Shivering thermogenesis in humans: Origin, contribution and metabolic requirement

ABSTRACT

As endotherms, humans exposed to a compensable cold environment rely on an increase in thermogenic rate to counteract heat lost to the environment, thereby maintaining a stable core temperature. This review focuses primarily on the most important contributor of heat production in cold-exposed adult humans, shivering skeletal muscles. Specifically, it presents current understanding on (1) the origins of shivering, (2) the contribution of shivering to total heat production and (3) the metabolic requirements of shivering. Although shivering had commonly been measured as a metabolic outcome measure, considerable research is still needed to clearly identify the neuroanatomical structures and circuits that initiate and modulate shivering and drives the shivering patterns (continuous and burst shivering). One thing is clear, the thermogenic rate in humans can be maintained despite significant inter-individual differences in the thermogenic contribution of shivering, the muscles recruited in shivering, the burst shivering rate and the metabolic substrates used to support shivering. It has also become evident that the variability in burst shivering rate between individuals, despite not influencing heat production, does play a key role in orchestrating metabolic fuel selection in the cold. In addition, advances in our understanding of the thermogenic role of brown adipose tissue have been able to explain, at least in part, the large inter-individual differences in the contribution of shivering to total heat production. Whether these differences in the thermogenic role of shivering have any bearing on cold endurance and survival remains to be established. Despite the available research describing the relative thermogenic importance of shivering skeletal muscles in humans, the advancement in our understanding of how shivering is initiated and modulated is needed. Such research is critical to consider strategies to either reduce its role to improve occupational performance or exploit its metabolic potential for clinical purposes.

KEYWORDS:

• thermogenesis
• shivering
• skeletal muscle
• energy metabolism
• electromyography
• carbohydrate utilization
• fatty acid oxidation

Endotherms face the vital challenge of maintaining an elevated and constant core temperature (T_core at 36–38°C) independently of climatic fluctuations. In addition to developing adequate behaviors to help maintain T_core, evolution has resulted in key biochemical, physiologic and morphological adaptations to modulate rates of heat loss (H_loss) and heat production (H_prod) in response to various levels of thermal stress. On both extremes of the spectrum, these adaptations have allowed some endotherms to optimize life in the hottest climates on Earth (e.g., high sweat rate, high surface-to-volume ratio), while others have evolved to thrive the most extreme cold conditions (i.e., high body insulation, low surface-to-volume ratio, highly thermogenic tissues). Within this thermoregulatory continuum, humans are generally well adapted for dissipating heat in warm climates but are particularly maladapted at conserving it in the cold. In fact, without proper shelter and/or clothing, cold exposure can rapidly result in temporary loss of function, permanent cell damage or even death. When ambient temperature decreases, the increase in H_loss experienced by individuals is (1) attenuated by a peripheral vasoconstriction and (2) compensated by an increase in the rate of heat production (H_prod) from the activation of nonshivering (NST) and shivering (ST) thermogenic processes. In this review, we focus mainly on the cold-induced activation of involuntary muscle contractions or ST, the main contributor of heat in cold-exposed adult humans. Building on previous reviews on this topic, we present recent findings on the: (1) origins of ST, (2) contribution of ST to total H_prod and (3) metabolic requirements of ST. We also bring forth new findings on the role played by NST mainly through the activation of brown adipose tissue (BAT). Novel research has shown that the activation of highly thermogenic BAT may explain in part the large inter-individual variability in ST response observed in cold exposed humans.

Origins of shivering

Establishing the origins of involuntary muscle contractions during ST in endotherms has been the subject of numerous studies in this field of physiology. Despite the breadth of research available, little is still known regarding the processes that regulate the initiation and control of ST. However, over the last decades, significant progress has been made in characterizing the central thermoregulatory circuits as a whole, using a variety of in situ and in vivo techniques. These include trans-synaptic retrograde and anterograde tracing techniques, neural substrate metabolism, electrophysiology, direct electrical brain stimulation, pharmacological stimulation and inhibition of neural pathways in various animal models (see ref. 1 for review) and using imagery techniques, including functional magnetic resonance imaging (fMRI)^2–5 and positron emission tomography/computerized tomography (PET/CT)^6–8 in humans. From these studies, it has been possible to derive the principal anatomic structures and neurophysiological substrates that make up the central thermoregulatory circuits. Figure 1 provides a general overview of the core pathways providing thermoregulatory control, including the sensory afferent axis (green boxes), the thermoregulatory control center and the efferent pathway (red box). This schematic reflects one proposed model, which posits that deep body temperature is perpetually controlled via a negative feedback loop, whereby deep (core) temperature serves as both the control variable (T_core) and the predominant feedback signal (i.e., from temperature-sensitive neurons in the brain or transient receptor potential (TRP) cation channels in the brain, spinal cord and viscera), while skin temperature provides a rapidly-responding auxiliary feedback signal (from TRP found in epidermis), providing negative or positive control.^9,10 Others suggest that thermoregulatory responses are under feedforward control driven by changes in skin temperature, thereby activating cold-defense effectors before any detectable change in T_core can occur.^11,12

Figure 1. Simplified schematic representation of cold-induced thermogenesis. Afferent input represented by green boxes, efferent output represented by red box, thermoregulatory control center (hypothalamus) and integrated response (core temperature) represented by black boxes. Model adapted from refs. 9,10,12.

Regardless of the preferred model used to illustrate this thermoregulatory control, it has become evident that each cold-defense effector response (i.e., vasomotor tone, BAT, ST) is independently controlled, with each effector being driven by different combinations of T_core and T_skin inputs. Further, recent evidence in mice suggests that under cooling conditions, it is the change in T_skin rather than the absolute T_skin that leads to the stimulation of temperature-sensing spinal dorsal root ganglia (DRG) neurons.¹³ How this ultimately influences the recruitment of each cold-defense thermoeffector or whether these cold-responding neurons also detect changes rather than absolute temperature in humans still remains to be determined. There are some indications in humans and monkeys that both the peripheral and central cold-sensitive receptors exhibit a vigorous increase in nerve impulse activity upon a decrease in T_skin followed by a steady-state continuous discharge when the temperature is held constant,^14,15 demonstrating an acute habituation effect. Both steady-state and nonsteady-state firing patterns can also be seen in the activation of the effector responses to an innocuous cold stimulus. Upon cooling in humans, ST electromyography (EMG) increases dramatically, before stabilizing to a lower amplitude.¹⁶ Unfortunately, in humans, brain mapping and functional imaging of the central control of thermoregulation has remained limited and somewhat controversial. Nevertheless, EMG measurements have been able to characterize distinct and important muscle recruitment patterns indicative of large inter-individual differences in human ST response.

Traditional methods for assessing changes in muscle recruitment during ST have included indwelling and surface EMG. From these electrophysiological signals, ST intensity and muscle recruitment pattern can be quantified and compared at different levels of cold exposure as well as between treatments and individuals (for example, ref. 17). At the whole-body level, as average T_skin and T_core decrease, ST intensity increases mainly in proximal trunk and leg muscles, whereas arm and lower leg muscles contribute little to H_prod.^18,19 When this increase in ST intensity and associated increase in H_prod is sufficient to compensate fully for the increase in H_loss, T_core remains fully defended as long as this compensation is maintained.^20–22 Changes in H_prod and T_core as a function of alterations in mean T_skin under such compensable cold conditions are presented in Figure 2. It is important to note that these changes in ST and H_prod can occur quickly when T_skin is modified.¹⁶ During uncompensable cold exposure, where H_loss surpasses the increase in ST intensity and associated increase in H_prod, both changes in T_skin and T_core become the ultimate driving force for the thermogenic response.²³ As T_core decreases to ∼35°C, increases in H_prod through ST can reach maximal values equivalent to ∼5 times resting metabolic rate (xRMR) or ∼40% of maximal oxygen consumption (VO₂max).^23,24 For example, when individuals were rewarmed following a 45 min cold water immersion at 7°C where average T_skin and T_core were reduced to ∼10°C and ∼34.5°C, respectively, H_prod reached maximal ST intensity.²⁴ Most importantly, if the decrease in T_core continues without compensation, ST stimulation is reduced or halted once a threshold T_core is reached (∼31°C;²⁵). Evidently, this reduction in ST and associated decrease in H_prod results in an even quicker reduction in T_core. Under such advanced stage of hypothermia, rewarming is not possible without an external source of heat. Exact physiologic reasons why the ST response is inhibited at a specific T_core and why maximal thermogenesis is three to four times lower during shivering compared with the metabolic rate attainable from muscle contractions through exercise are far from being well understood. However, maximal ST intensity reached by humans is consistent with values observed in avian species and large mammalian species, which rely almost entirely on this mode of H_prod during cold exposure.^26,27 Since high ST intensities interfere with voluntary movements,^28,29 it could be speculated that lower ST intensities produced sufficient heat to increase odds of survival without compromising locomotion and/or cold survival.

Figure 2. Changes in shivering intensity as a function of changes in mean skin temperature measured under compensable conditions (no change in T_core). Data adapted from refs. 20–22, 42, 43, 52, 56, 68.

On the methodological front, indwelling and surface EMG only provide superficial information on the recruitment of muscles and thus, cannot inform researchers on the role played by deeper parts of the muscles being monitored or by deep muscles. To assess the potential contribution of deep muscles to whole-body ST, Blondin et al.¹⁹ combined PET with ¹⁸fluro-dexoxyglucose (¹⁸FDG PET) and surface EMG. Using this approach, it was shown that, in superficial muscles, ST intensity measured by EMG covaried closely with increases in ¹⁸FDG uptake quantified by ¹⁸FDG PET. Examining deep muscles, this approach provided the first estimates of the activation of ST from changes in ¹⁸FDG uptake. Results showed that ¹⁸FDG uptake increased in deep muscles especially in the neck area (i.e., longus colli) and are therefore more active than many other surface muscles during ST. While it is well established that distal muscles contribute less to total ST, this new evidence indicates that both surface and deep proximal trunk muscles are involved in sustaining total H_prod in the cold.

Despite a general pattern emerging using ¹⁸FDG PET, muscle recruitment patterns are highly variable, even among morphologically similar men and women.^18,30–34 Indeed, cold exposure studies have shown that for the same given H_prod, some individuals rely almost entirely on upper body muscles, while others depend more on upper leg muscles.^17,18,30,31 Also, important inter-individual differences in motor unit recruitment have also been reported within the same muscles. Quantification of shivering EMG has revealed the presence of two distinct muscle recruitment patterns based on differences in intensity (2–5 versus 7–15% of maximal voluntary contractions (%MVC)) and rate of occurrence (8–10 versus 0.1–0.2 Hz).^31,35 Continuous, low-intensity ST is associated with the recruitment of type I fibers, whereas high-intensity bursts are linked with the recruitment of type II fibers.²⁸ The proportion of burst shivering rate to total ST activity is highly variable between muscles and between individuals for reasons that remain unknown. However, recently, Blondin et al.³⁶ have indicated that burst shivering rates are related in part to differences in muscle fiber composition between individuals. This observation is consistent with what is found in birds, where burst activity has also been linked to the fiber composition of a specific muscle.^37,38 Whether these differences in the proportion of bursts shivering to continuous shivering provide survival advantage remains unclear. In humans, ST research has consistently shown that thermogenic response remains unchanged even when different muscles are recruited and/or when high proportion of burst shivering activity is observed.^17,19,21,36 However, differences in burst shivering activity have important effects on whole body fuel selection and potentially on the depletion of carbohydrate (CHO) reserves (see Metabolic requirements of ST).

Contribution of ST to total heat production

Heat is a by-product of all exothermic biochemical reactions, which includes the combustion of metabolic substrates. As mentioned above, during cold exposure, metabolic processes are activated to stimulate H_prod in an attempt to prevent substantial decreases in T_core. Of all cold-activated mechanisms, the highest amount of heat is obtained by muscle contractions during voluntary movements or exercise. From 4 to 6 kJ min⁻¹ at RMR, humans can reach rates of thermogenesis 15 to 20-fold higher during maximal exercise. However, when exercise is not possible or advisable (e.g., overexertion, limited food supply, injury, risk of becoming disoriented), ST become the most important line of defense for compensating for increases in H_loss.

To assess differences in cold stress between various studies, ST responses may be characterized based on differences in thermogenesis from RMR to cold-induced MR. As such, changes in cold exposure can be reported as mild (1.0–2.0 xRMR; e.g., 19, 20, 22, 36, 39, 40–51), moderate (2.0–3.0; e.g., 34, 52, 53–61) and high (3.0–maximum; e.g., 21, 24, 62–64) (Figure 3). Of great importance, cold exposure intensities below moderate intensity for 2–3 h tend to be compensable, whereas high-intensity cold exposure may lead to a rapid decrease in T_core. In theory, under compensable conditions, T_core can be maintained as long as cold-induced H_prod is sustained. For example, during mild cold exposure in a thermal chamber at 7.5°C, we showed that T_core, H_prod and ST intensity at ∼1.8 xRMR can be sustained in men for up to 24 h independently of a negative energy balance (∼44% less than 24 h energy demand) and independently of a substantial change in metabolic fuel utilization⁶⁵ (see also Metabolic requirements of ST). However, it remains that even under this compensable condition, only four of the eight recruited subjects were willing to remain in the cold for the full 24 h. This exemplifies that the challenges for human survival even under mild, compensable conditions extend far beyond sustaining adequate thermogenic and physiologic responses.

Figure 3. Changes in thermogenic rate (times resting metabolic rate (xRMR)) as a function of changes in average skin temperature found in various cold exposure studies in lean healthy men. Thermogenic rates are characterized as mild (1–2 xRMR), moderate (2–3 xRMR) and high (>3 xRMR). Points represent data from end of cold exposure for following studies.^{19–22,24,34,36,39–64}

Above in Origins of ST, we reported that substantial inter-individual variability in ST muscle recruitment patterns (i.e., group of muscles recruited, rate of burst shivering activity) is found in humans. Adding to the complexity of these individual differences, recent work has also shown that the contribution of ST to total H_prod varies greatly between individual and is modulated by the presence of other thermogenic sources, such as BAT.²⁰ This tissue is essential to sustain endothermia, especially in newborn and small mammals. However, in humans, highly thermogenic BAT was thought to disappear after the first years of life. Recent studies have shown that not only is BAT present, but it is also metabolically active in some individuals.²⁰ Most importantly, Ouellet et al.²⁰ demonstrated that the presence of BAT was inversely correlated with whole-body ST activity in men. While it is clear that BAT contributes to NST in adult humans, its contribution to H_prod is unclear. Further work is also needed to determine the effects of cold acclimation or the administration of specific food (e.g., green tea, capsinoids) or pharmaceutical (e.g., nicotinic acid, β blockers) compounds in modulating the contribution of ST to total H_prod.

Metabolic requirements of ST

Combined CHO, lipids and proteins oxidation provide the necessary substrates to sustain ST (see 32, 66 for review). Metabolic research in the cold has clearly shown that the respective contribution of each fuel to ST can be highly variable and will depend on differences in the status of CHO reserves,¹⁷ in burst shivering rate³¹ and in the availability of exogenous substrates.^53–55 Over the last decades, metabolic research in the cold has focused on understanding whether, like exercise, the depletion of CHO reserves could limit ST endurance.⁶⁷ However, to date, mounting evidence indicates that adult humans are able to sustain ST and whole-body thermogenesis using a wide variety of metabolic fuels. When one fuel source is depleted or reduced, others compensate to maintain ATP production, ST intensity and whole body H_prod. The following sections overview the respective contribution of plasma glucose, muscle glycogen, lipids and proteins at various shivering intensities for different shivering patterns and with different levels of CHO availability.

Shivering intensity. As mentioned above, ST intensity is modulated by reducing mean T_skin and/or T_core. As ST intensifies, muscles rely progressively more on CHO, whereas the relative contribution of both lipids and proteins is reduced. The relative contribution of CHO to total thermogenic rate varies between ∼20% and 80% H_prod (see ³² for review) with rates of oxidation ranging from ∼130 to 500 mg kg⁻¹ h⁻¹.^21,34 The highest rates of whole-body CHO oxidation were found during passive rewarming, where men recovered passively from a T_core of ∼34.5–36.5°C following a 7°C water immersion.²⁴ Under these extreme conditions, CHO oxidation rates reached as much as ∼1.5 g min⁻¹ and accounted for three quarters of all the heat produced. It is also important to note that under all reported cold conditions, muscle glycogen reserves provide as much as ∼75–80% of the total glucose needed to sustain ST while the contribution of plasma glucose (hepatic) remains limited to ∼20–25% of total CHO oxidation.^21,52,68 Therefore, similar to exercise, muscle glycogen is always the greatest source of glucose for sustaining whole-body CHO oxidation and energy demands in the cold.

Lipids also play a key role to sustain H_prod in the cold. As previously stated, the relative contribution of lipids dominates at all intensities below 50% of maximal shivering intensity (∼20% VO₂max). However, for reasons that remain unknown, the oxidation rate of lipids rises to ∼140 mg kg⁻¹ h⁻¹ and remains constant as shivering intensifies. This maximal lipid oxidation rate in the cold is ∼3 times lower than what is found during exercise at a similar metabolic rate.⁶⁹ In women, lipid oxidation rates were reported to be higher, ranging between ∼190 and 200 mg kg⁻¹ h⁻¹ in the luteal and follicular phases of the menstrual cycle.³⁴ Consequently, in women, the relative contributions of lipids to total heat production were higher at both menstrual phases (∼75% H_prod) compared with men (∼50% H_prod).

Fatty acids are obtained from the lipolysis of triacylglycerol (TAG) stores located in adipose tissue, in the liver or in muscles. In the cold, the relative contribution of each compartment is still unknown. Using stable isotope tracer methods, Vallerand et al.⁵⁹ and Ouellet et al.²⁰ showed that the turnover rate of circulating fatty acids and rates of lipolysis increase proportionally to the increase in metabolic rate found during mild cold exposure. This indicates that circulating fatty acids play an important role to sustain ATP production in the cold. However, two earlier studies by Martineau and Jacobs^70,71 identified the effects of a reduction in plasma fatty acids availability in men with normal or reduced CHO reserves immersed at 18°C for 90 min. In both studies, the reduction in circulating fatty acids was induced by oral nicotinic acid administration (3.2 mg kg⁻¹ as niacin), an inhibitor of intracellular TG lipolysis, given before and during cold exposure. When fatty acid supply was suppressed in men with normal glycogen reserves, results showed that heat production remained unchanged presumably by increasing the use of intra-muscular lipids and glycogen reserves.⁷⁰ Similarly, in the second study, where both plasma fatty acid availability and glycogen reserves were reduced, changes in H_prod, immersion times and rectal temperature were not different from results found in CHO loaded and normal plasma fatty acid treatments.⁷¹ More recently, we showed that suppressing intracellular lipolysis by ingesting nicotinic acid, suppresses BAT oxidative metabolism, increases ST and alters fuel selection by increasing CHO utilization.³⁶ Independently of these important modifications in NST/ST responses and fuel selection, H_prod remained constant and unaffected by nicotinic acid. This further highlights the great versatility for sustaining thermogenic rate in cold exposed humans.

Finally, for proteins, it was customary, as for exercise, to assume that involuntary muscle contractions during ST were exclusively sustained by the oxidation of CHO and lipids. The contribution of protein oxidation was thought negligible at best. Proteins generally contribute as much as plasma glucose at low ST intensities (∼10% H_prod⁵²) and their role decreases progressively as ST intensifies.²¹ The initial contribution of this fuel to total ST is directly related to the individual's average dietary protein consumption. High protein intake will result in a higher initial contribution of proteins to total energy budget. In addition, as described later, any reductions in CHO availability have been shown to substantially increase the role played by proteins in the cold.⁶⁸

Shivering pattern. Previous work has shown that ST pattern and burst shivering activity does not modify H_prod but they can play a key role in orchestrating fuel selection.³¹ At a shivering intensity of ∼3.5 xRMR, Haman et al.³¹ reported that fuel selection in men ranged from 33% to 78% ${\dot{\text{H}}}_{\text{prod}}$ for CHO and from 14% to 60% ${\dot{\text{H}}}_{\text{prod}}$ for lipids. Detailed EMG analysis in eight large muscles revealed that burst shivering activity also exhibited large variability among individuals ranging from ∼2 to 8 bursts per minutes. Because burst activity was previously associated with the recruitment of glycolytic type II muscle fibers,^28,29 it was assumed that whole body CHO oxidation rate would be correlated with burst shivering activity. Results showed that both muscle glycogen and whole-body CHO oxidation co-vary closely with individual variations in burst activity, whereas the use of plasma glucose does not. This demonstrates that individuals with higher burst rates not only oxidize more glucose at the whole body level but would deplete muscle glycogen faster than the ones that display low bursting activity at the same given shivering intensity. However, it remains unclear whether CHO are essential for sustaining shivering for prolonged exposures.

CHO availability. Of all metabolic fuel stores, CHO account for only ∼1% of total reserves but still represent a large fraction of all the heat produced in the cold. Three independent studies modified CHO availability artificially by dietary and exercise manipulations to determine whether endogenous CHO reserves were essential to sustain ST. Following this decrease and increase in CHO stores, men were exposed to either moderate 2.5 xRMR³¹ or to high cold exposure 3.5 xRMR.^64,72 While in all studies glycogen depletion and loading caused a respective large shift in fuel use from lipid dominance (up to ∼80% ${\dot{\text{H}}}_{\text{prod}}$) to CHO dominance (up to ∼80% ${\dot{\text{H}}}_{\text{prod}}$), changes in T_core and T_skin remained unaffected by the modification in CHO availability. This indicates that decreases and increases in CHO availability result in a respective up- or downregulation in the use of lipids and proteins. This metabolic flexibility may not be surprising considering the relatively low metabolic rates achieved in the cold. Adding to this conclusion, Haman et al.^17,68 showed that, during mild shivering, humans are able to sustain a constant thermogenic rate by oxidizing these widely different fuel mixtures without modifying EMG ST pattern or muscle fiber recruitment (i.e., intensity, burst versus continuous ST). Therefore, the drastic switch in fuel metabolism found as a result of glycogen depletion and loading (CHO, ∼28 versus 65%; lipids, ∼53 versus 23%; proteins, ∼19 versus 12% ${\dot{\text{H}}}_{\text{prod}}$, respectively) can be maintained within the same muscle fibers. These results emphasize the importance of proteins and lipids in compensating for large decreases in CHO availability and indicate that CHO might not be essential to sustain ATP production during ST.

It is important to note that before obtaining detailed quantifications of the partitioning of CHO reserves for H_prod in the cold, Wissler⁶⁷ had proposed an elaborate model to predict shivering endurance based on empirical observations from Beckman, Reeves and Goldman.⁷³ In his model, shivering fatigue was determined from the onset of muscle cramping observed in a group of men exposed to 24°C water. Under conditions previously describe in glycogen depleted and glycogen loaded individuals,⁶⁸ the model of Wissler predicts that shivering at 200 W could be sustained for 33 to 42 h. In addition, Tikuisis et al.⁷⁴ later suggested that the Wissler model may underestimate true values. In a more recent paper, Haman et al.⁶⁵ estimated a 20 h time to glycogen depletion based on whole-body oxidation of glycogen; a value clearly shorter than that predicted by Wissler et al.⁶⁷ or Tikuisis et al..⁷⁴ Again, calculations of ST endurance based on time to glycogen depletion have two possible shortcomings: (1) glycogen may not be essential, and low-intensity shivering is sustainable solely on lipids and proteins and/or, (2) a significant shift in fuel selection to spare glycogen takes place after 2 h of shivering (in fact, a progressive increase in fat oxidation was observed by Tikuisis et al.⁷⁴ during prolonged shivering lasting for up to 4 h). No detailed information is currently available on fuel selection for shivering in excess of 4 h in fasted individual, and it is still unclear whether glycogen depletion coincides with muscle fatigue. Future studies should address this interesting problem. At the moment, however, research has demonstrated that lipid and protein oxidation can compensate for any reductions in CHO availability over a period of 90–120 min at shivering intensities ranging between ∼2.5 and 3.5 xRMR. Of course, longer exposure times and different shivering intensities are needed to validate this assumption.

Conclusion

This review focused on ST and presented the origins, contribution to total H_prod and metabolic requirements of this key thermogenic process for human survival in the cold. While much work remains to clearly understand the origins of the dual ST pattern (continuous and burst ST), research has shown that differences in burst shivering rate between individuals does not seem to influence H_prod but does play a key role in orchestrating metabolic fuel selection in the cold. In addition, advances in our understanding of the thermogenic role of BAT have been able to explain at least in part the large inter-individual differences in the contribution of ST to total H_prod. Whether these differences in the thermogenic role of ST have any bearing on cold endurance and survival remains to be established. However, it seems that under compensable conditions, fasted humans can sustain ST and H_prod using a variety of metabolic fuels and changes in endogenous CHO stores does not alter H_prod at low to moderate intensity for up to 4h. Clearly, improving the fundamental knowledge of the role played by muscles, BAT and other metabolic processes in sustaining thermogenic rate in cold humans is extremely important to provide additional breakthroughs in this field of research.

Disclosure of potential conflicts of interest

No potential conflicts of interest were disclosed.