moderately-severely obese individuals the prevalence of the MetS was higher in GHD than in GHS subjects. Thus, in obese subjects, GH status investigation might be considered in the clinical evaluation of their metabolic risk profile.

An obesity dietary quality index predicts abdominal obesity in women: potential opportunity for new prevention and treatment paradigms

Background. Links between dietary quality and abdominal obesity are poorly understood. Objective. To examine the association between an obesity-specific dietary quality index and abdominal obesity risk in women. Methods. Over 12 years, we followed 288 Framingham Offspring/Spouse Study women, aged 30-69 years, without metabolic syndrome risk factors, cardiovascular disease, cancer, or diabetes at baseline. An 11-nutrient obesity-specific dietary quality index was derived using mean ranks of nutrient intakes from 3-day dietary records. Abdominal obesity (waist circumference >88 cm) was assessed during follow-up. Results. Using multiple logistic regression, women with poorer dietary quality were more likely to develop abdominal obesity compared to those with higher dietary quality (OR 1.87; 95% CI, 1.01, 3.47; P for trend = .048) independent of age, physical activity, smoking, and menopausal status. Conclusions. An obesity-specific dietary quality index predicted abdominal obesity in women, suggesting targets for dietary quality assessment, intervention, and treatment to address abdominal adiposity.

Muscle protein synthesis and balance responsiveness to essential amino acids ingestion in the presence of elevated plasma free fatty acid concentrations

CONTEXT

Elevated plasma free fatty acid (FFA) concentrations are observed under various clinical circumstances and are associated with impaired glucose disposal in skeletal muscle.

OBJECTIVE

The aim of the study was to determine the effects of elevated plasma FFA concentrations on the response of protein synthesis and balance in muscle after essential amino acids (EAAs) ingestion.

DESIGN

Leg protein kinetics were determined in young healthy individuals before and after the ingestion of EAAs at 10 h after the initiation of either lipid (Liposyn/heparin+EAA) or saline (saline+EAA) infusions.

RESULTS

Plasma insulin responses where higher (P <0.05) in the Liposyn/heparin+EAA group than the saline+EAA group both before (14 +/- 4 vs. 6 +/- 1 microIU . ml(-1)) and after (1038 +/- 257 vs. 280 +/- 87 microIU . ml(-1) . 210 min(-1)) the EAA ingestion. After the EAA ingestion, the rates of both leg phenylalanine disappearance (Rd; nmol . min(-1) . kg lean leg mass(-1)) and muscle proteins fractional synthesis (FSR; % . h(-1)) increased (P <0.05) in both the Liposyn/heparin+EAA and saline+EAA groups, but these changes were not different between the two groups (Rd, 102 +/- 32 vs. 118 +/- 34; FSR, 0.014 +/- 0.005 vs. 0.018 +/- 0.007; P > 0.05). Although the leg phenylalanine rate of appearance (Ra; nmol . min(-1) . kg lean leg mass(-1)) was lower (381 +/- 47 vs. 518 +/- 40) and the balance was greater (-109 +/- 20 vs. -172 +/- 17) in the Liposyn/heparin+EAA group compared to the saline+EAA group before the EAA ingestion (P <0.05), the changes in both of these parameters were not different between groups after the EAA ingestion (P > 0.05).

CONCLUSIONS

Elevated plasma FFA concentrations do not interfere with the response of muscle protein synthesis and balance to a bolus ingestion of EAAs.

Impact of obesity on the growth hormone axis: evidence for a direct inhibitory effect of hyperinsulinemia on pituitary function

There is a negative relationship between obesity and GH. However, it is not known how metabolic changes, associated with obesity, lead to a reduction in GH output. This study examined the GH axis of two mouse models of obesity, the leptin-deficient (ob/ob) mouse and the diet-induced obese (DIO; high-fat fed) mouse. Both models displayed hyperglycemia and hyperinsulinemia with reduced expression of GH as well as reduced expression of pituitary receptors important for GH synthesis and release [GHRH receptor (DIO only) and the ghrelin receptor (ob/ob and DIO)]. These pituitary changes were not accompanied by changes in hypothalamic expression of GHRH or somatostatin; suggesting that alterations in pituitary function may be precipitated in part by direct effects of systemic signals. Of the metabolic and hormonal parameters examined (insulin, glucose, corticosterone, free fatty acids, ghrelin, and IGF-I), only insulin/glucose showed a significant, and negative, correlation with pituitary expression. Pituitaries of DIO mice remained responsive to the acute in vivo actions of insulin, as assessed by phosphorylation of Akt, despite systemic (skeletal muscle and fat) insulin resistance. In addition, treating primary pituitary cell cultures from lean mice with insulin reduced GH release as well as GH, GHRH receptor, and ghrelin receptor mRNA levels compared with vehicle-treated controls, where the magnitude of suppression of pituitary mRNA levels was similar to that observed in the DIO mouse. These results coupled with the fact that the pituitary expresses the insulin receptor at levels comparable to tissues classically considered insulin sensitive, indicates high circulating insulin levels can directly contribute to the suppression of GH synthesis and release in the obese state.

The evolution of human fatness and susceptibility to obesity: an ethological approach

Human susceptibility to obesity is an unusual phenomenon amongst animals. An evolutionary analysis, identifying factors favouring the capacity for fat deposition, may aid in the development of preventive public health strategies. This article considers the proximate causes, ontogeny, fitness value and evolutionary history of human fat deposition. Proximate causes include diet composition, physical activity level, feeding behaviour, endocrine and genetic factors, psychological traits, and exposure to broader environmental factors. Fat deposition peaks during late gestation and early infancy, and again during adolescence in females. As in other species, human fat stores not only buffer malnutrition, but also regulate reproduction and immune function, and are subject to sexual selection. Nevertheless, our characteristic ontogenetic pattern of fat deposition, along with relatively high fatness in adulthood, contrasts with the phenotype of other mammals occupying the tropical savannah environment in which hominids evolved. The increased value of energy stores in our species can be attributed to factors increasing either uncertainty in energy availability, or vulnerability to that uncertainty. Early hominid evolution was characterised by adaptation to a more seasonal environment, when selection would have favoured general thriftiness. The evolution of the large expensive brain in the genus Homo then favoured increased energy stores in the reproducing female, and in the offspring in early life. More recently, the introduction of agriculture has had three significant effects: exposure to regular famine; adaptation to a variety of local niches favouring population-specific adaptations; and the development of social hierarchies which predispose to differential exposure to environmental pressures. Thus, humans have persistently encountered greater energy stress than that experienced by their closest living relatives during recent evolution. The capacity to accumulate fat has therefore been a major adaptive feature of our species, but is now increasingly maladaptive in the modern environment where fluctuations in energy supply have been minimised, and productivity is dependent on mechanisation rather than physical effort. Alterations to the obesogenic environment are predicted to play a key role in reducing the prevalence of obesity.