Eric Jéquier

Leptin Signaling, Adiposity, and Energy Balance

Abstract: A chronic minor imbalance between energy intake and energy expenditure may lead to obesity. Both lean and obese subjects eventually reach energy balance and their body weight regulation implies that the adipose tissue mass is “sensed”, leading to appropriate responses of energy intake and energy expenditure. The cloning of the ob gene and the identification of its encoded protein, leptin, have provided a system signaling the amount of adipose energy stores to the brain. Leptin, a hormone secreted by fat cells, acts in rodents via hypothalamic receptors to inhibit feeding and increase thermogenesis. A feedback regulatory loop with three distinct steps has been identified: (1) a sensor (leptin production by adipose cells) monitors the size of the adipose tissue mass; (2) hypothalamic centers receive and integrate the intensity of the leptin signal through leptin receptors (LRb); (3) effector systems, including the sympathetic nervous system, control the two main determinants of energy balance—energy intake and energy expenditure. While this feedback regulatory loop is well established in rodents, there are many unsolved questions about its applicability to body weight regulation in humans. The rate of leptin production is related to adiposity, but a large portion of the interindividual variability in plasma leptin concentration is independent of body fatness. Gender is an important factor determining plasma leptin, with women having markedly higher leptin concentrations than men for any given degree of fat mass. The ob mRNA expression is also upregulated by glucocorticoids, whereas stimulation of the sympathetic nervous system results in its inhibition. Furthermore, leptin is not a satiety factor in humans because changes in food intake do not induce short‐term increases in plasma leptin levels. After its binding to LRb in the hypothalamus, leptin stimulates a specific signaling cascade that results in the inhibition of several orexigenic neuropeptides, while stimulating several anorexigenic peptides. The orexigenic neuropeptides that are downregulated by leptin are NPY (neuropeptide Y), MCH (melanin‐concentrating hormone), orexins, and AGRP (agouti‐related peptide). The anorexigenic neuropeptides that are upregulated by leptin are α‐MSH (α‐melanocyte‐stimulating hormone), which acts on MC4R (melanocortin‐4 receptor); CART (cocaine and amphetamine‐regulated transcript); and CRH (corticotropin‐releasing‐hormone). Obese humans have high plasma leptin concentrations related to the size of adipose tissue, but this elevated leptin signal does not induce the expected responses (i.e., a reduction in food intake and an increase in energy expenditure). This suggests that obese humans are resistant to the effects of endogenous leptin. This resistance is also shown by the lack of effect of exogenous leptin administration to induce weight loss in obese patients. The mechanisms that may account for leptin resistance in human obesity include a limitation of the blood‐brain‐barrier transport system for leptin and an inhibition of the leptin signaling pathways in leptin‐responsive hypothalamic neurons. During periods of energy deficit, the fall in leptin plasma levels exceeds the rate at which fat stores are decreased. Reduction of the leptin signal induces several neuroendocrine responses that tend to limit weight loss, such as hunger, food‐seeking behavior, and suppression of plasma thyroid hormone levels. Conversely, it is unlikely that leptin has evolved to prevent obesity when plenty of palatable foods are available because the elevated plasma leptin levels resulting from the increased adipose tissue mass do not prevent the development of obesity. In conclusion, in humans, the leptin signaling system appears to be mainly involved in maintenance of adequate energy stores for survival during periods of energy deficit. Its role in the etiology of human obesity is only demonstrated in the very rare situations of absence of the leptin signal (mutations of the leptin gene or of the leptin receptor gene), which produces an internal perception of starvation and results in a chronic stimulation of excessive food intake.

Effects of dietary fat on postprandial substrate oxidation and on carbohydrate and fat balances.

To study the effect of dietary fat on postprandial substrate utilization and nutrient balance, respiratory exchange was determined in seven young men for 1 h before and 9 h after the ingestion of one of three different breakfasts: i.e., bread, jam, and dried meat (482 kcal: 27% protein, 62% carbohydrate, and 11% fat); bread, jam, and dried meat plus 50 g of margarine containing long-chain triglycerides (LCT); or bread, jam, and dried meat plus 40 g medium-chain triglycerides (MCT) and 10 g LCT margarine (858 kcal: 15% protein, 35% carbohydrate, and 50% fat). Plasma glucose concentrations peaked 45 min after the start of the meals. When compared with the low fat meal, the LCT margarine supplement had no effect at any time on circulating glucose and insulin concentrations, nor on the respiratory quotient. When MCTs were consumed, plasma glucose and insulin concentrations remained lower and plasma FFA concentrations higher during the first 2 h. 9 h after the breakfasts, the amounts of substrates oxidized were similar in each case, i.e., approximately 320, 355, and 125 kcal for carbohydrate, fat, and protein, respectively. This resulted in comparable carbohydrate (mean +/- SD = -22 +/- 32, -22 +/- 37, and -24 +/- 22 kcal) and protein balances (-7 +/- 9, +7 +/- 7, and -8 +/- 11 kcal) after the low fat, LCT- and MCT-supplemented test meals, respectively. However, after the low fat meal, the lipid balance was negative (-287 +/- 60 kcal), which differed significantly (P less than 0.001) from the fat balances after the LCT- and MCT-supplemented meals, i.e., +60 +/- 33 and +57 +/- 25 kcal, respectively. The results demonstrate that the rates of fat and of carbohydrate oxidation are not influenced by the fat content of a meal.

Increased 24-Hour Energy Expenditure in Cigarette Smokers

Abstract We studied the effect of smoking on energy expenditure in eight healthy cigarette smokers who spent 24 hours in a metabolic chamber on two occasions, once without smoking and once while smoking 24 cigarettes per day. Diet and physical exercise (30 minutes of treadmill walking) were standardized on both occasions. Physical activity in the chamber was measured by use of a radar system. Smoking caused an increase in total 24-hour energy expenditure (from a mean value [±SEM] of 2230±115 to 2445±120 kcal per 24 hours; P<0.001), although no changes were observed in physical activity or mean basal metabolic rate (1545±80 vs. 1570±70 kcal per 24 hours). During the smoking period, the mean diurnal urinary excretion of norepinephrine (±SEM) increased from 1.25±0.14 to 1.82±0.28 μg per hour (P<0.025), and mean nocturnal excretion increased from 0.73±0.07 to 0.91±0.08 μg per hour (P<0.001). These short-term observations demonstrate that cigarette smoking increases 24-hour energy expenditure by approximately ...

The effect of graded doses of insulin on total glucose uptake, glucose oxidation, and glucose storage in man

The dose-response relationship between plasma insulin concentration and total glucose uptake, glucose oxidation, and glucose storage was examined in 22 healthy young volunteers by employing the euglycemic insulin clamp technique in combination with indirect calorimetry. Insulin was infused at five rates to achieve steady-state hyperinsulinemic plateaus of 62 ± 4, 103 ± 5, 170 ± 10, 423 ± 16, and 1132 ± 47 μU/ml. With increasing plasma insulin concentrations within the physiologic range, there was a linear increase in glucose uptake with a half maximally effective insulin concentration of 72 μU/ml. Glucose uptake by all tissues of the body reached 80% of its maximum value (12.6 mg/kg · min) at a plasma insulin concentration of ∼200 μU/ml. In contrast to total glucose uptake, glucose oxidation plateaued more quickly, achieved a maximum rate of only 4.0 mg/kg · min, and displayed a lower half maximally effective insulin concentration of 40 μU/ml. The increase in glucose uptake with progressively increasing plasma insulin levels was primarily the result of an increase in glucose storage, with a half maximally effective insulin concentration of 105 μU/ml and maximum rate of 8.7 mg/kg · min. Glucose storage represented over 60–70% of total glucose uptake at all insulin concentrations. After achieving maximum rates of insulin-mediated glucose uptake (plasma insulin concentration = 1132 μU/ml), hyperglycemia (+125 mg/dl) was superimposed on hyperinsulinemia to further enhance glucose transport. Under these conditions, total glucose uptake (32.5 mg/kg · min, P < 0.001) was markedly augmented but no significant increase in glucose oxidation was observed. These results indicate a true saturation of the glucose oxidation pathway. With pro-gressively increasing doses of insulin, the glucose storage represents the major route of glucose disposal.

The Effect of Ethanol on Fat Storage in Healthy Subjects

BACKGROUND Ethanol can account for up to 10 percent of the energy intake of persons who consume moderate amounts of ethanol. Its effect on energy metabolism, however, is not known. METHODS We studied the effect of ethanol on 24-hour substrate-oxidation rates in eight normal men during two 48-hour sessions in an indirect-calorimetry chamber. In each session, the first 24 hours served as the control period. On the second day of one session, an additional 25 percent of the total energy requirement was added as ethanol (mean [+/- SD], 96 +/- 4 g per day); during the other session, 25 percent of the total energy requirement was replaced by ethanol, which was isocalorically substituted for lipids and carbohydrates. RESULTS Both the addition of ethanol and the isocaloric substitution of ethanol for other foods reduced 24-hour lipid oxidation. The respective mean (+/- SE) decreases were 49.4 +/- 6.7 and 44.1 +/- 9.3 g per day (i.e., reductions of 36 +/- 3 percent and 31 +/- 7 percent from the oxidation rate during the control day; P less than 0.001 and P less than 0.0025). This effect occurred only during the daytime period (8:30 a.m. to 11:30 p.m.), when ethanol was consumed and metabolized. Neither the addition of ethanol to the diet nor the isocaloric substitution of ethanol for other foods significantly altered the oxidation of carbohydrate or protein. Both regimens including ethanol produced an increase in 24-hour energy expenditure (7 +/- 1 percent with the addition of ethanol, P less than 0.001; 4 +/- 1 percent with the substitution of ethanol for other energy sources, P less than 0.025). CONCLUSIONS Ethanol, either added to the diet or substituted for other foods, increases 24-hour energy expenditure and decreases lipid oxidation. Habitual consumption of ethanol in excess of energy needs probably favors lipid storage and weight gain.

7 Indirect calorimetry

Summary Indirect calorimery is a method which allows the non-invasive measurement of enery expenditure and substrate utilization in humans. The procedure is described and the main equations to calculate energy expenditure and substrate utilization are presented. The limitations of the method include physiological effects, such as hyperventilation, and the influence of metabolic processes such as gluconeogenesis, ketogenesis and lipogenesis. The general principle is that intermediate processes do not influence overall conclusions, provided that the intermediate substrates which are formed do not accumulate within the body or are not excreted. Continuous measurements of metabolic rate and respiratory quotient using the ventilated hood system have been carried out during the last 5 years to study carbohydrate and lipid metabolism in lean subjects, in obese and diabetic patients. By using the euglycaemic insulin clamp technique or by giving oral glucose loads, it has been shown that the main effect of insulin on carbohydrate metabolism is to stimulate glucose storage. By raising plasma free fatty acid levels with a neutral fat infusion in lean subjects, both glucose oxidation and glucose storage were impaired during euglycaemic insulin clamps. Glucose storage was found to be markedly impaired in non-diabetic obese patients, during euglycaemic insulin clamps in the presence of elevated lipid oxidation. In obese diabetic patients, the impairment in glucose storage was more pronounced than in non-diabetic obese; this defect was particularly marked during euglycaemic insulin clamps, but it was also present after an oral glucose load. It is concluded that impairment of glucose storage is a major defect of glucose utilization in type II diabetes.

Suppression of Alcohol-Induced Hypertension by Dexamethasone

BACKGROUND Alcohol consumption is associated with an increased incidence of hypertension and stroke, but the triggering mechanisms are unclear. In animals, alcohol causes activation of the sympathetic nervous system and also stimulates the release of corticotropin-releasing hormone (CRH), which has sympatho-excitatory effects when administered centrally. METHODS To determine whether alcohol evokes sympathetic activation and whether such activation is attenuated by the inhibition of CRH release, we measured blood pressure, heart rate, and sympathetic-nerve action potentials (using intraneural microelectrodes) in nine normal subjects before and during an intravenous infusion of alcohol (0.5 g per kilogram of body weight over a period of 45 minutes) and for 75 minutes after the infusion. Each subject received two infusions, one after the administration of dexamethasone (2 mg per day) and one after the administration of a placebo for 48 hours. RESULTS The infusion of alcohol alone evoked a marked (P < 0.001) and progressive increase in the mean (+/- SD) rate of sympathetic discharge, from 16 +/- 3 bursts per minute at base line to 30 +/- 8 bursts per minute at the end of the two-hour period. This sympathetic activation was accompanied during the second hour by an increase in mean arterial pressure of 10 +/- 5 mm Hg (P < 0.001). After the administration of dexamethasone, the alcohol infusion had no detectable sympathetic effect. The dexamethasone-induced suppression of sympathetic activation was associated with a decrease in mean arterial pressure of 7 +/- 6 mm Hg (P < 0.001) during the alcohol infusion and with suppression of the pressor effect during the second hour. CONCLUSIONS Alcohol induces pressor effects by sympathetic activation that appear to be centrally mediated. It is possible that these alcohol-induced hemodynamic and sympathetic actions could participate in triggering cardiovascular events.

Estimation of speed and incline of walking using neural network

A portable data logger is designed to record body accelerations during human walking. Five subjects walk first on a treadmill at various speeds on the level, and at positive and negative inclines. Then, the subjects performed a self-pace walking on an outdoor test circuit involving roads of various inclines. The recorded signals are parameterized, and the pattern of walking at each gait cycle is found. These patterns are presented to two neural networks which estimate the incline and the speed of walking. The results show a good estimation of the incline and the speed for all of the subjects. The correlation between predicted and actual inclines is r=0.98, and the maximum of speed-predicted error is 16%. To the best of our knowledge these results constitute the first speed and incline estimation of level and slope-unconstrained walking. >

Effects of isoenergetic glucose-based or lipid-based parenteral nutrition on glucose metabolism, de novo lipogenesis, and respiratory gas exchanges in critically ill patients.

OBJECTIVE To compare the effects of isocaloric, isonitrogenous carbohydrate nutrition vs. lipid-based total parenteral nutrition on respiratory gas exchange and intermediary metabolism in critically ill patients. DESIGN Prospective, clinical trial. SETTING Surgical intensive care unit in a major university hospital in Switzerland. PATIENTS Sixteen patients admitted to the surgical intensive care unit. INTERVENTIONS Patients were randomized to receive isocaloric isonitrogenous total parenteral nutrition (TPN) containing 75% (TPN-glucose) or 15% (TPN-lipid) glucose over a 5-day period. MEASUREMENTS AND MAIN RESULTS Indirect glucose metabolism was assessed from plasma carbon-13 (13C)-labeled glucose and 13C-labeled CO2 production during a tracer infusion of uniformly 13C-labeled glucose, and de novo lipogenesis was estimated from the incorporation of 13C into palmitate-very low density lipoproteins (VLDL) during a tracer infusion of 1-(13)C acetate. Compared with TPN-lipid, TPN-glucose increased plasma glucose more (by 26% vs. 7%, p < .05), increased insulin more (by 284% vs. 40%, p < .01), and increased total CO2 more (by 15% vs. 0%, p < .01). Both nutrient mixtures failed to inhibit endogenous glucose production and net protein oxidation, suggesting absence of suppression of gluconeogenesis. Fractional de novo lipogenesis was markedly increased by TPN-glucose to 17.4% vs. 3.3% with TPN lipids. CONCLUSIONS The rate of glucose administration commonly used during TPN of critically ill patients does not suppress endogenous glucose production or net protein loss, but markedly stimulates de novo lipogenesis and CO2 production. Increasing the proportion of fat may be beneficial, provided that lipid emulsion has no adverse effects.

Glucose-induced Thermogenesis in Nondiabetic and Diabetic Obese Subjects

The glucose-induced thermogenesis (GIT) following a 100-g oral glucose load has been measured by continuous indirect calorimetry in 55 nondiabetic and diabetic obese subjects of various ages and compared with two control groups of 17 young and 13 elderly nonobese subjects. The obese subjects were divided into four groups: group A, normal glucose tolerance; group B, impaired glucose tolerance; group C, diabetes with increased insulin response; group D, diabetes with reduced insulin response. The glucose-induced thermogenesis measured during 3 h represented 8.6 ± 0.7% of the energy content of the load in the young control group. In all obese groups, the glucose-induced thermogenesis was significantly lower than in the young control group, i.e., 6.6 ± 0.9%, 6.4 ± 0.6%, 3.7 ± 0.7%, and 2.2 ± 0.4% in groups A, B, C, and D, respectively. Since the obese diabetics were older than the other groups, their glucose-induced thermogenesis was compared with that of the elderly control group; the latter (5.8 ± 0.3%) was significantly lower (P < 0.05) than that of the young control group. The obese diabetics also had a significantly lower glucose-induced thermogenesis than the elderly control group (P < 0.02 and P < 0.001 for groups C and D, respectively). When corrected for glucosuria and after taking into account the glucosuria and the changes in the glucose space, the corrected glucose-induced thermogenesis (i.e., related to glucose “uptake”), was still significantly reduced in group A versus the young control group (6.6 ± 0.9 versus 8.6 ± 0.7%, P < 0.05), and in group D versus the elderly (matched for age) control group (4.2 ± 0.7 versus 5.8 ± 0.3%, P < 0.05). It is concluded that the postprandial thermogenesis induced by glucose ingestion is decreased in the presence of insulin resistance and/or reduced insulin response to the glucose load in obese subjects. In addition, age itself contributes to decrease glucose-induced thermogenesis.