Corticotropin-releasing hormone directly stimulates thermogenesis in skeletal muscle possibly through substrate cycling between de novo lipogenesis and lipid oxidation

The mechanisms by which CRH and related peptides (i.e. the CRH/urocortin system) exert their control over thermogenesis and weight regulation have until now focused only upon their effects on brain centers controlling sympathetic outflow. Using a method that involves repeated oxygen uptake determinations in intact mouse skeletal muscle, we report here that CRH can act directly on skeletal muscle to stimulate thermogenesis, an effect that is more pronounced in oxidative than in glycolytic muscles and that can be inhibited by a selective CRH-R2 antagonist or blunted by a nonselective CRH receptor antagonist. This thermogenic effect of CRH can also be blocked by interference along pathways of de novo lipogenesis and lipid oxidation, as well as by inhibitors of phosphatidylinositol 3-kinase or AMP-activated protein kinase. Taken together, these studies demonstrate that CRH can directly stimulate thermogenesis in skeletal muscle, and in addition raise the possibility that this thermogenic effect, which requires both phosphatidylinositol 3-kinase and AMP-activated protein kinase signaling, might occur via substrate cycling between de novo lipogenesis and lipid oxidation. The effect of CRH in directly stimulating thermogenesis in skeletal muscle underscores a potentially important peripheral role for the CRH/urocortin system in the control of thermogenesis in this tissue, in its protection against excessive intramyocellular lipid storage, and hence against skeletal muscle lipotoxicity and insulin resistance.

Cardiovascular responses to water drinking: does osmolality play a role?

Water drinking activates the autonomic nervous system and induces acute hemodynamic changes. The actual stimulus for these effects is undetermined but might be related to either gastric distension or to osmotic factors. In the present study, we tested whether the cardiovascular responses to water drinking are related to water's relative hypoosmolality. Therefore, we compared the cardiovascular effects of a water drink (7.5 ml/kg body wt) with an identical volume of a physiological (0.9%) saline solution in nine healthy subjects (6 male, 3 female, aged 26 +/- 2 years), while continuously monitoring beat-to-beat blood pressure (finger plethysmography), cardiac intervals (electrocardiography), and cardiac output (thoracic impedance). Total peripheral resistance was calculated as mean blood pressure/cardiac output. Cardiac interval variability (high-frequency power) was assessed by spectral analysis as an index of cardiac vagal tone. Baroreceptor sensitivity was evaluated using the sequence technique. Drinking water, but not saline, decreased heart rate (P = 0.01) and increased total peripheral resistance (P < 0.01), high-frequency cardiac interval variability (P = 0.03), and baroreceptor sensitivity (P = 0.01). Neither water nor saline substantially increased blood pressure. These responses suggest that water drinking simultaneously increases sympathetic vasoconstrictor activity and cardiac vagal tone. That these effects were absent after drinking physiological saline indicate that the cardiovascular responses to water drinking are influenced by its hypoosmotic properties.

Substrate cycling between de novo lipogenesis and lipid oxidation: a thermogenic mechanism against skeletal muscle lipotoxicity and glucolipotoxicity

Life is a combustion, but how the major fuel substrates that sustain human life compete and interact with each other for combustion has been at the epicenter of research into the pathogenesis of insulin resistance ever since Randle proposed a 'glucose-fatty acid cycle' in 1963. Since then, several features of a mutual interaction that is characterized by both reciprocality and dependency between glucose and lipid metabolism have been unravelled, namely: the inhibitory effects of elevated concentrations of fatty acids on glucose oxidation (via inactivation of mitochondrial pyruvate dehydrogenase or via desensitization of insulin-mediated glucose transport),the inhibitory effects of elevated concentrations of glucose on fatty acid oxidation (via malonyl-CoA regulation of fatty acid entry into the mitochondria), and more recentlythe stimulatory effects of elevated concentrations of glucose on de novo lipogenesis, that is, synthesis of lipids from glucose (via SREBP1c regulation of glycolytic and lipogenic enzymes). This paper first revisits the physiological significance of these mutual interactions between glucose and lipids in skeletal muscle pertaining to both blood glucose and intramyocellular lipid homeostasis. It then concentrates upon emerging evidence, from calorimetric studies investigating the direct effect of leptin on thermogenesis in intact skeletal muscle, of yet another feature of the mutual interaction between glucose and lipid oxidation: that of substrate cycling between de novo lipogenesis and lipid oxidation. It is proposed that this energy-dissipating substrate cycling that links glucose and lipid metabolism to thermogenesis could function as a 'fine-tuning' mechanism that regulates intramyocellular lipid homeostasis, and hence contributes to the protection of skeletal muscle against lipotoxicity.

Ectopic fat storage in heart, blood vessels and kidneys in the pathogenesis of cardiovascular diseases

In humans and most animal models, the development of obesity leads not only to increased fat depots in classical adipose tissue locations but also to significant lipid deposits within and around other tissues and organs, a phenomenon known as ectopic fat storage. The purpose of this review is to explore the possible locations of ectopic fat in key target-organs of cardiovascular control (heart, blood vessels and kidneys) and to propose how ectopic fat storage can play a role in the pathogenesis of cardiovascular diseases associated with obesity. In animals fed a high-fat diet, cardiac fat depots within and around the heart impair both systolic and diastolic functions, and may in the long-term promote heart failure. Accumulation of fat around blood vessels (perivascular fat) may affect vascular function in a paracrine manner, as perivascular fat cells secrete vascular relaxing factors, proatherogenic cytokines and smooth muscle cell growth factors. Furthermore, high amounts of perivascular fat could mechanically contribute to the increased vascular stiffness seen in obesity. Finally, accumulation of fat in the renal sinus may limit the outflow of blood and lymph from the kidney, which would alter intrarenal physical forces and promote sodium reabsorption and arterial hypertension. Taken together, ectopic fat storage in key target-organs of cardiovascular control may impair their functions, contributing to the increased prevalence of cardiovascular diseases in obese subjects.

Redistribution of glucose from skeletal muscle to adipose tissue during catch-up fat: a link between catch-up growth and later metabolic syndrome

Catch-up growth, a risk factor for later obesity, type 2 diabetes, and cardiovascular diseases, is characterized by hyperinsulinemia and an accelerated rate for recovering fat mass, i.e., catch-up fat. To identify potential mechanisms in the link between hyperinsulinemia and catch-up fat during catch-up growth, we studied the in vivo action of insulin on glucose utilization in skeletal muscle and adipose tissue in a previously described rat model of weight recovery exhibiting catch-up fat caused by suppressed thermogenesis per se. To do this, we used euglycemic-hyperinsulinemic clamps associated with the labeled 2-deoxy-glucose technique. After 1 week of isocaloric refeeding, when body fat, circulating free fatty acids, or intramyocellular lipids in refed animals had not yet exceeded those of controls, insulin-stimulated glucose utilization in refed animals was lower in skeletal muscles (by 20-43%) but higher in white adipose tissues (by two- to threefold). Furthermore, fatty acid synthase activity was higher in adipose tissues from refed animals than from fed controls. These results suggest that suppressed thermogenesis for the purpose of sparing glucose for catch-up fat, via the coordinated induction of skeletal muscle insulin resistance and adipose tissue insulin hyperresponsiveness, might be a central event in the link between catch-up growth, hyperinsulinemia and risks for later metabolic syndrome.

The direct effect of leptin on skeletal muscle thermogenesis is mediated by substrate cycling between de novo lipogenesis and lipid oxidation

We report here studies that integrate data of respiration rate from mouse skeletal muscle in response to leptin and pharmacological interference with intermediary metabolism, together with assays for phosphatidylinositol 3-kinase (PI3K) and AMP-activated protein kinase (AMPK). Our results suggest that the direct effect of leptin in stimulating thermogenesis in skeletal muscle is mediated by substrate cycling between de novo lipogenesis and lipid oxidation, and that this cycle requires both PI3K and AMPK signaling. This substrate cycling linking glucose and lipid metabolism to thermogenesis provides a novel thermogenic mechanism by which leptin protects skeletal muscle from excessive fat storage and lipotoxicity.

Adaptive thermogenesis and uncoupling proteins: a reappraisal of their roles in fat metabolism and energy balance

After decades of controversies about the quantitative importance of autoregulatory adjustments in energy expenditure in weight regulation, there is now increasing recognition that even subtle variations in thermogenesis could, in dynamic systems and over the long term, be important in determining weight maintenance in some and obesity in others. The main challenge nowadays is to provide a mechanistic explanation for the role of adaptive thermogenesis in attenuating and correcting deviations of body weight and body composition, and in the identification of molecular mechanisms that constitute its effector systems. This workshop paper reconsiders what constitutes adaptive changes in thermogenesis and reassesses the role of the sympathetic nervous system (SNS) and uncoupling proteins (UCP1, UCP2, UCP3, UCP5/BMCP1) as the efferent and effector components of the classical one-control system for adaptive thermogenesis and fat oxidation. It then reviews the evidence suggesting that there are in fact two distinct control systems for adaptive thermogenesis, the biological significance of which is to satisfy--in a lifestyle of famine-and-feast--the needs to suppress thermogenesis for energy conservation during weight loss and weight recovery even under environmental stresses (e.g., cold, infection, nutrient imbalance) when sympathetic activation of thermogenesis has equally important survival value.

Skeletal muscle mitochondrial oxidative capacity and uncoupling protein 3 are differently influenced by semistarvation and refeeding

We investigated, in skeletal muscle mitochondria isolated from semistarved and refed rats, the relation between the protein expression of uncoupling protein 3 (UCP3) and mitochondrial oxidative capacity, assessed as state 4 and state 3 respiration rates in presence of substrates that are either non-lipids (glutamate, succinate) or lipids (palmitoyl CoA, palmitoylcarnitine). During semistarvation, when whole-body thermogenesis is diminished, state 3 respiration was lower than in fed controls by about 30% independently of substrate types, while state 4 respiration was lower by 20% only during succinate oxidation, but UCP3 was unaltered. After 5 days of refeeding, when thermogenesis is still diminished, neither state 4, state 3 nor UCP3 were lower than in controls. Refeeding on a high-fat diet, which exacerbates the suppression of thermogenesis, resulted in a two-fold elevation in UCP3 but no change in state 4 or state 3 respiration. These results during semistarvation and refeeding, in line with those previously reported for fasting, are not in support of the hypothesis that UCP3 is a mediator of adaptive thermogenesis pertaining to weight regulation, and underscore the need for caution in interpreting parallel changes in UCP3 and mitochondrial oxidative capacity as the reflection of mitochondrial uncoupling by UCP3.

A role for suppressed thermogenesis favoring catch-up fat in the pathophysiology of catch-up growth

Catch-up growth is a risk factor for later obesity, type 2 diabetes, and cardiovascular diseases. We show here that after growth arrest by semistarvation, rats refed the same amount of a low-fat diet as controls show 1) lower energy expenditure due to diminished thermogenesis that favors accelerated fat deposition or catch-up fat and 2) normal glucose tolerance but higher plasma insulin after a glucose load at a time point when their body fat and plasma free fatty acids (FFAs) have not exceeded those of controls. Isocaloric refeeding on a high-fat diet resulted in even lower energy expenditure and thermogenesis and increased fat deposition and led to even higher plasma insulin and elevated plasma glucose after a glucose load. Stepwise regression analysis showed that plasma insulin and insulin-to-glucose ratio after the glucose load are predicted by variations in efficiency of energy use (i.e., in thermogenesis) rather than by the absolute amount of body fat or plasma FFAs. These studies suggest that suppression of thermogenesis per se may have a primary role in the development of hyperinsulinemia and insulin resistance during catch-up growth and underscore a role for suppressed thermogenesis directed specifically at catch-up fat in the link between catch-up growth and chronic metabolic diseases.

Skeletal muscle mitochondrial efficiency and uncoupling protein 3 in overeating rats with increased thermogenesis

To establish whether changes in skeletal muscle mitochondrial efficiency contribute to increased energy expenditure and decreased metabolic efficiency of overeating rats with increased thermogenesis, we measured basal proton leak, fatty acid-induced uncoupling and uncoupling protein 3 (UCP3) content in subsarcolemmal and intermyofibrillar skeletal muscle mitochondria. Intermyofibrillar, but not subsarcolemmal, mitochondria from rats with increased thermogenesis exhibited a lower proton leak compared with controls. In both mitochondrial populations from rats with increased thermogenesis, fatty acid-induced uncoupling was increased significantly and a small recoupling effect of GDP was detected. In addition, intermyofibrillar and subsarcolemmal mitochondria from rats with increased thermogenesis showed higher UCP3 contents than controls. These results point out that metabolic efficiency in subsarcolemmal and intermyofibrillar mitochondria from rats with increased thermogenesis is differently regulated. In fact, in intermyofibrillar mitochondria both basal proton leak and fatty acid-induced uncoupling are altered, while in subsarcolemmal mitochondria only fatty acid-induced uncoupling increases. Both mitochondrial populations in skeletal muscle cells from rats with increased thermogenesis display an increased fatty acid-induced uncoupling and UCP3 content, which could contribute to avoiding obesity.

Skeletal muscle heterogeneity in fasting-induced upregulation of genes encoding UCP2, UCP3, PPARgamma and key enzymes of lipid oxidation

The uncoupling protein homologs UCP2 and UCP3 have been proposed as candidate genes for the regulation of lipid metabolism. Within the context of this hypothesis, we have compared, from fed and fasted rats, changes in gene expression of skeletal muscle UCP2 and UCP3 with those of carnitine palmitoyltransferase I and medium-chain acyl-CoA dehydrogenase, two key enzymes regulating lipid flux across the mitochondrial beta-oxidation pathway. In addition, changes in gene expression of peroxisome proliferator-activated receptor gamma, a nuclear transcription factor implicated in lipid metabolism, were also investigated. The results indicate that in response to fasting, the mRNA levels of UCP2, UCP3, carnitine palmitoyltransferase I and medium-chain acyl-CoA dehydrogenase are markedly increased, by three- to sevenfold, in the gastrocnemius and tibialis anterior (fast-twitch muscles, predominantly glycolytic or oxidative-glycolytic), but only mildly increased, by less than twofold, in the soleus (slow-twitch muscle, predominantly oxidative). Furthermore, such muscle-type dependency in fasting-induced transcriptional changes in UCP2, UCP3, carnitine palmitoyltransferase and medium-chain acyl-CoA dehydrogenase persists when the increase in circulating levels of free fatty acids during fasting is abolished by the anti-lipolytic agent nicotinic acid - with blunted responses only in the slow-twitch muscle contrasting with unabated increases in fast-twitch muscles. Independently of muscle type, however, the mRNA levels of peroxisome proliferator-activated receptor gamma are not altered during fasting. Taken together, these studies indicate a close association between fasting-induced changes in UCP2 and UCP3 gene expression with those of key regulators of lipid oxidation, and are hence consistent with the hypothesis that these UCP homologs may be involved in the regulation of lipid metabolism. Furthermore, they suggest that in response to fasting, neither the surge of free fatty acids in the circulation nor induction of the peroxisome proliferator-activated receptor gamma gene may be required for the marked upregulation of genes encoding the UCP homologs and key enzymes regulating lipid oxidation in fast-twitch muscles.

Pathways from weight fluctuations to metabolic diseases: focus on maladaptive thermogenesis during catch-up fat

It has long been known that obesity is a high risk factor for cardiovascular diseases. In more recent years, the analysis of several large epidemiological databases has also revealed that, independently of excess weight, large fluctuations in body weight at some point earlier in life represent an independent risk factor for type 2 diabetes and hypertension-two major contributors to cardiovascular diseases. High cardiovascular morbidity and mortality have indeed been reported in men and women who in young adulthood experienced weight fluctuations (involving the recovery of body weight after weight loss due to disease, famine or voluntary slimming), or when weight fluctuations occurred much earlier in life and involved catch-up growth after fetal or neonatal growth retardation. This paper addresses the pathways from weight fluctuations to chronic metabolic diseases by focusing on the phenomenon of accelerated fat recovery (ie catch-up fat) after weight loss or growth retardation. Arguments are put forward that, during catch-up growth or weight recovery on our modern refined foods, the mechanisms of adaptive thermogenesis that regulate catch-up fat are pushed beyond the limits for which they were meant to operate and turn maladaptive. The consequences are enhanced susceptibilities towards skeletal muscle insulin resistance and overactive sympathetic activity, both of which are major contributors to the pathogenesis of chronic metabolic diseases. Since weight fluctuation earlier in life (independently of excess weight later in life) is an independent risk factor for metabolic diseases, the mechanisms by which body fat is acquired would seem to be at least as important as the consequences of excess fat per se in the pathogenesis of diabetes, hypertension and cardiovascular diseases.

Leptin directly stimulates thermogenesis in skeletal muscle

Using a method involving repeated oxygen uptake (MO(2)) determinations in skeletal muscle ex vivo, the addition of leptin was found to increase MO(2) in soleus muscles from lean mice. These effects were found to be inhibited by phosphatidylinositol 3-kinase inhibitors, absent in muscles from obese Lepr(db) mice which have the dysfunctional long form of leptin receptor, and blunted in muscles from diet-induced obese mice in the fed state but not during fasting. These findings indicate that leptin has direct thermogenic effects in skeletal muscle, and that these effects require both the long form of leptin receptors and phosphatidylinositol 3-kinase signalling.

An adipose-specific control of thermogenesis in body weight regulation

Much of our understanding about 'adaptive thermogenesis' as a control system in mammalian weight regulation derives from studies of experimental starvation and overfeeding, and these have served to characterize its functional role as an 'attenuator' of energy imbalance. By applying a system-analysis approach in evaluating data on the energetics of starvation and refeeding, evidence is presented here in support of the hypothesis that there are in fact two distinct control systems underlying adaptive thermogenesis. In one of them, the efferent limb is primarily under the control of the sympathetic nervous system (SNS), whose functional state is dictated by overlapping or interacting signals arising from a variety of environmental stresses, including food deprivation, deficiency of essential nutrients, excess energy intake and exposure to cold or to infections; it is hence referred to as the non-specific control of thermogenesis, and is likely to occur primarily in organs/tissues with a high specific metabolic rate (eg liver, kidneys, brown fat). The other is independent of the functional state of the SNS and is dictated solely by signals arising from the state of depletion of the adipose tissue fat stores; it is hence referred to as the adipose-specific control of thermogenesis, and is postulated to occur primarily in the skeletal muscle. While suppression of this adipose-specific thermogenesis during both starvation and refeeding leads to energy conservation, the energy spared during refeeding is directed specifically at the replenishment of the fat stores, so that it functions as an 'accelerator' of fat recovery. These two distinct control systems for adaptive thermogenesis have been incorporated in a compartmental model of body weight and body composition regulation. This is used to provide a mechanistic explanation as to how, during weight recovery, they can operate simultaneously but in opposite directions--with activation of thermogenesis under non-specific control being energy-dissipating, while suppression of thermogenesis under adipose-specific control being energy-conserving--and could hence explain the paradox of a high efficiency of fat recovery co-existing with an overall state of enhanced thermogenesis and hypermetabolism. Elucidating the components of the adipose-specific control of thermogenesis (ie its sensors, signals and effector mechanisms) will have important implications for our understanding of body composition regulation, and hence for the development of more effective strategies in the management of cachexia and obesity.

Uncoupling protein 3 and fatty acid metabolism

A role for uncoupling protein (UCP) 3 in fatty acid metabolism is reviewed within the context of our proposal, first put forward in 1998, that this homologue of UCP1 may be involved in the regulation of lipids as fuel substrate rather than in the mediation of thermogenesis. Since then, the demonstrations of muscle-type differences in UCP3 gene regulation in response to dietary manipulations (starvation, high-fat feeding) or to pharmacological interferences with the flux of lipid substrates between adipose-tissue stores and skeletal-muscle mitochondrial oxidation are all in accord with this proposed role for UCP3 in regulating lipids as fuel substrate. However, given the current limitations of gene-knockout technology for evaluating/interpreting the functional importance of genes encoding mitochondrial membrane proteins, the transition from 'associative' to 'cause-and-effect' evidence for a physiological role of UCP3 in regulating fatty acid metabolism will have to await the development of assays that are sensitive to changes in UCP3 activity. Furthermore, in evaluating the physiological regulators of UCP3, the available evidence points to the existence of adipose-derived factor(s) which, independently of circulating levels of free fatty acids, initiates events leading to the transcription of genes encoding UCP3 and key enzymes of lipid oxidation in the fast glycolytic or fast oxidative-glycolytic muscles, i.e. in the bulk of the skeletal-muscle mass. It is proposed that in tissues where UCP3 co-exists with UCP2 (skeletal muscle, brown adipose tissue, heart) they may act in concert in the overall regulation of lipid oxidation, concomitant to the prevention of lipid-induced oxidative damage.

Fasting-induced changes in the expression of genes controlling substrate metabolism in the rat heart

During fasting, when overall metabolism changes, the contribution of glucose and fatty acids (FA) to cardiac energy production alters as well. Here, we examined if the heart is able to adapt to such fasting-induced changes by modulation of its gene expression. Rats were fed ad libitum or fasted for 46 h, resulting in reduced circulating glucose levels and a 3-fold rise in FA. Besides changes in the cardiac activity or content of proteins involved in glucose or FA metabolism, mRNA levels also altered. The cardiac expression of genes coding for glucose-handling proteins (glucose transporter GLUT4, hexokinase I and II) was up to 70% lower in fasted than in fed rats. In contrast, the mRNA levels of various genes involved in FA transport and metabolism (FA translocase/CD36, muscle-type carnitine palmitoyl transferase 1, long-chain acyl-CoA dehydrogenase) and of the uncoupling protein UCP-3 increased over 50% in hearts of fasted rats. Surprisingly, mRNA levels of the fatty acid- activated transcription factors PPARalpha and PPARbeta/delta were reduced in hearts of fasted rats, whereas in livers, fasting led to a marked rise in PPARalpha mRNA. Reducing FA levels by nicotinic acid administration during the final 8 h of fasting did not affect the expression of the majority of metabolic genes, but totally abolished the induction of UCP-3. In conclusion, the adult rat heart responds to changes in nutritional status, as provoked by 46 h fasting, through adjustment of glucose as well as FA metabolism at the level of gene expression.

Differences in proton leak kinetics, but not in UCP3 protein content, in subsarcolemmal and intermyofibrillar skeletal muscle mitochondria from fed and fasted rats

We have investigated the effect of 24-h fasting on basal proton leak and uncoupling protein (UCP) 3 expression at the protein level in subsarcolemmal and intermyofibrillar skeletal muscle mitochondria. In fed rats, the two mitochondrial populations displayed different proton leak, but the same protein content of UCP3. In addition, 24-h fasting, both at 24 and 29 degrees C, induced an increase in proton leak only in subsarcolemmal mitochondria, while UCP3 content increased in both the populations. From the present data, it appears that UCP3 does not control the basal proton leak of skeletal muscle mitochondria.

Uncoupling proteins: their roles in adaptive thermogenesis and substrate metabolism reconsidered

During the past few years, there have been two major developments, if not revolutions, in the field of energy balance and weight regulation. The first at the molecular level, which was catalysed by developments in DNA screening technology together with the mapping of the human genome, has been the tremendous advances made in the identification of molecules that play a role in the control of food intake and metabolic rate. The second, at the systemic level, which centered upon the use of modern technologies or more robust analytical techniques for assessing human energy expenditure in response to starvation and overfeeding, has been the publication of several papers providing strong evidence that adaptive thermogenesis plays a much more important role in the regulation of body weight and body composition than previously thought. Within these same few years, several new members of the mitochondrial carrier protein family have been identified in a variety of tissues and organs. All apparently possess uncoupling properties in genetically-modified systems, with two of them (uncoupling protein (UCP) 2 and UCP3) being expressed in adipose tissues and skeletal muscles, which are generally recognised as important sites for variations in thermogenesis and/or in substrate oxidation. Considered as breakthrough discoveries, the cloning of these genes has generated considerable optimism for rapid advances in our molecular understanding of adaptive thermogenesis, and for the identification of new targets for pharmacological management of obesity and cachexia. The present paper traces first, from a historical perspective, the landmark events in the field of thermogenesis that led to the identification of these genes encoding candidate UCP, and then addresses the controversies and on-going debate about their physiological importance in adaptive thermogenesis, in lipid oxidation or in oxidative stress. The general conclusion is that UCP2 and UCP3 may have distinct primary functions, with UCP3 implicated in regulating the flux of lipid substrates across the mitochondria and UCP2 in the control of mitochondrial generation of reactive oxygen species. The distinct functions of these two UCP1 homologues have been incorporated in a conceptual model to illustrate how UCP2 and UCP3 may act in concert in the overall regulation of lipid oxidation concomitant to the prevention of lipid-induced oxidative damage.

Uncoupling proteins: their roles in adaptive thermogenesis and substrate metabolism reconsidered

During the past few years, there have been two major developments, if not revolutions, in the field of energy balance and weight regulation. The first at the molecular level, which was catalysed by developments in DNA screening technology together with the mapping of the human genome, has been the tremendous advances made in the identification of molecules that play a role in the control of food intake and metabolic rate. The second, at the systemic level, which centered upon the use of modern technologies or more robust analytical techniques for assessing human energy expenditure in response to starvation and overfeeding, has been the publication of several papers providing strong evidence that adaptive thermogenesis plays a much more important role in the regulation of body weight and body composition than previously thought. Within these same few years, several new members of the mitochondrial carrier protein family have been identified in a variety of tissues and organs. All apparently possess uncoupling properties in genetically-modified systems, with two of them (uncoupling protein (UCP) 2 and UCP3) being expressed in adipose tissues and skeletal muscles, which are generally recognised as important sites for variations in thermogenesis and/or in substrate oxidation. Considered as breakthrough discoveries, the cloning of these genes has generated considerable optimism for rapid advances in our molecular understanding of adaptive thermogenesis, and for the identification of new targets for pharmacological management of obesity and cachexia. The present paper traces first, from a historical perspective, the landmark events in the field of thermogenesis that led to the identification of these genes encoding candidate UCP, and then addresses the controversies and on-going debate about their physiological importance in adaptive thermogenesis, in lipid oxidation or in oxidative stress. The general conclusion is that UCP2 and UCP3 may have distinct primary functions, with UCP3 implicated in regulating the flux of lipid substrates across the mitochondria and UCP2 in the control of mitochondrial generation of reactive oxygen species. The distinct functions of these two UCP1 homologues have been incorporated in a conceptual model to illustrate how UCP2 and UCP3 may act in concert in the overall regulation of lipid oxidation concomitant to the prevention of lipid-induced oxidative damage.