Postprandial thermogenesis has been proposed to be an important contributor to body weight regulation, particularly in light of the fact that increases in energy expenditure induced by a meal are blunted in obesity (12, 14–28); however, the mechanism by which meal-induced thermogenesis occurs in lean individuals but is blunted in obese subjects remains poorly understood.

Sympathetic activation has been correlated with increases in body temperature in numerous prior studies (6, 9, 41, 62–64), but the mechanism by which sympathetic activity is stimulated postprandially and the mechanism by which it may alter energy expenditure remain under debate. Our study establishes that increases in body temperature following meal ingestion, which mostly reflect increases in energy expenditure — due to increased mitochondrial uncoupling by UCP1 activation in brown and beige adipose tissue (65–71) and possibly sarcolipin induction in skeletal muscle (69–71) — can be dissociated under certain conditions from energy expenditure. It is likely that the two processes, postprandial increases in body temperature and postprandial increases in energy expenditure, are regulated independently and that their dissociation is not the result of a specific regulatory action, but rather the divergence of two independent regulatory mechanisms. The current study demonstrates that meals comprising carbohydrate or protein, but not fat, increase plasma leptin, catecholamines, and body temperature; however, all meals increase energy expenditure, as assessed by indirect calorimetry. These data therefore argue against the hypothesis that meal thermogenesis and postprandial energy expenditure are components of the same phenomenon (33).

In order to elucidate the mechanism of feeding-induced increases in body temperature, we first hypothesized that a postprandial rise in temperature would be initiated by a physiologic change that signals to the body that substrate has been provided. We hypothesized that the increase in plasma leptin concentrations that has been shown in numerous studies to occur upon refeeding, reversing the starvation-induced hypoleptinemia that occurs in lean animals and humans (72, 73), may provide this signal. Consistent with this hypothesis, an increase in plasma leptin concentrations was associated with increased body temperature in refed rats (Figure 1D); and an infusion of recombinant leptin to increase plasma leptin concentrations to physiologic levels observed in refed rats increased body temperature to refed levels (Figure 4C), while pretreatment with a leptin antagonist abrogated feeding-induced increases in body temperature (Figure 3H), demonstrating a causal link between leptin and postprandial increases in body temperature. To confirm this hypothesis, we employed inducible fat-specific OGT KO mice. Mice with lifelong deletion of this gene in adipose tissue showed reductions in white adipose leptin mRNA and plasma leptin concentrations under conditions of high-fat diet but not chow feeding after a 6-hour fast (74); however, postprandial leptin secretion was not studied in this report. In addition, since it is likely that compensatory mechanisms develop in the setting of lifelong deletion of OGT in adipose tissue, the inducible model offers certain advantages in testing the physiologic impact of postprandial leptin secretion. We show here that OGT FKO mice failed to secrete leptin under hyperglycemic-hyperinsulinemic conditions, and that this lack of glucose/insulin-induced hyperleptinemia was correlated with an absent thermogenic response to glucose. In this model, we show that leptin mediated hyperglycemia-hyperinsulinemia–induced hyperthermia: after a leptin bolus, OGT FKO mice demonstrated a normal increase in body temperature (Supplemental Figure 5, E–I). Similarly, ob/ob mice, which lack the ability to secrete leptin, showed no catecholamine or temperature response to refeeding, but both increased in response to injection with recombinant leptin, again directly implicating leptin in postprandial increases in body temperature (Figure 3, B–D). However, blocking leptin action with a small molecule antagonist reversed the feeding-induced increases in catecholamines and body temperature while increasing food intake and preventing the reversal of hypercorticosteronemia with refeeding (Figure 3, F–H) (54). Taken together, these data indicate that an increase in plasma leptin concentration is both necessary and sufficient to cause increases in body temperature under postprandial conditions and that OGT plays an important role in mediating hyperglycemia-hyperinsulinemia stimulation of WAT leptin secretion. In addition, the similar effect of leptin on body temperature in rats and mice bolsters the potential translatability of our findings.

Next, we investigated the mechanism by which an increase in plasma leptin concentration mediates postprandial increases in body temperature. Feeding-induced increases in plasma leptin concentrations were correlated with increases in plasma catecholamine concentrations: refeeding and leptin infusion increased plasma catecholamine concentrations in sham-operated but not ADX rats (Figure 1, B, C, L and M, Figure 4B, and Supplemental Figure 8E). β 3 -Adrenergic activity in BAT was necessary for increases in body temperature: body temperature of Adrβ 3 BKO mice failed to increase in response to a glucose meal, in contrast to that of their WT littermates (Figure 5F). These data are consistent with previous studies in which β-adrenergic blockade was found to abrogate the postprandial thermogenic response in humans and animals (43, 46, 47, 49, 50, 75–77) but in contrast to similar studies showing no impact of beta blockers on meal thermogenesis (48, 51, 52, 78, 79). The central mechanism of leptin stimulation of β-adrenergic activity was confirmed by ICV injection of leptin: leptin (10 μg) injected into the ICV space did not alter jugular venous plasma leptin concentrations, but it increased plasma catecholamine concentrations and body temperature to postprandial levels (Figure 1E and Supplemental Figure 2, A and F). Although both ICV and systemic leptin infusion, as well as refeeding, increased plasma thyroid hormone concentrations, these effects were dissociated from body temperature: both refeeding and leptin infusion increased T3 and T4 concentrations in ADX rats but failed to increase body temperature. These results support the hypothesis that basal body temperature and energy expenditure are regulated differently from postprandial body temperature and energy expenditure: whereas thyroid function is well established to play an important role in maintenance of basal body temperature and energy expenditure, it appears to play a less important role in the increases in both parameters that occur following a meal. However, it does remain possible that leptin may alter tissue deiodinase activity and therefore conversion of T4 to T3; further studies will be required to address this point. Taken together, these data demonstrate that leptin stimulates adrenomedullary secretion of catecholamines through a CNS-mediated process, likely through signaling in the arcuate nucleus, and that β-adrenergic activity is required for mediation of leptin-induced postprandial increases in body temperature. Surprisingly, both rats treated with an antagonist primarily suppressing β 1 -adrenergic activity and mice lacking the β 3 -adrenergic receptor in BAT showed a similar lack of a temperature response to refeeding, potentially due to β 1 -β 3 crosstalk — particularly in rodents living at subthermoneutrality throughout their lives, which may have beiged WAT sensitive to β 1 — and/or an effect of atenolol to suppress β 3 in addition to β 1 activity (80).

Having established that leptin causes postprandial thermogenesis via increases in β-adrenergic activity, we next asked which tissue(s) are primarily responsible for catecholamines’ effect of stimulating thermogenesis and hypothesized that BAT may be a key mediator of the thermogenic effect of leptin. Consistent with a role for BAT thermogenesis, interscapular BATectomy reduced the thermogenic effect of food and of leptin by approximately 60% (Figure 5B), indicating that interscapular BAT thermogenesis accounts for the majority of the postprandial thermogenic response. Next, we asked whether the stimulation of thermogenesis by catecholamines requires increased adipose tissue lipolysis to provide a substrate for catecholamine-induced BAT thermogenesis. Consistent with a requirement for catecholamine-driven lipolysis to mediate postprandial increases in body temperature, treatment with a small molecule inhibitor of ATGL, atglistatin, abrogated the thermogenic effect of leptin, while infusion of Liposyn/heparin to restore BAT long-chain acyl-CoA concentrations in epinephrine- and atglistatin-treated rats replicated the thermogenic effect of epinephrine (Figure 5E). Taken together, these studies demonstrate that β-adrenergic stimulation of lipolysis is required for leptin-induced, BAT-mediated increases in body temperature. Furthermore, they demonstrate the necessity for leptin-induced stimulation of adrenomedullary secretion of catecholamines in leptin-induced adipose tissue lipolysis as opposed to leptin stimulation of lipolysis through direct innervation of WAT; this was demonstrated by the lower plasma NEFA concentrations observed in ADX-high corticosterone treated rats after refeeding despite identical plasma insulin and corticosterone concentrations but the absence of circulating catecholamines. These data would suggest that direct sympathetic innervation of WAT may be required for tonic, low rates of lipolysis, but not for leptin’s promotion of lipolysis after refeeding. It is also possible that branched-chain amino acids contribute to BAT-mediated increases in temperature, as demonstrated by a recent study (61) and consistent with the reduction in plasma leucine and isoleucine concentrations in leptin-treated and refed rats; however, since atglistatin treatment abrogated the temperature response to epinephrine, it is likely that under fasted-refed conditions, fatty acids derived from WAT lipolysis are the predominant energy source fueling BAT thermogenesis.

As meal thermogenesis itself has been proposed to inhibit food intake (37, 40, 41, 81), we measured refed food intake in rats treated with epinephrine and found that after a 48-hour fast it was markedly reduced in epinephrine-infused rats; however, this effect was not dependent upon increases in body temperature, as demonstrated by the fact that neither BATectomy nor atglistatin treatment with or without fatty acid replacement altered food intake in epinephrine-infused animals (Figure 5F). Taken together, these data demonstrate that catecholamine-induced increases in body temperature, which occur with meal ingestion, can be dissociated from reduction of food intake and that increased circulating catecholamine concentrations per se, but not the resulting increase in body temperature, may mediate the impact of β-adrenergic activity of suppressing food intake. These results also highlight the yin-yang nature of the adrenal gland in the regulation of food intake by leptin: the transition from high (>2.4 ng/mL) to low (<1 ng/mL) plasma concentrations of leptin stimulates the adrenal cortex to secrete corticosterone, which in turn stimulates food intake (54), whereas the transition from low to high plasma concentrations of leptin concentrations stimulates the adrenal medulla to secrete catecholamines, which in turn inhibit food intake. This logic would allow circulating leptin levels to serve as both an “on” signal (with low leptin increasing appetite due to hypercorticosteronemia) and an “off” signal (as high leptin suppresses appetite by increasing catecholamine concentrations) for food intake.

The gut hormone secretin has recently been proposed to mediate both thermogenesis and postprandial satiety (37). In this study, we sought to determine whether secretin’s effects of promoting postprandial thermogenesis might also be dependent on the adrenal medulla–adipose tissue axis. Consistent with previous findings (37, 82–84), oral carbohydrate and protein meals caused a transient increase in plasma secretin concentrations. However, we found that these effects could be dissociated from the thermogenic response: an isocaloric intravenous glucose infusion caused a similar thermogenic response without any increase in plasma secretin concentrations (Figure 6F). To further explore the physiologic effect of secretin on body temperature, we treated 48-hour-fasted lean rats with an i.p. injection of secretin to mimic postprandial secretin concentrations and found that this intervention caused a transient, modest increase in plasma glucose, insulin, leptin, and catecholamine concentrations in sham-operated rats at 15 minutes but not ADX or atenolol-treated rats (Figure 6), demonstrating that secretin’s effect of causing a small increase in body temperature was mediated by adrenomedullary secretion of plasma catecholamine concentrations. These data are in contrast to the findings of Li et al. (37), who demonstrated that pretreatment with propranolol did not affect BAT temperature in secretin-treated mice. It is possible that these divergent results may be explained by the short duration of the thermogenic response to secretin (Figure 6J): the previous study reported temperature AUC over 2.5 hours, a timescale over which the impact of secretin is expected to be minimal. Most importantly, the thermogenic effect of secretin is minor compared with the effect of leptin and thus may contribute modestly to meal thermogenesis but cannot explain the majority of the temperature response to refeeding in awake rats.

Next, we aimed to examine why the thermogenic response to refeeding is blunted in obesity. To that end, we studied rats both before and after high-fat feeding, which increased body weight by approximately 25%. This increase in body weight caused hyperleptinemia such that fasting plasma leptin concentrations were increased 6-fold and fasting plasma catecholamine concentrations were increased 3-fold from those measured in the same animals prior to the induction of obesity. In obese animals, neither refeeding nor ICV leptin infusion was associated with a thermogenic response beyond the already high baseline body temperatures (Figure 1, I and J). These data suggest that the lack of a temperature response to refeeding may be attributable to the failure of leptin to drop below the threshold at which catecholamines and temperature are reduced in a physiologically meaningful way. The correlation between plasma epinephrine and plasma leptin concentrations showed a 95% maximal epinephrine response to leptin and a 95% maximal temperature response to epinephrine at ~2.4 ng/mL (Supplemental Figure 11, A–C), approximately the plasma leptin concentrations measured in fasting obese rodents. These data demonstrate that there is a relatively low threshold (~2.4 ng/mL in rats) for plasma leptin concentration at which the response to leptin on adrenomedullary secretion of catecholamines is maximized such that there is no physiologic response to reductions in plasma leptin that still exceed this threshold. Taken together, these data suggest that “leptin resistance,” i.e., an inability to increase energy expenditure and/or inhibit food intake in response to leptin, may not be a real phenomenon but may simply reflect the maximal effects of leptin to stimulate adrenomedullary catecholamine secretion and suppress adrenocortical corticosterone secretion, which both plateau at plasma leptin concentrations of approximately 2.4 ng/mL.

Having demonstrated that obese rats exhibit impaired meal-induced increases in body temperature, because fasting does not decrease plasma leptin concentrations below the threshold at which leptin responsiveness occurs, we next asked whether feeding-induced increases in body temperature could be restored following weight normalization on a VLCD. Associated with normalization of plasma leptin concentrations to those measured in lean rats, the VLCD lowered fasting plasma catecholamine concentrations and body temperature, restoring the thermogenic effect of refeeding in formerly obese rats (Figure 2, C–F). These results demonstrate that obesity increases total energy expenditure, while normalization of body weight and body fat content with hypocaloric feeding restores the normal diurnal variation of energy expenditure with feeding and fasting.

Results of the current study as well as previous studies linking alterations in meal thermogenesis to obesity raise the question of whether chronic alterations in postprandial body temperature may alter body weight gain over time. To answer this question, we compared weight change in rats given their total daily calories in 2 carbohydrate-rich boluses through a chronically implanted intragastric catheter, so that food administration could be precisely controlled, as compared with the same amount of total daily calories continuously infused over 24 hours. Bolus feeding increased prandial leptin and catecholamine spikes, thereby increasing body temperature and limiting weight gain over the 10-day infusion period (Figure 7, A–D). The alterations in weight gain with bolus feeding were mediated by increases in plasma catecholamines in bolus-fed rats: treatment of bolus-fed rats with atenolol abrogated the protective effect of bolus feeding. Thus, these data provide a mechanism by which time-restricted feeding/intermittent fasting may improve metabolic health (85–87). In contrast, the timing of intragastric bolus feeding (once daily in the morning or evening) had no impact on any of these parameters. Based on these data, it is possible that previous studies reporting differences in weight gain observed with alteration of feeding time in rodents allowed to consume food per orem may reflect small differences in food consumption when meals are given ad libitum, and/or alterations in energy expenditure induced by awakening the rodents at times when they may not typically be active; in the absence of these factors, as in our study, feeding time does not alter postprandial body temperature and has no impact on weight gain. Further studies will be required to determine to what extent these findings translate to humans, in whom the role of BAT in whole-body energetics remains the subject of active investigation.

In summary, our data establish a mechanism by which body temperature increases postprandially through a leptin–brain–adrenal medulla–adipose tissue axis. Specifically, we show the following: (a) Through a central mechanism, meal-stimulated increases in leptin promote adrenomedullary secretion of catecholamines, which in turn are necessary and sufficient for meal-induced increases in body temperature through β-adrenergic agonism. (b) The dose-response curve for leptin stimulation of adrenomedullary secretion of catecholamines and thermogenesis plateaus at approximately 2.4 ng/mL. (c) Adipose O-linked β-d-N-acetylglucosamine is required for glucose-induced hyperleptinemia and postprandial increases in body temperature. (d) Upon weight normalization, formerly obese rodents regain normal diurnal feeding/fasting variations in plasma leptin, catecholamines, and body temperature. (e) Both increases in plasma epinephrine and stimulation of adipose tissue lipolysis leading to increases in plasma fatty acid concentrations and increased BAT acyl-CoA concentrations — derived mostly from WAT lipolysis — are required for postprandial increases in body temperature. Branched-chain amino acids may also contribute to BAT thermogenesis. (f) Alterations in thyroid hormone may play some role in the regulation of body temperature by central leptin administration, but can be dissociated from temperature regulation under postprandial conditions in ADX animals. (g) Secretin is not required for meal-induced thermogenesis. (h) Meals comprising carbohydrate or protein, but not fat, increase plasma leptin, catecholamines, and body temperature; however, all meals increase energy expenditure, as assessed by indirect calorimetry. These data demonstrate that meal thermogenesis and postprandial energy expenditure are not one phenomenon. (i) Epinephrine suppresses food intake, independently of changes in adipose tissue lipolysis and thermogenesis. (j) Rats fed with high-carbohydrate meal boluses are protected from weight gain relative to those fed continuously, due to increased β-adrenergic activity–induced increases in body temperature; however, meal timing has no impact on weight change.

Taken together, these data provide insights into leptin biology and demonstrate that activation of the leptin–adrenal medulla–adipose tissue axis contributes to the maintenance of metabolic homeostasis by regulating postprandial thermogenesis. These results also provide a potential mechanism by which time-restricted feeding may improve metabolic health.