Inverse association between sympathetic nervous system activity and bone mass in middle aged overweight individuals

BACKGROUND/PURPOSE

Sympathetic nervous system activation in obesity is associated with impaired cardiovascular and metabolic function. Animal studies have shown a direct link between sympathetic nervous activation and bone health but little is known about this link in humans. This study examined whether sympathetic activation may impact bone health in overweight adults.

METHODS

This cross sectional study included 96 overweight or obese middle-aged adults (51 males, mean body mass index: 32.8 kg/m², mean age: 55.3 years). Multivariate linear regression models evaluated associations between whole body and leg bone mineral density (BMD) and bone mineral content (BMC) derived from dual-energy X-ray absorptiometry and muscle sympathetic nervous system activity (MSNA) measured by microneurography.

RESULTS

Older age, male sex and higher weight were associated with higher leg and body BMC and BMD. After adjustment for age, sex and weight, MSNA was significantly inversely associated with total BMC (p = 0.012) and with leg BMC (p < 0.01) but was not associated with either total or leg BMD (p = 0.159 and p = 0.063 respectively). When the analysis was sex specific, the relationships between MSNA and total and leg BMC were only significant in males.

CONCLUSIONS

Our study indicates that in middle aged overweight or obese males, sympathetic activation may have a deleterious effect on bone mineral content.

Endothelial Function in Healthy Young Individuals Is Associated with Dietary Consumption of Saturated Fat

Background: A diet rich in fat, in particular saturated fat (SF), may be linked to cardiovascular disease development, possibly due to a detrimental effect of fat on endothelial function (EF).

Objective: We aimed to determine whether the habitual SF intake [as a ratio to total fat (the sum of saturated, polyunsaturated, and monounsaturated fat)] might influence endothelial function in young, overweight but otherwise healthy adults.

Design: Sixty-nine young adults (49 males, mean age: 23 ± 1 years, mean BMI: 29.1 ± 0.8 kg/m²) were classified into three tertiles according to their habitual SF intake consumption (low SF: <39%, medium SF 39.1–43.7%, and high SF: >43.7% of total fat). Endothelial function was assessed using digital amplitude tonometry.

Results: The three groups of individuals were comparable for total energy intake and calories from: fat, protein, and carbohydrates. There was no difference in anthropometric and hemodynamic variables among the groups. Those in the high SF group presented with impaired endothelial function [reactive hyperemia index (RHI): high SF: 1.60 ± 0.08 compared to 2.23 ± 0.16 in the medium SF and 2.12 ± 0.14 in the low SF group, P < 0.01]. Regression analysis, including gender, age, ethnicity, body mass index indicated that the ratio of SF to total fat was an independent predictor of the RHI (P < 0.05).

Conclusion: The habitual consumption of a diet high in SF in relation to polyunsaturated and monounsaturated fat was strongly associated with impaired endothelial function in young overweight adults, potentially contributing to increased risk of developing cardiovascular disease.

Diet low in advanced glycation end products increases insulin sensitivity in healthy overweight individuals: a double-blind, randomized, crossover trial

BACKGROUND

The consumption of advanced glycation end products (AGEs) has increased because of modern food processing and has been linked to the development of type 2 diabetes in rodents.

OBJECTIVE

We determined whether changing dietary AGE intake could modulate insulin sensitivity and secretion in healthy, overweight individuals.

DESIGN

We performed a double-blind, randomized, crossover trial of diets in 20 participants [6 women and 14 men; mean ± SD body mass index (in kg/m(2)): 29.8 ± 3.7]. Isoenergetic- and macronutrient-matched diets that were high or low in AGE content were alternately consumed for 2 wk and separated by a 4-wk washout period. At the beginning and end of each dietary period, a hyperinsulinemic-euglycemic clamp and an intravenous glucose tolerance test were performed. Dietary, plasma and urinary AGEs N(€)-(carboxymethyl)lysine (CML), N(€)-(carboxyethyl)lysin (CEL), and methylglyoxal-derived hydroimadazolidine (MG-H1) were measured with the use of mass spectrometry.

RESULTS

Participants consumed less CML, CEL, and MG-H1 during the low-AGE dietary period than during the high-AGE period (all P < 0.05), which was confirmed by changes in urinary AGE excretion. There was an overall difference in insulin sensitivity of -2.1 mg · kg(-1) · min(-1) between diets (P = 0.001). Insulin sensitivity increased by 1.3 mg · kg(-1) · min(-1) after the low-AGE diet (P = 0.004), whereas it showed a tendency to decrease by 0.8 mg · kg(-1) · min(-1) after the high-AGE diet (P = 0.086). There was no difference in body weight or insulin secretion between diets (P = NS).

CONCLUSIONS

A diet that is low in AGEs may reduce the risk of type 2 diabetes by increasing insulin sensitivity. Hence, a restriction in dietary AGE content may be an effective strategy to decrease diabetes and cardiovascular disease risks in overweight individuals. This trial was registered at clinicaltrials.gov as NCT00422253.

Metabolic syndrome: a sympathetic disease?

Metabolic syndrome is associated with adverse health outcomes and is a growing problem worldwide. Although efforts to harmonise the definition of metabolic syndrome have helped to better understand the prevalence and the adverse outcomes associated with the disorder on a global scale, the mechanisms underpinning the metabolic changes that define it are incompletely understood. Accumulating evidence from laboratory and human studies suggests that activation of the sympathetic nervous system has an important role in metabolic syndrome. Indeed, treatment strategies commonly recommended for patients with metabolic syndrome, such as diet and exercise to induce weight loss, are associated with sympathetic inhibition. Pharmacological and device-based approaches to target activation of the sympathetic nervous system directly are available and have provided evidence to support the important part played by sympathetic regulation, particularly for blood pressure and glucose control. Preliminary evidence is encouraging, but whether therapeutically targeting sympathetic overactivity could help to prevent metabolic syndrome and attenuate its adverse outcomes remains to be determined.

Specific plasma lipid classes and phospholipid fatty acids indicative of dairy food consumption associate with insulin sensitivity

BACKGROUND

Reports have suggested that the consumption of dairy foods may reduce risk of type 2 diabetes on the basis of evidence of raised circulating ruminant fatty acids.

OBJECTIVE

We determined whether certain phospholipid species and fatty acids that are associated with full-fat dairy consumption may also be linked to diminished insulin resistance.

DESIGN

Four variables of insulin resistance and sensitivity were defined from oral-glucose-tolerance tests in 86 overweight and obese subjects with metabolic syndrome. Plasma phospholipids, sphingolipids, and fatty acids were determined by using a lipidomic analysis and gas chromatography-mass spectrometry to provide objective markers of dairy consumption. Food records provided information on dairy products. Associations were determined by using linear regression analyses adjusted for potential confounders age, sex, systolic blood pressure, waist:hip ratio, or body mass index (BMI) and corrected for multiple comparisons.

RESULTS

Lysophosphatidylcholine, lyso-platelet-activating factor, and several phospholipid fatty acids correlated directly with the number of servings of full-fat dairy foods. Lysophosphatidylcholine and lyso-platelet-activating factor were also associated directly with insulin sensitivity when accounting for the waist:hip ratio (Matsuda index unadjusted, P < 0.001 for both; adjusted for multiple comparisons, P < 0.02 for both) and inversely with insulin resistance (fasting insulin unadjusted, P < 0.001 for both; adjusted, P = 0.04 and P < 0.05, respectively; homeostasis model assessment of insulin resistance adjusted, P = 0.04 for both; post-glucose insulin area under the plasma insulin curve during the 120 min of the test adjusted, P < 0.01 for both). The substitution of BMI for the waist:hip ratio attenuated associations modestly. Phospholipid fatty acid 17:0 also tended to be associated directly with insulin sensitivity and inversely with resistance.

CONCLUSION

Variables of insulin resistance were lower at higher concentrations of specific plasma phospholipids that were also indicators of full-fat dairy consumption. This trial was registered at clinicaltrials.gov as NCT00163943.

Feasibility of catheter-based renal nerve ablation and effects on sympathetic nerve activity and blood pressure in patients with end-stage renal disease

BACKGROUND AND OBJECTIVES

Sympathetic activation is a hallmark of ESRD and adversely affects cardiovascular prognosis. Efferent sympathetic outflow and afferent neural signalling from the failing native kidneys are key mediators and can be targeted by renal denervation (RDN). Whether this is feasible and effective in ESRD is not known.

DESIGN, SETTING, PARTICIPANTS AND MEASUREMENTS

In an initial safety and proof-of-concept study we attempted to perform RDN in 12 patients with ESRD and uncontrolled blood pressure (BP). Standardized BP measurements were obtained in all patients on dialysis free days at baseline and follow up. Measures of renal noradrenaline spillover and muscle sympathetic nerve activity were available from 5 patients at baseline and from 2 patients at 12 month follow up and beyond.

RESULTS

Average office BP was 170.8 ± 16.9/89.2 ± 12.1 mmHg despite the use of 3.8 ± 1.4 antihypertensive drugs. All 5 patients in whom muscle sympathetic nerve activity and noradrenaline spillover was assessed at baseline displayed substantially elevated levels. Three out of 12 patients could not undergo RDN due to atrophic renal arteries. Compared to baseline, office systolic BP was significantly reduced at 3, 6, and 12 months after RDN (from 166 ± 16.0 to 148 ± 11, 150 ± 14, and 138 ± 17 mmHg, respectively), whereas no change was evident in the 3 non-treated patients. Sympathetic nerve activity was substantially reduced in 2 patients who underwent repeat assessment.

CONCLUSIONS

RDN is feasible in patients with ESRD and associated with a sustained reduction in systolic office BP. Atrophic renal arteries may pose a problem for application of this technology in some patients with ESRD.

Dyslipidemia is associated with sympathetic nervous activation and impaired endothelial function in young females

BACKGROUND

Dyslipidemia is one the most well-established risk factors for cardiovascular disease development. Moreover, hypercholesterolemia and plasma cholesterol level in the high to normal range are established triggers for impairment in endothelial function. Evidence indicates that endothelial function is closely linked with sympathetic nervous activity in healthy individuals. We therefore investigated whether both endothelial and sympathetic functions may be impaired in young females with abnormal plasma cholesterol levels.

METHODS

Baseline endothelial function (digital pulse amplitude) and muscle sympathetic nervous activity (microneurography) were retrospectively analyzed in 14 young healthy females with dyslipidemia as indicated by total cholesterol ≥197mg/dL, high-density lipoprotein ≤39mg/dL, or low-density lipoprotein >116mg/dL, and in 13 females with lipids in the healthy range.

RESULTS

Subjects with dyslipidemia had significantly impaired endothelial function compared to those with a normal cholesterol profile (reactive hyperemia index; 1.61±0.10 vs. 2.32±0.14, P < 0.001), increased muscle sympathetic nervous activity (after adjusting for body mass and age, 36±3 vs. 27±3 bursts per 100 heartbeats, P = 0.049) and elevated high-sensitivity C-reactive protein (4.13±0.77 vs. 1.92±0.61mg/L, P = 0.03).

DISCUSSION

Our results indicate that young healthy females with dyslipidemia present with a strong impairment of endothelial function and increased sympathetic drive. The sympathetic activation observed in the subjects with an elevated cholesterol profile may play a role in the development of cardiovascular disease development.

Cholesterol efflux from macrophages is influenced differentially by plasmas from overweight insulin-sensitive and -resistant subjects

OBJECTIVE

In vitro measurements of cholesterol efflux from macrophages have recently been shown to associate with cardiovascular risk. We investigated whether cholesterol efflux from macrophages incubated with plasmas from overweight/obese subjects with metabolic syndrome was influenced by the presence of insulin resistance.

METHODS

Plasmas were obtained from 47 men and women with metabolic syndrome, of whom 25 were found to be insulin resistant (IR) and 22 insulin sensitive (IS) (Matsuda, De Fronzo equation based on oral glucose tolerance test). Activated human macrophage THP-1 cells in which cholesterol had been radiolabelled were incubated with the subjects' plasmas to allow calculation of % cholesterol efflux.

RESULTS

Body mass index and waist measurements, as well as plasma lipid levels, did not differ between the two groups. Homeostatic model assessment-insulin resistance value as well as plasma insulin and leptin concentrations were higher in IR subjects. Cholesterol efflux was found to be significantly greater with plasmas from IR subjects (9.1%) than from IS subjects (6.7%) (P=0.005). Further, cholesterol efflux was significantly inversely associated with insulin sensitivity index (P<0.001), directly with arterial insulin concentration (P<0.001) and directly with cholesteryl ester transfer protein (CETP) mass (P=0.044).

CONCLUSION

Plasmas from overweight subjects with insulin resistance induced greater in vitro cholesterol efflux compared with IS subjects. Efflux inversely correlated with insulin sensitivity suggesting an increase in reverse cholesterol transport in the IR state that may lead to greater transfer of cholesterol to apoB lipoproteins from high-density lipoproteins via CETP as a factor in the association between IR and atherosclerosis.

Ghrelin modulates sympathetic nervous system activity and stress response in lean and overweight men

Ghrelin is a growth hormone-releasing peptide secreted by the stomach with potent effects on appetite. Experimental and clinical studies indicate that ghrelin also influences cardiovascular regulation and metabolic function and mediates behavioral responses to stress. We investigated the effects of ghrelin on blood pressure (BP), sympathetic nervous system activity, and mental stress responses in lean (n=13) and overweight or obese (n=13) individuals. Subjects received an intravenous infusion of human ghrelin (5 pmol/kg per minute for 1 hour) and saline in a randomized fashion. Ghrelin decreased systolic (-6 and -11 mm Hg) and diastolic BP (-8 mm Hg for both), increased muscle sympathetic nervous system activity (18±2 to 28±3 bursts per min, P<0.05 and from 21±2 to 32±3 bursts per min, P<0.001) in lean and overweight or obese subjects, respectively, without a significant change in heart rate, calf blood flow, or vascular resistance. Ghrelin induced a rise in plasma glucose concentration in lean individuals (P<0.05) and increased cortisol levels in both groups (P<0.05). Stress induced a significant change in mean BP (+22 and +27 mm Hg), heart rate (+36 and +29 bpm), and muscle sympathetic nervous system activity (+6.1±1.6 and +6.8±2.7 bursts per min) during saline infusion in lean and overweight or obese subjects, respectively. During ghrelin infusion, the changes in BP and muscle sympathetic nerve activity in response to stress were significantly reduced in both groups (P<0.05). In conclusion, ghrelin exerts unique effects in that it reduces BP and increases muscle sympathetic nervous system activity and blunts cardiovascular responses to mental stress. These responses may represent a combination of peripheral (baroreflex-mediated) and central effects of ghrelin.

Sympathetic nervous system activity is associated with obesity-induced subclinical organ damage in young adults

Excess weight is established as a major risk factor for cardiovascular diseases, particularly in young individuals. To get a better understanding of the pathophysiology underlying increased cardiovascular disease risk, we evaluated early signs of organ damage and their possible relationship to sympathetic nervous activity. Eighteen lean (body mass index <25 kg/m(2)) and 25 overweight or obese (body mass index >25 kg/m(2)) healthy university students were included in the study. We comprehensively assessed subclinical target organ damage, including the following: (1) assessment of renal function; (2) left ventricular structure and systolic and diastolic function; and (3) endothelial function. Muscle sympathetic nervous activity was assessed by microneurography. Participants with excess weight had decreased endothelial function (P<0.01), elevated creatinine clearance (P<0.05), increased left ventricular mass index (P<0.05), increased left ventricular wall thickness (P<0.01), lower systolic and diastolic function (P<0.01), and elevated muscle sympathetic nervous activity (P<0.001) compared with lean individuals. In multiple regression analysis, endothelial function was inversely related to muscle sympathetic nervous activity (R(2)=0.244; P<0.05), whereas creatinine clearance and left ventricular mass index were positively related to muscle sympathetic nervous activity, after adjustment for body mass index, sex, and blood pressure (R(2)=0.318, P<0.01 and R(2)=0.312, P<0.05, respectively). Excess weight in young individuals is associated with subclinical alterations in renal and endothelial function, as well as in the structure of the heart, even in the absence of hypertension. Sympathetic activity is closely associated with cardiovascular and renal alterations observed in these subjects.

Sympathetic activation in chronic renal failure

The potential involvement of sympathetic overactivity has been neglected in this population despite accumulating experimental and clinical evidence suggesting a crucial role of sympathetic activation for both progression of renal failure and the high rate of cardiovascular events in patients with chronic kidney disease. The contribution of sympathetic neural mechanisms to the occurrence of cardiac arrhythmias, the development of hypertension, and the progression of heart failure are well established; however, the exact mechanisms contributing to heightened sympathetic tone in patients with chronic kidney disease are unclear. This review analyses potential mechanisms underlying sympathetic activation in chronic kidney disease, the range of adverse consequences associated with this activation, and potential therapeutic implications resulting from this relationship.

Differing pattern of sympathoexcitation in normal-weight and obesity-related hypertension

Hypertension in normal-weight and obese individuals is characterized by activation of the sympathetic nervous system. Measurement of spillover of the sympathetic transmitter, norepinephrine, to plasma indicates that the regional pattern of sympathetic activation in the 2 "variants" of essential hypertension differs, excluding the heart in obesity-related hypertension. Whether sympathetic nerve firing characteristics also differ is unknown. We studied multiunit and single fiber sympathetic nerve firing properties in patients with normal-weight hypertension and obesity-related hypertension, comparing these with nerve characteristics in normal-weight and obese people with normal blood pressure. Both normal-weight hypertensive (n=10) and obese hypertensive (n=14) patients had increased total multiunit muscle sympathetic nerve activity compared with the normal-weight (n=11) and obese (n=11) people with normal blood pressure (65+/-4 versus 47+/-6 bursts per 100 heartbeats, P<0.01 in the normal-weight groups and 68+/-4 versus 53+/-3 bursts per 100 beats, P<0.01 in the obese groups). Sympathetic activation in normal-weight hypertension was characterized by increased firing rate of single vasoconstrictor fibers (70+/-8 versus 28+/-3 spikes per 100 beats; P<0.001), increased firing probability per heartbeat (39+/-3% versus 20+/-3%; P<0.001), and higher incidence of multiple spikes per heartbeat (30+/-4% versus 17+/-4%; P<0.05). Sympathetic activation in obesity-related hypertension differed, involving recruitment of previously silent fibers, which fired at a normal rate. The pattern of sympathetic activation in normal-weight and obesity-related hypertension differs in terms of both the firing characteristics of individual sympathetic fibers and the sympathetic outflows involved. The underlying central nervous system mechanism and the adverse consequences of the 2 modes of sympathetic activation may differ.

Gender differences in sympathetic nervous activity: influence of body mass and blood pressure

BACKGROUND

Obesity and high blood pressure (BP) commonly coexist in patients, and both conditions are associated with elevated sympathetic nervous activity. We tested whether the sympathetic nervous system was differently affected in men and women by the body mass index (BMI), BP, leptin and weight loss.

METHODS

We measured muscle sympathetic nerve activity (MSNA, microneurography), BP and plasma leptin concentrations in 167 age-matched normotensive and hypertensive men and women divided into three subgroups: lean, BMI < 25 kg/m; overweight, BMI > or = 25 and < 30 kg/m; and obese, BMI > or = 30 kg/m. These measurements were repeated in a subgroup of 19 obese subjects who underwent a 12-week diet.

RESULTS

Women with hypertension had increased MSNA compared with their normotensive counterparts (37 +/- 2 versus 25 +/- 2 bursts/min, analysis of variance, P < 0.001) and MSNA was significantly related to BP (P < 0.05, r = 0.236) but not to BMI. MSNA in men with hypertension was no different from that in normotensive subjects (33 +/- 2 versus 30 +/- 2 bursts/min), but MSNA was significantly related to BMI (P < 0.05, r = 0.249). Diet resulted in the same degree of weight loss in men and women, but induced a decrease in MSNA only in men (43 +/- 3 to 34 +/- 3 bursts/min, P < 0.01). The plasma leptin concentration was higher in women than in men, and for both groups it was related to BMI not BP (r = 0.497, P < 0.001 in women and r = 0.483, P < 0.001 in men).

CONCLUSION

These data demonstrate a gender difference in the regulation of the sympathetic nervous system, in which MSNA mainly relates to BP in women and to BMI in men.