Integration of energy intake and expenditure in animals and man: the autonomic and adrenal hypothesis

Evaluation of Cardiovascular Response to Isometric Handgrip Exercise in Obese Individuals

Background The variability in hemodynamic responses to isometric handgrip exercise in people with different body mass indices (BMIs) is noteworthy due to the frequent incidence of hypertension, obesity, and related cardiovascular illnesses in India. This investigation may be necessary to anticipate and prevent excessive heart strain during static activities. Therefore, this study aimed to evaluate the impact of isometric handgrip exercise on cardiorespiratory responses in an Indian population with varying BMI levels. The objective was to compare the cardiovascular responses of obese and non-obese individuals to an isometric handgrip exercise test (heart rate, systolic blood pressure, diastolic blood pressure, rate pressure product, and mean arterial pressure). Methodology This study was conducted from April 2021 to October 2022. Measurements were obtained using a pedestal-style weighing scale with a maximum capacity of 120 kg and an accuracy of 100 g at the Department of Physiology, Indira Gandhi Institute of Medical Sciences, Patna. Sphygmomanometer was used for measuring blood pressure, and a stopwatch was used to time the duration of each prolonged isometric contraction performed by the participants using a handgrip dynamometer. Baseline measurements of heart rate, blood pressure, cardiac output, and heart rate variability were done during the final three minutes of the rest phase and once more throughout the 10-minute recovery period that followed the isometric handgrip exercise. Results The average heart rate increased during the isometric handgrip exercise in the obese group (13.02 ± 1.88 beats per minute). Systolic blood pressure increased by 16.40 ± 2.65 mmHg and 23.66 ± 3.14 mmHg in the obese group and the normal weight group, respectively. The diastolic blood pressure increased similarly in the overweight and normal weight groups, measuring 18.64 ± 3.63 mmHg and 12.14 ± 1.95 mmHg, respectively. Furthermore, the mean blood pressure increased by 20.45 ± 3.13 mmHg and 13.67 ± 1.64 mmHg, respectively, both in the normal and overweight groups. Conclusions Obese individuals had greater resting heart rates, systolic blood pressure, and diastolic blood pressure than non-obese individuals. Following an isometric handgrip workout, non-obese individuals reported greater increases in heart rate, systolic blood pressure, and diastolic blood pressure than obese individuals.

Autonomic Reactivity Differs in Young Adults Classified using Revised Indian and WHO Guidelines for Obesity

INTRODUCTION

Many studies have reported alteration in autonomic activity in obesity. However, there is paucity of literature comparing autonomic reactivity using different guidelines of obesity. As Indian guidelines were revised recently and WHO states that countries should use all categories of BMI for reporting purposes, it is prudent to compare physiological state in different categories of BMI.

AIM

The aim of the present study was to compare the autonomic alteration in young adults using revised Indian and WHO guidelines for obesity.

MATERIALS AND METHODS

A battery of autonomic tests (Valsalva Manoeuvre (VM), Deep Breathing Test (DBT), Lying to Standing Test (LST) and Hand Grip isometric exercise Test (HGT) was conducted on 34 overweight and obese and 30 normal weight volunteers categorised using revised Indian guidelines of body mass index. Same participants were regrouped and analysed using WHO guidelines of BMI and waist hip ratio (WHR).

RESULTS

For analysis, participants were grouped into 3 categories of normal, overweight and obese using revised Indian guidelines for obesity. Same participants were regrouped according to WHO guidelines. E:I ratio during DBT, 30:15 ratio during LST, Valsalva ratio during VM and increase in DBP during HGT were compared in different subgroups. There was no difference in sympathetic and parasympathetic activities in participants classified according to revised Indian guidelines. In participants classified using WHO criteria, sympathetic reactivity in overweight subjects was significantly less as compared to normal subjects (p<0.05).

CONCLUSION

Autonomic alterations might be more related to body fat percent rather than BMI. Indian guidelines are based on the observation that Asian population has more adipose tissue in WHO range of BMI. As the guidelines of BMI are applicable to all age groups and do not consider physical activity profile, they might still not be a good predictor of body fat.

Origins and early development of the concept that brown adipose tissue thermogenesis is linked to energy balance and obesity

Brown adipose tissue (BAT) was identified as a thermogenic organ in 1961, and in 1978 shown to be the major site of thermoregulatory non-shivering thermogenesis in rats acclimated to the cold. Investigations in the mid-late 1970s established the uncoupling of oxidative phosphorylation through a proton conductance pathway across the mitochondrial inner membrane as the mechanism for heat production in BAT, this being regulated by UCP1 which was first discovered as a 32,000 M_r cold-inducible protein. These developments came when those concerned with nutritional energetics were proposing that thermogenesis is a significant factor in energy balance and the aetiology of obesity. A link with BAT was first demonstrated in obese ob/ob mice, which were shown to have decreased thermogenic activity in the tissue, and in rats exhibiting diet-induced thermogenesis (DIT) during overfeeding on a cafeteria diet where an activation of brown fat was evident. These pioneering observations led to extensive studies on BAT in different animal models of obesity, both genetic (particularly ob/ob and db/db mice, fa/fa rats) and experimentally-induced. In each case, indices of BAT activity and capacity (mitochondrial content, GDP binding, amount of UCP1) indicated that the tissue plays a role in DIT and that obesity is characterised by reduced thermogenesis. Links between BAT and whole-body energetics were also made in physiological situations such as lactation and fasting. Studies in the 1980s also provided clear evidence for the presence of BAT in adult humans, particularly through the detection of UCP1, and its activation in patients with phaeochromocytoma. Interest in BAT in energetics and obesity waned by the 1990s; the current major renewal of interest has undoubtedly been contingent on the pioneering developments that emerged some 40 years ago.

Understanding the metabolic syndrome: a modeling perspective

The prevalence of obesity is growing at an alarming rate, placing many at risk for developing diabetes, hypertension, sleep apnea, or a combination of disorders known as "metabolic syndrome". The evidence to date suggests that metabolic syndrome results from an imbalance in the mechanisms that link diet, physical activity, glucose-insulin control, and autonomic cardiovascular control. There is also growing recognition that sleep-disordered breathing and other forms of sleep disruption can contribute significantly to autonomic dysfunction and insulin resistance. Chronic sleep deprivation resulting from sleep-disordered breathing or behavioral causes can lead to excessive daytime sleepiness and lethargy, which in turn contribute to increasing obesity. Analysis of this complex dynamic system using a model-based approach can facilitate the delineation of the causal pathways that lead to the emergence of the metabolic syndrome. In this paper, we provide an overview of the main physiological mechanisms associated with obesity and sleep-disordered breathing that are believed to result in metabolic and autonomic dysfunction, and review the models and modeling approaches that are relevant in characterizing the interplay among the multiple factors that underlie the development of the metabolic syndrome.

Insulin Levels, Physical Activity, and Urinary Catecholamine Excretion of Obese and Non-Obese Rhesus Monkeys

The hypothesis that spontaneous obesity in rhesus monkeys is associated with abnormalities in energy expenditure was tested. Obese (n=7) and non-obese (n=5) monkeys were described in terms of body size and composition, food intake, and physical activity. Additionally, the relationships among fasting and stimulated insulin levels in serum, C-peptide levels in serum and urine, and urinary catecholamines were examined. Obese animals had primarily abdominal deposition of excess body fat, as indicated by markedly elevated abdominal circumferences and skin-fold thicknesses. Food intake did not differ between groups. Physical activity was much lower in the obese group. Obese monkeys had markedly higher serum insulin and C-peptide levels in the fasted state and in response to an intravenous glucose challenge. Urinary excretion of C-peptide and catecholamines was measured during successive 2-day periods of ad libitum feeding, food deprivation, and refeeding in order to examine potential differences between groups in sympathoadrenal activity and their relationship to insulin secretion. C-peptide excretion was greater for obese and decreased for both groups during food deprivation. Urinary dopamine (DA), norepinephrine (NE), and epinephrine (E) levels were significantly greater for obese animals in all conditions. DA excretion was lowest during deprivation and E excretion was lowest during refeeding, whereas NE excretion was relatively unaffected by feeding condition. The overall patterns of C-peptide and catecholamine excretion were qualitatively similar for both groups, and there were no reliable differences between obese and non-obese in their responses to the feeding manipulation. The results suggest that hyperinsulinemia associated with obesity in rhesus monkeys is linked to increased catecholamine secretion and a resistance to catecholaminergic action.

Autonomic nervous system dysfunction in obesity and Prader-Willi syndrome: current evidence and implications for future obesity therapies

The autonomic nervous system (ANS) controls essential functions like breathing, heart rate, digestion, body temperature and hormone levels. Evidence suggests that ANS dysfunction is associated with adult and childhood obesity and plays a role in the distribution of total body fat and the development of obesity-related complications in humans. This review summarizes our current understanding of ANS involvement in the pathogenesis of obesity and Prader-Willi syndrome. Available evidence of ANS dysfunction in the control of energy balance is limited and, in some cases, contradictory. Further investigation in this area is warranted in order to better understand the important contributions of the ANS to regulation of body fat, development of obesity and its comorbidities. Results from these studies will guide the development of novel obesity therapeutics targeting specific ANS dysfunction.

The difference in the storage capacities for carbohydrate and for fat, and its implications in the regulation of body weight

The two-compartment model presented here suggests that weight maintenance can be achieved by a regulation of food intake geared primarily toward the maintenance of stable glycogen levels, rather than toward the preservation of the overall energy balance. This concept is reminiscent of the glucostatic theory of food intake regulation proposed by Mayer. It is viewed here as being linked to changes in the body's carbohydrate stores, which represent an integration of carbohydrate and lipid fluxes, rather than to changes in blood glucose levels, whose substantial variations during the day are dependent on various circumstantial events. The model illustrates that the fat to carbohydrate ratio of the diet may have considerable potential influence on steady state body composition, even though carbohydrates and fats are both able to meet the body's energy substrate requirements. It appears that failure of appropriately reducing the range within which glycogen levels are maintained when the diet's fat content rises will require an expansion of the adipose tissue mass to raise FFA levels and fat oxidation to a rate commensurate with the proportion of fat in the diet. Maintenance of glycogen reserves below their level of saturation is made less likely by the high palatability and ubiquitous availability of foods in affluent societies. Thus, one can understand the high incidence of obesity among populations consuming mixed diets with a relatively high fat content, without having to attribute this to some defect(s) in the mechanism(s) controlling food intake.

Neuropeptide Y and its receptors as potential therapeutic drug targets

Neuropeptide Y (NPY) is a 36-amino-acid peptide that exhibits a large number of physiological activities in the central and peripheral nervous systems. NPY mediates its effects through the activation of six G-protein-coupled receptor subtypes named Y(1), Y(2), Y(3), Y(4), Y(5), and y(6). Evidence suggests that NPY is involved in the pathophysiology of several disorders, such as the control of food intake, metabolic disorders, anxiety, seizures, memory, circadian rhythm, drug addiction, pain, cardiovascular diseases, rhinitis, and endothelial cell dysfunctions. The synthesis of agonists and antagonists for these receptors could be useful to treat several of these diseases.

Symptoms of hypoglycaemia in people with diabetes

The symptoms of hypoglycaemia are fundamental to the early detection and treatment of this side-effect of insulin and oral hypoglycaemic therapy in people with diabetes. The physiology of normal responses to hypoglycaemia is described and the importance of symptoms of hypoglycaemia is discussed in relation to the treatment of diabetes. The symptoms of hypoglycaemia are described in detail. The classification of symptoms is considered and the usefulness of autonomic and neuroglycopenic symptoms for detecting hypoglycaemia is discussed. The many external and internal factors involved in the perception of symptoms are reviewed, and symptoms of hypoglycaemia experienced by people with Type 2 diabetes are addressed. Age-specific differences in the symptoms of hypoglycaemia have been identified, and are important for clinical and research practice, particularly with respect to the development of acquired hypoglycaemia syndromes in people with Type 1 diabetes that can result in impaired awareness of hypoglycaemia. In addition, the routine assessment of hypoglycaemia symptoms in the diabetic clinic is emphasized as an important part of the regular review of people with diabetes who are treated with insulin.

The relationship between heart rate variability and measures of body habitus

There is a well-recognized relationship between autonomic nervous system function and body habitus although few studies have addressed the role of the parasympathetic nervous system. A decrease in parasympathetic nervous-system-mediated heart rate variability in obesity may in part explain the mortality and morbidity that are associated with the obese state. We used multiple linear regression techniques to explore the relationship between measures of heart rate variability and anthropometric indices in 597 male participants in the Normative Aging Study. After adjustment for age and log10 heart rate, weight and body mass index were significant predictors of both the expiratory to inspiratory ratio (E/I ratio) and the difference between maximum and minimum heart rate (HRMax-Min). The abdomen-to-hip ratio and percentage body fat were not significant predictors of measures of heart rate variability. A one standard deviation change in the anthropometric index (weight, body mass index) resulted in a decrease in the E/I ratio of 0.010-0.014 and a decrease in the HRMax-Min of 0.486-0.715 beats/min. A change in the anthropometric index across the distribution (5-95 percentile) resulted in a decrease in the E/I ratio of 0.032-0.037 and a decrease in the HRMax-Min of 1.56-2.39 beats/min. These results indicate that heart rate variability and overall body size are correlated. This association could in part explain the mortality and morbidity that is associated with the obese state.

Growth and maturation of primary-cultured adipocytes from lean and ob/ob mice

Stromal vascular cells from epididymal fat pads of lean and obese mice were cultured in a medium (alpha-MEM) containing fetal bovine serum (FBS) and cell replication followed for 11 days. In both types of cells, confluence occurred at 4-5 days, after which virtual growth arrest occurred in lean-mouse cells while replication continued, albeit at a slower rate in obese-mouse cells. Little or no lipid accumulation or glycerol-3-phosphate dehydrogenase (GPDH) activity was observed under these conditions. When a differentiation mixture consisting of insulin, corticosterone and isobutylmethylxanthine was added to the serum-containing alpha-MEM, a proportion of the lean-mouse cells accumulated triglycerides and GPDH activity increased significantly, indicating differentiation. By contrast, little or no differentiation occurred in obese-mouse cells. When cells grown in serum-containing alpha-MEM were transferred to a serum-free defined medium at confluence, extensive differentiation and maturation occurred in lean-mouse cells but not in obese-mouse cells. Similar experiments were conducted in cells isolated from the retroperitoneal fat pad. Although the growth pattern was similar to that of epididymal preadipocytes, the retroperitoneal lean- and obese-mouse cells differentiated more readily than epididymal cells, as shown by the GPDH specific activity. These data suggest that cells from obese mice are resistant to differentiation under conditions that support extensive differentiation in lean-mouse cells.

The effects of different percentages of dietary fat intake, exercise, and calorie restriction on body composition and body weight in obese females

PURPOSE

The goal of this study was to determine the effect of varying the amount of dietary fat, while holding calories at 1,200 kcals/day, on body weight and percent body fat in 35 obese women.

DESIGN

A pretest, midtest, posttest experimental design was employed, and subjects were randomly divided into one of four dietary fat groups, with 10%, 20%, 30%, or 40% of caloric intake as dietary fat.

INTERVENTION

Subjects consumed 1,200 kcals/day and a specified percentage of total energy as fat, depending on their dietary group. Protein was held constant at 20%. All subjects engaged in a five day/week walking program.

SETTING

Participants were recruited from the general community using newspaper advertisements.

SUBJECTS

Thirty-five obese women 25 to 45 years of age (means=38 +/- 4.97) served as subjects. All were at least 20% above ideal weight and 30% to 52% body fat.

MEASURES

Percent body fat, body weight, and anthropomorphic measurements were taken at baseline, six and 12 weeks. Dietary intake was recorded daily by each subject, and exercise walking logs were maintained by each participant.

RESULTS

All subjects lost body weight and body fat; however, there were no significant differences in the rate or amount of body weight or percent body fat lost across the four groups during the intervention.

CONCLUSIONS

It appears that during calorie restriction and exercise for 12 weeks, percent of calories derived from dietary fats does not influence loss of body weight or percent body fat in adult obese women.

Altered brain cholinergic enzymes activity in the genetically obese rat

Genetically obese male Zucker rats (fa/fa) and their lean littermates (Fa/-) were used in this experiment. Fourteen-week-old obese and lean littermates were sacrificed and choline acetyltransferase (ChAT) and acetylcholinesterase (AChE) enzymes were assayed in specific brain regions. The assays of these enzymes indicate that obese animals had a significantly lower ChAT activity in the cerebellum, pons, and cerebral cortex and a significant increase in ChAT activity in the thalamus and hypothalamus. Meanwhile, the cerebral cortex, cerebellum, midbrain, thalamus and hypothalamus of the obese animals showed significantly higher AChE activity than their lean littermates. It was concluded from this study that obesity may be associated with changes in the enzymes of the brain cholinergic system.

Nutrient balance and obesity: an approach to control of food intake in humans

This article has examined the regulated systems that control nutrient balance. From this analysis, the following conclusions may be suggested: 1. Each nutrient is regulated separately in a feedback system. 2. The control of glucose is regulated by the size of the glycogen stores; the size of the fat depots, by the rate of hepatic fatty acid oxidation; and protein, by the size of the protein depots. 3. Obesity can occur as a result of hyperphagia or from repartitioning the deposition of nutrients. In either case, there is a relative or absolute reduction in the activity of the sympathetic nervous system, requiring adequate levels of circulating corticosteroids.

Body fat and the activity of the autonomic nervous system

The cause of most cases of human obesity is unknown. Specific alterations in the activity of the autonomic nervous system may mediate and perhaps cause obesity in animal models. We therefore looked for alterations in autonomic activity in human obesity. Fifty-six healthy men with various percentages of body fat underwent autonomic testing while at rest. Significant correlations were found between the percentage of body fat and the variation in the R-R interval after beta-adrenergic blockade (r = -0.30, P less than 0.03), the heart rate (r = 0.30, P less than 0.03), the plasma norepinephrine concentration (r = -0.30, P less than 0.05), the plasma epinephrine concentration (r = -0.49, P less than 0.001), and the pupillary latency period (r = 0.39, P less than 0.01). Each of these variables reflects the activity of the sympathetic nervous system or parasympathetic nervous system or both. Depressions in sympathetic and parasympathetic activity were significantly but weakly associated with increasing percentages of body fat. These associations indicate that in obese persons, autonomic changes, though not necessarily causal, involve several organ systems. We suggest that autonomic alterations are important in human obesity, as they are in animal obesity. A disordered homeostatic mechanism may promote excessive storage of energy by decreasing sympathetic activity, while defending against weight gain by decreasing parasympathetic activity. The use of autonomic profiles holds promise for classifying human obesity and identifying obese patients at increased risk for various disorders.

Effect of lateral hypothalamic lesion on brown adipose tissue of Zucker lean and obese rats

Acute (10-day) lateral hypothalamic (LH) lesion induced a reduction of food intake in both lean and obese Zucker rats which averaged about 50% over the course of the first 10 days. The aphagia associated with a fall in body weight in both genotypes which was greater than their respective pair-fed controls, indicating a change in energetic efficiency. The reduced level of BAT protein, mitochondria and GDP binding observed in the obese rat was restored after LH lesion, suggesting the reestablishment of a normal sympathetic drive to the tissue. The markedly lower plasma insulin concentration in the LH lesioned obese rat is consistent with a reduction in parasympathetic activity in these animals. Food restriction in the sham lean rat reduced BAT protein content and mitochondrial GDP binding, whereas no such changes were observed in the food restricted obese rat. This demonstrates the insensitivity of the obese rat to dietary signals and may imply that LH lesion restores diet-induced BAT thermogenesis in the obese rat.

Effects of ethanol and tertiary-butanol on energy expenditure and substrate utilization in the rat

The acute effects of ethanol and tertiary-butanol, an alcohol which is not metabolized via the alcohol dehydrogenase pathway, on whole body metabolism were studied using indirect calorimetry. Ethanol, but not t-butanol, increased energy expenditure in food-deprived rats. Both ethanol and t-butanol reduced respiratory quotient (RQ), an index of overall body energy substrate utilization. The lowered RQ indicates an increased dependence upon lipids as an energy source. Taken together, these data suggest that ethanol, probably within a narrow dose range, can enhance energy expenditure in the rat, either via a metabolite (e.g., acetaldehyde) or through a consequence of its oxidation. The increase in lipid mobilization seen after acute treatment with ethanol, on the other hand, appears to be independent of its oxidation.

Peptides and feeding

This report reviews the hypothesis that peptides play a role in appetite modulation, stressing that the available evidence is predominantly pharmacological and thus caution needs to be taken in assigning physiological significance at this time. Two peptide systems have been postulated--a peripheral satiety system, typified by the gastrointestinal hormone cholecystokinin and a central feeding system driven by the opioid peptides and neuropeptide Y. This review also discusses the putative role of peptides in the anorexia of aging, drinking elicited by feeding and as mediators of the autonomic effects seen in association with ventromedial hypothalamic lesions.