Furthermore, the adaptive increase of BAT temperature in response to cold is greater in women than in men, irrespective of the ovarian phase, and positively correlated with estradiol levels (161). Finally, although the study of BAT activity during the ovarian cycle should be addressed with more detail, evidence has shown that basal BAT temperature increases during the luteal phase and this increase is positively correlated with progesterone levels. Thus, although at an ambient temperature of 28 °C, levels of UCP1 are similar between male and female rats, at colder temperatures (22 °C and 18 °C) UCP1 levels are significantly higher in females. This has important implications in our ‘obesogenic’ society, living most/all our time in our TNZ range of temperatures and surrounded by food, which is always accessible and highly palatable. During short exposures to cold, an almost perfect fit is shown, with intake adjustment compensating accurately for temperature-induced changes in expenditure. Food intake is required to meet physiological needs of nutrients, but also as a source of energy, including what is needed for thermal homeostasis. When analyzing the effect of ambient temperature on energy homeostasis, the other variable to consider is energy intake. Thus, a diminished response of BAT to food intake and to cold temperature has been observed in practically all genetic forms of obesity (172). Sympathetic denervation of the arm in humans with palmar hyperhidrosis improves skeletal muscle work efficiency 44 in arm muscles, and chemical sympathectomy attenuates leptin-mediated increases in energy expenditure in rats 45. Thyroid hormone increases energy expenditure by increasing heart rate, blood pressure, muscle ATP consumption (largely by stimulating production of muscle ATPase). Discrepancies among studies may reflect differences in subject populations regarding exercise, gender, age, or weight loss regimens, as well as the degree of weight stability at the time of study. A greater decrease in serum COR was noted in men with higher baseline COR levels, whereas the decrease observed in men with lower baseline COR levels was significantly smaller. Nindl et al. (2001), Daly et al. (2005) and Brownlee et al. (2005) confirmed the presence of a relationship between COR and TES during sample recovery, which could suggest that a critical concentration of COR has to be achieved in order to substantially influence circulating TES levels. Cortisol affects metabolism by maintaining blood glucose levels at a sufficiently high level during physiological stress. COR is a catabolic hormone that is secreted by the adrenal cortex in response to physiological stress. In turn, Kukkonen-Harjula and Kauppinen (1988) demonstrated sauna-induced changes in TES secretion. The production of some hormones increases during physiological stress when physical effort exceeds the body’s regulatory capabilities and induces an excessive response that triggers neurohormonal changes (Jaskólski & Jaskólska, 2006). Testosterone (TES), cortisol (COR), dehydroepiandrosterone sulfate (DHEA-S), and prolactin (PRL) levels were measured before and after the sauna bath. The aim of the study was to determine the effect of repeated hot thermal stress and cold water immersion on the endocrine system of young adult men with moderate and high levels of physical activity (PA). Pharmacotherapy activating the leptin-signaling pathway may help weight-reduced individuals to sustain their weight loss98. Yet, in humans (lean or obese) and rodents who are not leptin-deficient, induction of weight loss requires doses of leptin that produce plasma leptin concentrations over 10 times normal 95, 96. Administration of leptin to leptin-deficient rodents and humans in doses that restore circulating leptin concentrations to their physiological range increases energy expenditure 93, decreases energy intake, increases sympathetic nervous system activity 94, and normalizes hypothalamic-pituitary-adrenal, thyroid, and gonadal function1, 29, 82. The hyperphagic, hypometabolic phenotype of weight-reduced humans is similar to that of leptin-deficient or -unresponsive humans and rodents 80.
