Body composition as related to heat regulation in women

Thermal discomfort with cold extremities in relation to age, gender, and body mass index in a random sample of a Swiss urban population

Background

The aim of this epidemiological study was to investigate the relationship of thermal discomfort with cold extremities (TDCE) to age, gender, and body mass index (BMI) in a Swiss urban population.

Methods

In a random population sample of Basel city, 2,800 subjects aged 20-40 years were asked to complete a questionnaire evaluating the extent of cold extremities. Values of cold extremities were based on questionnaire-derived scores. The correlation of age, gender, and BMI to TDCE was analyzed using multiple regression analysis.

Results

A total of 1,001 women (72.3% response rate) and 809 men (60% response rate) returned a completed questionnaire. Statistical analyses revealed the following findings: Younger subjects suffered more intensely from cold extremities than the elderly, and women suffered more than men (particularly younger women). Slimmer subjects suffered significantly more often from cold extremities than subjects with higher BMIs.

Conclusions

Thermal discomfort with cold extremities (a relevant symptom of primary vascular dysregulation) occurs at highest intensity in younger, slimmer women and at lowest intensity in elderly, stouter men.

Invited Review: Aging and human temperature regulation

This mini-review focuses on the effects of aging on human temperature regulation. Although comprehensive reviews have been published on this topic (Kenney WL. Exercise and Sport Sciences Reviews, Baltimore: Williams & Wilkins, 1997, p. 41-76; Pandolf KB. Exp Aging Res 17: 189-204, 1991; Van Someren EJ, Raymann RJ, Scherder EJ, Daanen HA, and Swaab DF. Ageing Res Rev 1: 721-778, 2002; and Young AJ. Exp Aging Res 17: 205-213, 1991), this mini-review concisely summarizes the present state of knowledge about human temperature regulation and aging in thermoneutral conditions, as well as during hypo- and hyperthermic challenges. First, we discuss age-related effects on baseline body core temperature and phasing rhythms of the circadian temperature cycle. We then examine the altered physiological responses to cold stress that result from aging, including attenuated peripheral vasoconstriction and reduced cold-induced metabolic heat production. Finally, we present the age-related changes in sweating and cardiovascular function associated with heat stress. Although epidemiological evidence of increased mortality among older adults from hypo- and hyperthermia exists, this outcome does not reflect an inability to thermoregulate with advanced age. In fact, studies that have attempted to separate the effects of chronological age from concurrent factors, such as fitness level, body composition, and the effects of chronic disease, have shown that thermal tolerance appears to be minimally compromised by age.

Aging and human cold tolerance

Hypothermia is widely considered to be a more serious threat for older than for younger persons because of older person's impaired ability to defend body temperature during cold exposure. Some epidemiological studies indicate that the incidence of death from hypothermia increases with age, but surveys of body temperature normally maintained by older persons while in their own homes do not indicate a large incidence of hypothermia. More reliable comparisons of thermoregulatory responses to cold stress in younger and older subjects have been performed under controlled conditions in laboratory experiments. Generally, older men appear less able than younger men to defend their core temperature during experimental cold exposures. Cold exposure may elicit a slightly smaller rise in metabolic heat production, and the cutaneous vasoconstrictor response to cold may be less responsive in old than in young men. These aging effects may, however, be limited to men. In a recent study, older women appeared to defend core temperature during cold exposure as well as, or better than, younger women. Preventable changes in body composition and physical fitness rather than aging per se may contribute to impaired thermoregulatory responses to cold observed in older workers.

Seasonal variation in physiological responses to mild cold air in young and older men

Eight men aged 60-65 years and six men aged 20-25 years, wearing only swimming trunks, were exposed to an air temperature of 17 degrees C and 45% R.H. in each of the four seasons. The increase in the rate of metabolic heat production (% delta M) for the older group in the cold test was significantly higher in summer and autumn than in winter and spring (P < 0.05), but did not differ in the young group between seasons. Compared to the young group the % delta M was significantly greater for the older group (due to a marked increase in four individuals) in summer and autumn (P < 0.04). At the end of the period of cold exposure, the decrements of rectal temperature (delta Tre), mean skin temperature (Tsk; due to a marked decrease in four individuals) and foot skin temperature (Tfoot) were significantly greater for the older group compared to the young group at all times of the year (P < 0.003). Seasonal variations in the two groups were similar, e.g., the delta Tre gradually became smaller from summer to winter (P < 0.05) and then increased slightly in the spring (P = 0.07). Tfoot for both groups decreased from summer to autumn (P < 0.01) and remained unchanged subsequently. No seasonal variations were observed for Tsk in either group. The increase in diastolic blood pressure (BPd) during the test was significantly smaller in winter in both groups (P < 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)

Thermoregulatory responses of young and older men to cold exposure

Nine young (20-25 years) and ten older (60-71 years) men, matched for body fatness and surface area:mass ratio, underwent cold tests in summer and winter. The cold tests consisted of a 60-min exposure, wearing only swimming trunks, to an air temperature of 17 degrees C (both seasons) and 12 degrees C (winter only). Rectal (Tre) and mean skin (Tsk) temperatures, metabolic heat production (M), systolic (BPs) and diastolic (BPd) blood pressures and heart rate (fc) were measured. During the equilibrium period (28 degrees C air temperature) there were no age-related differences in Tre, Tsk, BPs, BPd, or fc regardless of season, although M of the older men was significantly lower (P < 0.003). The decrease in Tre and Tsk (due to the marked decrease in six of the older men) and the increase in BPs and BPd were significantly greater (P < 0.004) for the older men during all the cold exposures. The rate of increase in M was significantly greater (P < 0.01) for the older group when exposed to 12 degrees C in winter and 17 degrees C in summer (due to the marked increase in four of the older men). This trend was not apparent during the 17 degrees C exposure in winter. There was no age-related difference in fc during the exposures. Significant decreases in Tre and Tsk and increases in M, BPs and BPd during the 12 degrees C exposure were observed for the older group (P < 0.003) compared to their responses during the 17 degrees C exposure in winter. In contrast, Tre, M, BPs in the young group were not affected as much by the colder environment. It was concluded that older men have more variable responses and some appear more or less responsive to mild and moderate cold air than young men.

Changes of body temperature and heat in cardiac surgical patients

Changes in body temperature were assessed in ten adult patients undergoing surgery involving cardiopulmonary bypass (CPB) and induced hypothermia. Intraoperatively, in comparable time intervals before CPB and after rewarming, the patients lost body heat. Between the time of induction of anaesthesia and CPB, the temperature of blood in the pulmonary artery fell 1.46 (SD 0.28 degrees C); between CPB and the end of surgery the fall was 1.55 (SD 0.86 degrees C). The extent of spontaneous hypothermia did not correlate with the amount of subcutaneous fat. Hypothermia was induced to obtain a stable deep body temperature of 27.2 (SD 1.3) degrees C, when mean skin temperature averaged 2 degrees C higher. The CPB machine returned approximately 2000 kJ of heat in the rewarming period, to produce pulmonary artery and mean skin temperatures of 37.1 (SD 0.7) degrees C and 31.4 (SD 2.1) degrees C respectively. Intraoperative deep body temperatures demonstrated the expected exponential relationship with metabolic rate. Postoperatively, increase in metabolic rate was associated with rising deep body and skin temperatures. Low resistance to the flow of heat toward the skin surface was demonstrated by low postoperative values for thermal insulation, which may indicate good peripheral perfusion seen during continuing vasodilator therapy.

Peripheral temperature monitoring during cardiopulmonary bypass operation

Almost one-third of 24 adult patients undergoing hypothermic cardiopulmonary bypass (CPB) for elective cardiac operation were found to have upper extremity skin and muscle temperatures of 30.0 degrees C or less at termination of CPB despite the return of nasopharyngeal temperature to normal values. Within 45 minutes, the mean nasopharyngeal temperature of these patients fell spontaneously from 37.1 degrees +/- 0.3 degrees C (+/- standard deviation) to 35.1 degrees +/- 0.4 degrees C, a significantly greater fall (p less than 0.005) than was observed for patients with extremity temperatures greater than 30.0 degrees C. Persistent hypothermia of the upper extremities correlated statistically with large body mass; it appears that these patients incur disproportionately large caloric debts during hypothermic CPB. Inadvertent hypothermia after CPB can be minimized if both core and extremity temperatures are utilized to provide an assessment of the adequacy of warming prior to return to spontaneous circulation.

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.

Physiologic regulation of body energy storage

Both new and published data (rats, mice, and human beings) on three parameters--fat mass, fat-free body mass (FFBM), and total body mass in some cases--are evaluated. Steady state values of the parameters are analyzed for changes in response to specific perturbing agents and for their frequency distributions. Temporal sequences of values on individuals are examined for evidence of regulatory responses. The results lead to the hypothesis that the FFBM is regulated, but probably not as a unit, and that mass of fat is regulated with a high priority near the range extremes but with a much lower priority in the mid-range. Properties and advantages of such a mechanism are discussed.