Basal heat production and elimination of thirteen normal women at temperatures from 22 degrees C. to 35 degrees C

The thermoneutral zone in women takes an "arctic" shift compared to men

Conventionally, women are perceived to feel colder than men, but controlled comparisons are sparse. We measured the response of healthy, lean, young women and men to a range of ambient temperatures typical of the daily environment (17 to 31 °C). The Scholander model of thermoregulation defines the lower critical temperature as threshold of the thermoneutral zone, below which additional heat production is required to defend core body temperature. This parameter can be used to characterize the thermoregulatory phenotypes of endotherms on a spectrum from "arctic" to "tropical." We found that women had a cooler lower critical temperature (mean ± SD: 21.9 ± 1.3 °C vs. 22.9 ± 1.2 °C, P = 0.047), resembling an "arctic" shift compared to men. The more arctic profile of women was predominantly driven by higher insulation associated with more body fat compared to men, countering the lower basal metabolic rate associated with their smaller body size, which typically favors a "tropical" shift. We did not detect sex-based differences in secondary measures of thermoregulation including brown adipose tissue glucose uptake, muscle electrical activity, skin temperatures, cold-induced thermogenesis, or self-reported thermal comfort. In conclusion, the principal contributors to individual differences in human thermoregulation are physical attributes, including body size and composition, which may be partly mediated by sex.

Opportunities and challenges in the therapeutic activation of human energy expenditure and thermogenesis to manage obesity

The current obesity pandemic results from a physiological imbalance in which energy intake chronically exceeds energy expenditure (EE), and prevention and treatment strategies remain generally ineffective. Approaches designed to increase EE have been informed by decades of experiments in rodent models designed to stimulate adaptive thermogenesis, a long-term increase in metabolism, primarily induced by chronic cold exposure. At the cellular level, thermogenesis is achieved through increased rates of futile cycling, which are observed in several systems, most notably the regulated uncoupling of oxidative phosphorylation from ATP generation by uncoupling protein 1, a tissue-specific protein present in mitochondria of brown adipose tissue (BAT). Physiological activation of BAT and other organ thermogenesis occurs through β-adrenergic receptors (AR), and considerable effort over the past 5 decades has been directed toward developing AR agonists capable of safely achieving a net negative energy balance while avoiding unwanted cardiovascular side effects. Recent discoveries of other BAT futile cycles based on creatine and succinate have provided additional targets. Complicating the current and developing pharmacological-, cold-, and exercise-based methods to increase EE is the emerging evidence for strong physiological drives toward restoring lost weight over the long term. Future studies will need to address technical challenges such as how to accurately measure individual tissue thermogenesis in humans; how to safely activate BAT and other organ thermogenesis; and how to sustain a negative energy balance over many years of treatment.

Quantification of the Capacity for Cold-Induced Thermogenesis in Young Men With and Without Obesity

OBJECTIVE

Cold exposure increases energy expenditure (EE) and could have a role in combating obesity. To understand this potential, we determined the capacity for cold-induced thermogenesis (CIT), the EE increase above the basal metabolic rate at the individualized coldest tolerable temperature before overt shivering.

DESIGN

During a 13-day inpatient protocol, we quantitated the EE of 12 lean men and 9 men with obesity at various randomly ordered ambient temperatures in a room calorimeter. Subjects underwent brown fat imaging after exposure to their coldest tolerable temperature.

RESULTS

CIT capacity was 300 ± 218 kcal/d (mean ± SD) or 17 ± 11% in lean men and 125 ± 146 kcal/d or 6 ± 7% in men with obesity (P = 0.01). The temperature below which EE increased, lower critical temperature (Tlc), was warmer in lean men than men with obesity (22.9 ± 1.2 vs 21.1 ± 1.7°C, P = 0.03), but both had similar skin temperature (Tskin) changes and coldest tolerable temperatures. Whereas lean subjects had higher brown fat activity, skeletal muscle activity increased synchronously with CIT beginning at the Tlc in both groups, indicating that muscle is recruited for CIT in parallel with brown fat, not sequentially after nonshivering thermogenesis is maximal.

CONCLUSIONS

Despite greater insulation from fat, men with obesity had a narrower range of tolerable cool temperatures available for increasing EE and less capacity for CIT than lean men, likely as a result of greater basal heat production and similar perception to Tskin cooling. Further study of the reduced CIT capacity in men with obesity may inform treatment opportunities for obesity.

Cold-induced thermogenesis in humans

A basic property of endothermic thermoregulation is the ability to generate heat by increasing metabolism in response to cold ambient temperatures to maintain a stable core body temperature. This process, known as cold-induced thermogenesis (CIT), has been measured in humans as early as 1780 by Antoine Lavoisier, but has found renewed interest because of the recent 'rediscovery' of thermogenic, cold-activated brown adipose tissue (BAT) in adult humans. In this review, we summarize some of the key findings of the work involving CIT over the past two centuries and highlight some of the seminal studies focused on this topic. There has been a substantial range of variability in the reported CIT in these studies, from 0 to 280% above basal metabolism. We identify and discuss several potential sources of this variability, including both methodological (measurement device, cold exposure temperature and duration) and biological (age and body composition of subject population) discrepancies. These factors should be considered when measuring CIT going forward to better assess whether BAT or other thermogenic organs are viable targets to combat chronic positive energy balance based on their relative capacities to elevate human metabolism.

Basal metabolic rate studies in humans: measurement and development of new equations

OBJECTIVE

To facilitate the Food and Agriculture Organization/World Health Organization/United Nations University Joint (FAO/WHO/UNU) Expert Consultation on Energy and Protein Requirements which met in Rome in 1981, Schofield et al. reviewed the literature and produced predictive equations for both sexes for the following ages: 0-3, 3-10, 10-18, 18-30, 30-60 and >60 years. These formed the basis for the equations used in 1985 FAO/WHO/UNU document, Energy and Protein Requirements. While Schofield's analysis has served a significant role in re-establishing the importance of using basal metabolic rate (BMR) to predict human energy requirements, recent workers have subsequently queried the universal validity and application of these equations. A survey of the most recent studies (1980-2000) in BMR suggests that in most cases the current FAO/WHO/UNU predictive equations overestimate BMR in many communities. The FAO/WHO/UNU equations to predict BMR were developed using a database that contained a disproportionate number--3388 out of 7173 (47%)--of Italian subjects. The Schofield database contained relatively few subjects from the tropical region. The objective here is to review the historical development in the measurement and application of BMR and to critically review the Schofield et al. BMR database presenting a series of new equations to predict BMR.

DESIGN

This division, while arbitrary, will enable readers who wish to omit the historical review of BMR to concentrate on the evolution of the new BMR equations.

SETTING

BMR data collected from published and measured values.

SUBJECTS

A series of new equations (Oxford equations) have been developed using a data set of 10,552 BMR values that (1) excluded all the Italian subjects and (2) included a much larger number (4018) of people from the tropics.

RESULTS

In general, the Oxford equations tend to produce lower BMR values than the current FAO/WHO/UNU equations in 18-30 and 30-60 year old males and in all females over 18 years of age.

CONCLUSIONS

This is an opportune moment to re-examine the role and place of BMR measurements in estimating total energy requirements today. The Oxford equations' future use and application will surely depend on their ability to predict more accurately the BMR in contemporary populations.

Influence of localized auxiliary heating on hand comfort during cold exposure

There is a need for a hand-heating system that will keep the hands warm during cold exposure without hampering finger dexterity. The purpose of this study was to examine the effects of torso heating on the vasodilative responses and comfort levels of cooled extremities during a 3-h exposure to -15 degreesC air. Subjects were insulated, but their upper extremities were left exposed to the cold ambient air. The effect of heating the torso [torso-heating test (THT)] on hand comfort was compared with a control condition in which no torso heating was applied, but Arctic mitts were worn [control test (CT)]. The results indicate that mean finger temperature, mean finger blood flow, mean toe temperature, mean body skin temperature, body thermal comfort, mean finger thermal comfort, and rate of body heat storage were all significantly (P < 0.05) higher on average (n = 6) during THT. Mean body heat flow was significantly (P < 0.05) lower during THT. There were no significant differences (P >/= 0.05) in rectal temperature between CT and THT. Mean unheated body skin temperature and mean unheated body heat flow (both of which did not include the torso area in the calculation of mean body skin temperature and mean body heat flow) were also calculated. There were no significant differences (P >/= 0.05) in mean unheated body skin temperature and mean unheated body heat flow between CT and THT. It is concluded that the application of heat to the torso can maintain finger and toe comfort for an extended period of time during cold exposure.

Entropy principle for human development, growth and aging

Entropy productions within nude subjects in respiration calorimeters are calculated from the corresponding energetic data obtained by Du Bois et al. (1952, J. Nutr. 48, 257-293.). The entropy production for men is constant at environmental temperatures from 24-34 degrees C. The metabolic entropy production comprises 98.6% of the total entropy production. The entropy production for women shows a minimum at 30 degrees C (the middle of the neutral zone), a small rise in the cold zone and a trend toward a rise in the warm zone; the average entropy production for women is 8.7% smaller than that for men. The entropy production rises from 0-2 years of age, and decreases rapidly from 2-25 years of age and then gradually to 85 years of age. The entropy production does not seem to achieve a minimum or a level in the lives of men and women. Based on these results, a three-stage hypothesis of entropy production in human life is proposed.

Seasonal changes in circadian rhythms of body temperatures in humans living in a dry tropical climate

Seven volunteers (3 females and 4 males; 3 Caucasians and 4 Africans) participated in two 24 h sessions during the cool dry (CD) and the hot dry (HD) seasons of the sahelian tropical climate. Body temperatures were taken on portable cassette recorders for 24 h. Rectal (Tre) and mean skin (Tsk) temperatures decreased in the HD compared to the CD conditions, meeting one of the criteria for adaptation to heat. No ethnic differences in thermal responses were found. Males and females differed in their body temperature rhythms and in their reactions to heat. Body temperatures were higher in females than in males. Males reacted to heat with a decrease in Tre, without change in the Tre-Tsk gradient. Females showed a decrease in both Tre and Tsk, more marked for Tsk, with an increase in the Tre-Tsk gradient. It was concluded that males showed seasonal acclimatization to heat via a decrease in metabolism confirmed by a decrease in plasma levels of thyroid stimulating hormone (TSH) in the HD condition. Females showed a mixed metabolic and thermolytic type of acclimatization, with an absence of variation in plasma TSH levels. In conclusion, the steady rise in temperature between the CD and HD conditions was sufficient to trigger an acclimatization to heat similar in Caucasian and African subjects, although exposure to the external climate differed widely.

Determinants of maternal temperature during labor

Intrapartum maternal temperatures and neonatal temperature immediately after birth were measured in 50 cases. Maternal temperatures were determined in part by the amount of hyperventilation, perspiration, and physical activity, with those who were calm and less active having the higher temperatures. Thus, with hyperventilating parturients, it was difficult to define "normal" maternal temperatures. The neonatal rectal temperature immediately after birth seemed best correlated with the maternal vaginal temperature. Two examples are presented which suggest that cases of fetal distress associated with tachycardia and decreased placental exchange may be examples of isolated fetal hyperthermia.

Sex differences in human thermoregulatory response to heat and cold stress

The current literature on male-female differences in response to thermal stress has been reviewed. Morphologically, women average 20% smaller body mass, 14% more body fat, 33% less lean body mass, but only 18% less surface area than men. Women have greater body insulation when vasoconstricted (except hands and feet) and a larger peripheral heat sink, but at the cost of (1) greater body fat burden, (2) less muscle mass and strength, and (3) smaller circulating blood volumes which requires greater physiological strain to balance heat production and loss. Under heat stress, women generally show (1) relatively more peripheral blood pooling, (2) greater heart rate increases, (3) more frequent circulatory embarassment, (4) lower maximal sweat rates, (5) higher skin temperatures with greater body heat storage, and (6) poorer maintenance of circulating blood volume with more impact from dehydration. Proportionately fewer women than men can be successfully heat acclimated. In the cold, women generally have (1) less capability for maximum heat production by either exercise or shivering, (2) a more extensively vasoconstricted periphery, (3) lower foot, hand, and mean skin temperatures, (4) greater surface heat losses, especially from the geometrically thinner extremities, (5) increased rates of extremity, but not core, cooling, and (6) relatively greater risk of cold injury.

Comparison of thermal exchanges in men and women under neutral and hot conditions

1. The thermoregulatory responses in unacclimatized men and women were analysed and compared by means of standard heat exposure tests which allowed evaporative losses, body temperatures, heat storage and the complete thermal balance to be continuously recorded in all subjects. 2. The most pronounced differences were observed in delay before onset of sweating. Sweating always occurred faster with lower body temperatures in men than in women. The period immediately following ovulation was characterized by an increase in onset delay and a decrease in the sensitivity in sweating response compared with the pre-ovulation period. 3. The evaporative rate in the steady state did not change significantly in the post-ovulation period and was found to be higher in men who consequently had lower mean skin temperatures. Skin conductances, different under thermo-neutral conditions, were the same in men and women under hot conditions. 4. Women showed a more definite increase of body temperatures and heat storage than men due to delayed sweating and decreased sweating sensitivity. The body heat content in the steady state increased more markedly in women than in men. Furthermore, the highest heat storage values were found during the post-ovulation period. A high degree of correlation was found between body heat content and absolute onset delay. 5. The sweating kinetics and the transient phase just before the steady state of the thermal balance appeared to be decisive factors in the differentiation of the thermoregulatory behaviour in women before and after ovulation. Heat storage achieved during the transient phase and not made up by adequate evaporation seemed to be the cause of the large increase of body temperatures and heat content shown by women in hot environments.

The effects of intravenous protein--calorie supplementation on the tissue composition of postoperative body weight loss

The components of postoperative body weight loss were analysed in eleven adult male patients who had uncomplicated abdominal surgery of moderate severity namely vagotomy and a drainage procedure for chronic duodenal ulceration. Four patients (Group I) had the routine postoperative regimen which consisted of intravenous isotonic dextrose and dextrose-saline solutions. Another group of four patients (Group IIa) had supplementation of this regimen with 50% dextrose and an amino-acid/sorbitol preparation whilst a group of three (Group IIb) patients had supplementation with only amino-acid/sorbitol preparation.

Group I patients exhibited the expected losses of lean tissue and body fat, but these were partially obscured by retention of extracellular water. Group IIa exhibited excessive loss of extracellular water which was parallel to an increased urine flow and this loss completely overshadowed the reduction in lean tissue and body fat losses produced by the protein and calorie supplementation. Group IIb patients exhibited the smallest amount of weight loss because they had a reduction in lean tissue and body fat losses whilst they still exhibited a mild antidiuretic response. It is suggested that the amino-acid/sorbitol preparation provides the safest form of supplementation, particularly in the face of increased extra-renal losses of water such as may be dictated by a warm tropical environment.

ACUTE EXPOSURE OF HUMAN SUBJECTS TO AN AMBIENT TEMPERATURE OF 10 °C

Acute exposures to cold (10 °C) for 1 hour were carried out in early April on five unclothed healthy male subjects to follow their shivering responses, oxygen consumption, ventilation, skin and oral temperatures. Exposure to the cold resulted in immediate (within 5 minutes) and sustained increases in oxygen consumption, ventilation, and heat production. These subjects did not show generalized shivering, however, until after 30 minutes of exposure, even though short bursts of shivering were noted before this time. The immediate increase in heat production without shivering is indicative of a non-shivering thermogenesis which may be the result of cold-acclimatization in these subjects. Seasonal acclimatization would be maximal at the time of year during which the experiments were carried out.