The primary aim of this study was to quantify the dynamic changes in EE and to investigate their associations with changes in hormonal axes, substrate utilization, and autonomic nervous system parameters in healthy subjects of both sexes exposed to a sustainable reduction in environmental temperature. We thus targeted whole-body adaptive thermogenesis, irrespective of the anatomical or tissue distribution. The two experimental conditions (19 and 24 °C) were chosen because they are tolerable and closely represent the range of temperatures in climate-controlled buildings and homes. To accurately measure the changes in hormonal homeostasis and substrate utilization, we performed multiple blood sampling concomitant to the EE measurement. We further explored the acute effects of a standard meal on these physiological parameters.
Our data demonstrate that a 5 °C reduction in the environmental temperature resulted in a consistent increase of about 6% in EE without differences between the sexes. The observed changes are due to an increase in non-shivering thermogenesis without any measurable increase in spontaneous physical activity. The relative magnitude of this increase in EE was generally similar to that measured by others (20
) upon exposure to colder temperatures for longer periods.
Not surprisingly, the exposure to 19 °C resulted in a decrease in skin temperature due to superficial vasoconstriction, which was consistent with previous findings (20
). Contrary to earlier experiments (20
) performed at a colder temperature (16 °C), we did not observe changes in the core body temperature. A postmeal increase in temperature, indicating a measurable thermic effect of food, was present in both the abdomen and the neck regions at 24 and 19 °C. However, this response was not observed at 19 °C in either the thigh or the core body temperatures, suggesting that the thermic effect of food is somewhat blunted upon exposure to 19 °C.
We postulate that the increase in sympathetic nervous system activity, as evidenced by the relative elevation of plasma and urinary catecholamine and by the increase in the low frequency heart rate variability, is a major effector for the increase in EE in response to 19 °C. The dynamic changes in plasma concentrations of DOPAC and DHPG mirrored that of norepinephrine, indicating that the overall catecholamine turnover was up-regulated. Consistent with previous findings (21
), the observed reduction in the heart rate and the increase in blood pressure are most likely secondary to peripheral vasoconstriction.
Contrary to previous observations (7
), we did not observe any substantial difference in substrate utilization, although we observed a trend toward a reduction in fasting respiratory quotient (RQ) at 19 °C. This discrepancy could be explained by differences in experimental conditions, since the lower temperature was milder in this study. The increase in plasma free fatty acids (FFA) is consistent with catecholamine-stimulated lipolysis. The marginal increase in glycerol levels in the subcutaneous adipose tissue further supports this explanation.
The effects of catecholamines on glucose metabolism are more complex. The increase in fasting insulin observed at 19 °C is likely secondary to catecholamine-mediated hepatic gluconeogenesis (22
); however, it did not extend into the postprandial period and the glucose AUC was lower during this part of the study. Furthermore, the raise in FFA likely contributes to the state of relative insulin resistance observed during fasting. Taken together, these findings can be interpreted as a state of relative insulin resistance and catecholamine-stimulated hepatic gluconeogenesis, with an overall increase in total consumption of metabolic fuels. The marginal decrease in glucose levels in the subcutaneous adipose tissue further strengthens this explanation.
At 19 °C, we also observed an increase in free T4
(and serum T3
in men), consistent with a shift toward the activation of the pituitary–thyroid axis. Alternatively, one could speculate that the increase in free T4
(as measured by analog assay) is at least in part secondary to an interference due to the relative increase in FFA levels (23
). Nonetheless, it is worth noting that sustained exposure to β-adrenergic stimulation generates an increase in serum T3
), in keeping with an activation of the type-2 deiodinase-mediated T4
). It is thus conceivable that prolonged exposure to mild cold temperature will result in a significant activation of the thyroid axis (with a further increase in EE) as a long-term compensatory response to cold exposure. Finally, although no changes were demonstrated in the ACTH levels, the small but significant increase in urinary free cortisol (and serum cortisol in men) implies activation of the hypothalamic–pituitary–adrenal axis consistent with a stress response. The changes in the glucocorticoid and thyroid axes were particularly evident in males. Although we cannot rule out gender as a primary cause of these differences, it is possible that the observed changes are at least in part attributable to the higher proportion of fat-free mass in males. Conversely, the greater increase in urinary cortisol observed in females is probably related to the lower fat-free mass (and hence relatively lower urinary creatinine), since the excretion was measured as cortisol/creatinine ratio.
Our data indicate that subtle modulations in the environmental temperature, well within the limits of tolerability, result in significant changes in EE and in many hormonal axes. The magnitude of the observed increase in EE is remarkable, since it is similar or superior to pharmacological interventions aimed at increasing EE and ultimately to weight loss (27
). Indeed, projecting the point estimate increase in EE from this study over the 24-h period would represent 20% of the negative energy balance commonly prescribed in weight loss interventions (31
). It is worth noting that the strict adherence to such regimens is uncommon, and that the self-assessment of the dietary intake is usually biased toward under-reporting (33
); hence the actual deficit in energy balance required to achieve a sustained weight loss would probably be significantly less than what is commonly prescribed, remarkably similar to the one we observed (2
It is possible that compensatory mechanisms, such as an increase in energy intake (appetite), would prevent weight loss (34
). Furthermore, our data indicate that exposure to 19 °C leads to a mild increase in blood pressure, a state of relative insulin resistance, and a marginal increase in cortisol whose long-term consequences could potentially trump the beneficial effects of increasing resting EE.
This study is a proof-of-concept that manipulations in the environmental temperature, well within the range of tolerability, result in a significant increase in EE and in measurable changes in hormonal homeostasis. One plausible mechanism is catecholamine induction of brown adipose tissue (BAT)-like activity. While no changes were observed in the BAT-specific transcripts in the subcutaneous adipose tissue, it is possible that longer duration of exposure to 19 °C could stimulate transcription of BAT-specific genes (35
). However, the actual contribution of BAT to the maintenance of energy balance in humans has not been empirically demonstrated, and it is possible that other tissues such as skeletal muscle may play a major role in adaptive thermogenesis in adult humans (37
The findings of this study are particularly robust since, to the best of our knowledge, this is the first to study the simultaneous characterization of the changes in EE, sympathetic nervous system activity, and hormonal axes by frequent blood sampling in response to minimal perturbations of the environmental temperature in a relatively large number of volunteers of both the sexes. The study design and the use of a diet-controlled run-in period virtually eliminate the possibility of bias due to carry-over effect, and any confounder due to anticipation. Furthermore, we carefully calibrated our whole room indirect calorimeters at each temperature to ensure accurate measurements of physiological changes in EE and substrate oxidation. One obvious limitation of the study is represented by the brevity of the intervention; we were thus unable to demonstrate any difference in clinically significant endpoints, such as BMI, body composition, and carbohydrate metabolism parameters. Furthermore, we could not evaluate the effects of counter-regulatory mechanisms. Since our study population was limited to non-obese, relatively young individuals, it is also possible that subjects with a higher percentage of fat mass, such as overweight and elderly individuals, may have a blunted response to this intervention (38
). When we explored the individual changes in EE with respect to fat mass and changes in plasma norepinephrine, we observed a non-significant negative trend that supports this hypothesis (data not shown). Although we performed multiple comparisons on our dataset, one should consider that the primary endpoint was defined a priori
in the setting of the statistical power analysis and that the findings in the various secondary endpoints are in keeping with the study hypothesis and primary endpoint findings. Thus, the possibility of type-1 error appears extremely unlikely.
In conclusion, our study demonstrates that minimal modulation in the environmental temperature results in a significant and potentially clinically relevant increase in EE. Further studies are needed to investigate the long-term effects of mild cold exposure on clinically relevant end-points and its applicability as an intervention aimed to promote weight loss.