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PubMed1997

Pituitary, gonadal and adrenal hormones after prolonged residence at extreme altitude in man.

High altitude-induced alterations in pituitary, gonadal and adrenal hormones were studied in (i) eugonadal men from the armed forces who were resident at sea level (SL), (ii) SL residents staying at an altitude of 3542 m for periods ranging from 3 to 12 months (acclimatized lowlanders, ALL), (iii) ALL who stayed at 6300 m for 6 months, (iv) ALL who trekked from 3542 to 5080 m and stayed at an altitude of more than 6300 m in the glacier region for 6 months, and (v) high-altitude natives (HAN) resident at an altitude of 3300-3700 m. Circulating levels of LH, FSH, prolactin, cortisol, testosterone, dihydrotestosterone (DHT) and progesterone in ALL at 3542 m and in HAN were not significantly different (p > 0.05) from the SL control values. When the ALL living at 3542 m trekked to an extreme altitude of 5080 m, their testosterone levels showed a significant decrease (p < 0.01) compared to the preceding altitude values but had returned to SL values when measured after 6 months' continuous stay at 6300 m. As with testosterone, the levels of DHT and oestradiol-17 beta (E2) after prolonged stay at extreme altitude were also not significantly different (p > 0.05) from the SL values. The LH levels after trekking to 5080 m were significantly higher (p < 0.01) than at an altitude of 3542 m, but decreased to levels found at 3542 m or SL after prolonged residence at extreme altitude. Plasma levels of ACTH, prolactin, FSH and cortisol on arrival at 5080 m, and after a 6-month stay at extreme altitude, were not significantly different (p > 0.05) from the SL values. Plasma progesterone levels tended to increase on arrival at 5080 m but a significant increase (p < 0.001) was evident only after a 6-month stay at extreme altitude. These observations suggest that prolonged residence at lower as well as at extreme altitude does not appreciably alter blood levels of pituitary, gonadal or adrenal hormones except for plasma levels of progesterone. The exact mechanism and significance of this increase remains unknown, but may be important in increasing the sensitivity of the hypoxic ventilatory response and activation of haemoglobin synthesis.

Acclimatization↗
PubMed2001

Hemoglobin mass and peak oxygen uptake in untrained and trained residents of moderate altitude.

Blood composition, hemoglobin mass (CO rebreathing method) and VO2peak were measured in 15 untrained (UT-Bogotá) and 14 trained males (TR-Bogotá) living at 2600 m of altitude, and in 14 untrained lowlanders (UT-Berlin). [Hb] amounted to 15.3 + 0.2(SE) g/dl in UT-Berlin, 17.4 + 0.2 g/dl in UT-Bogotá and 16.0 + 0.2 g/dl in TR-Bogotá. Hb mass was significantly higher in UT-Bogotá (13.2 + 0.4 g/kg, P < 0.01) and in TR-Bogotá (14.7 + 0.5 g/kg, P < 0.001) than in UT-Berlin (11.7 + 0.2 g/kg). In TR-Bogotá also plasma volume was expanded. Erythropoietin concentrations in UT-Bogotá and TR-Bogotá were not significantly increased. There was a positive correlation between blood volume and VO2peak for the pooled values of all subjects, if the oxygen uptake of UT-Berlin was corrected for an ascent to 2600 m. For the Hb mass - VO2peak relation two groups are indicated pointing to two types of altitude acclimatization with different Hb mass increases but similar distribution of aerobic performance capacity. We suggest that different genetic properties in a population of mixed ethnic origin might play a role.

Acclimatization↗
PubMed2002

Erythropoiesis and performance after two weeks of living high and training low in well trained triathletes.

The purpose of our study was to evaluate hematologic acclimatization during 2 weeks of intensive normoxic training with regeneration at moderate altitude (living high-training low, LHTL) and its effects on sea-level performance in well trained athletes compared to another group of equally trained athletes under control conditions (living low - training low, CONTROL). Twenty-one triathletes were ascribed either to LHTL (n = 11; age: 23.0 +/- 4.3 yrs; VO 2 max: 62.5 +/- 9.7 [ml x min -1 x kg -1]) living at 1956 m of altitude or to CONTROL (n = 10; age: 18.7 +/- 5.6 yrs; VO 2 max: 60.5 +/- 6.7 ml x min -1 x kg -1) living at 800 m. Both groups performed an equal training schedule at 800 m. VO 2 max, endurance performance, erythropoietin in serum, hemoglobin mass (Hb tot, CO-rebreathing method) and hematological quantities were measured. A tendency to improved performance in LHTL after the camp was not significant (p < 0.07). Erythropoietin concentration increased temporarily in LHTL (Delta 14.3 +/- 8.7 mU x ml -1; p < 0.012). Hb tot remained unchanged in LHTL whereas was slightly decreased from 12.5 +/- 1.3 to 11.9 +/- 1.3g x kg -1 in CONTROL (p < 0.01). As the reticulocyte number tended to higher values in LHTL than in CONTROL, it seems that a moderate stimulation of erythropoiesis during regeneration at altitude served as a compensation for an exercise-induced destruction of red cells.

Acclimatization↗
PubMed1992

Anaerobic performance at altitude.

Anaerobic metabolism is usually evaluated by the determination of the anaerobic capacity and the maximal anaerobic mechanical external power (Wmax). Conflicting results are reported on anaerobic capacity evaluated by maximal oxygen deficit and debt, and maximal blood lactate concentration during acute or chronic hypoxia (acclimatized subjects). Data on muscle biopsies (lactate concentration, changes in ATP, phosphocreatine and glycogen stores, glycolytic enzyme activities) and the few studies on lactate flux give in most cases evidence of a non-alteration of the anaerobic capacity for altitudes up to 5,500 m. No differences are observed in Wmax measured at high altitudes up to 5,200 m during intense short-term exercises: (1) jumps on a force platform which is a good indicator of alactic Wmax, and (2) 7-10 s sprints (i.e. force-velocity test) which solicit alactic metabolism but also lactic pathway. For exercises of duration equal or more than 30 s (i.e. Wingate test), there are conflicting results because a lower participation of aerobic metabolism during this test at high altitude can interfere with anaerobic performance. In conclusion, we can admit that anaerobic performances are not altered by high altitudes up to 5,200 m if the length of exposure does not exceed 5 weeks. After this period, muscle mass begins to decrease.

Acclimatization↗
PubMed1992

Are Tibetans better adapted?

Evidence is reviewed from our recent (1987-1991) investigations which demonstrate better high-altitude adaptation among Tibetans than in acclimatized newcomers or other lifelong high-altitude residents. Characteristics of oxygen transport contributing to the Tibetans' remarkable exercise performance are described.

Acclimatization↗
PubMed1992

Nutrition and high altitude exposure.

Altitude exposure leads to considerable weight loss. The different hypotheses that have been put forward to explain this phenomenon are discussed reviewing the literature: 1) a primary decrease of food intake due to loss of appetite caused, directly or indirectly, by hypoxia, changes of menus, comfort and habits, 2) a discrepancy between energy intake and energy expenditure due to an increased basal metabolic rate and/or high levels of activity which are not matched by an increased food intake, 3) a loss of body water due to increased insensible loss through increased ventilation in the mountain environment, decreased liquid intake, and/or changes in water metabolism, 4) an impaired absorption of nutrients from the gastrointestinal tract, and 5) a loss of muscle mass due to lack of physical exercise and/or direct effects of hypoxia on protein synthesis. It is concluded that altitude weight loss is due to an initial loss of water and subsequently to loss of fat mass and muscle wasting. Up to altitudes around 5000 m the weight loss from fat and muscle seems to be largely avoidable by maintaining adequate intake in a comfortable setting. Primary anorexia, lack of comfort and palatable food, detraining, and possible direct effects of hypoxia on protein metabolism seem to inevitably lead to weight loss during longer exposures at higher altitudes. In order to minimize losses it is advisable to acclimatize properly, to reduce the length of stay at extreme altitude as much as possible and to maintain a high and varied nutrient intake.

Acclimatization↗
PubMed1998

Rehydration strategies--balancing substrate, fluid, and electrolyte provision.

The combination of heat stress, dehydration, and exercise imposes perhaps the most-severe physiological challenge for the human body short of disease or serious bleeding. Exercise in the heat requires the body to attempt to cope simultaneously with competing demands for cardiovascular homeostasis, thermoregulatory control, and maintenance of muscle energetics. When dehydration is superimposed upon this scenario (as is often the case during most forms of exercise), the results can be catastrophic for both health and performance. Fluid replacement reduces the risk of heat illness and improves exercise performance by preventing or reducing dehydration and by providing a convenient means of ingesting carbohydrate. The fact that even low levels of dehydration (e.g., equivalent to a 2% loss of body weight) impair cardiovascular and thermoregulatory response, and reduces the capacity for exercise, is beyond scientific dispute. For these reasons, optimal performance is possible only when dehydration is minimized by ingesting ample volumes of fluid during exercise. Recent research has demonstrated that consuming fluid in direct proportion to sweat loss (or close to it) maintains important physiological functions and significantly improves exercise performance, even during exercise lasting only one hour. Preventing dehydration enables the cardiovascular system to maintain blood pressure and cardiac output, thereby sustaining the increase in skin blood flow and sweating that are essential for optimal temperature regulation. Remaining well hydrated during exercise also preserves muscle function, reducing the reliance on muscle glycogen as a fuel source. Carbohydrate ingestion also improves exercise performance, an effect that is independent of and additive to preventing dehydration. The practical application of this knowledge requires that athletes follow a more-aggressive fluid-replacement regimen than is now usually the case. Successful implementation of this regimen requires that coaches, athletes, and support personnel are made aware of the practical benefits of adequate fluid replacement, that appropriate fluid-replacement strategies are developed and implemented, and that athletes have the opportunity to train themselves to ingest larger volumes of fluid more frequently. Success during competition in warm weather is more likely for those athletes who are highly fit and well acclimatized to training and competing in the heat, and who diligently avoid even low levels of dehydration. The competitive advantage will definitely shift in favor of those athletes whose coaches and trainers recognize the fundamental value of fitness, acclimation, and hydration, coupled with other strategies for keeping athletes cooled and fueled.

Acclimatization↗
PubMed1998

Heat acclimation--mechanisms of adaptation to exercise in the heat.

Repeated exposures to exercise and heat produce acclimatization, changes in physiological function by which the tolerance to heat stress is improved. The main issues to be discussed are the possible mechanisms for the increase in plasma volume, the increase in sweating rate and the endocrine responses to exercise with acclimation to both dry and humid heat. This will be discussed on the basis of the literature and our recent and ongoing experiments. We have tried to analyze this by comparing different acclimation procedures: exercise at 50-60% V/O2max, 60-90 min, dry heat 40 degrees C, 20% RH; 40-50% VO2max, 45-51 min, humid heat 35 degrees C, 85% RH; and 70-75% VO2max, 30-35 min, dry heat 40 degrees C, 20% RH. Subjects exercised in dry or humid heat for 8-12 consecutive days. Acclimation was achieved by all procedures, as indicated by a lower heart rate, increased plasma volume and sweating rate. We hypothesize that it is the repeated exposures to high core temperature that induce the changes, possibly via endocrine factors activated by the rising core temperature and the prolonged exercise. The increased sensitivity of the sweat glands for thermal and hormonal stimuli after acclimation may be obtained through an increase in receptor density for neural and humoral stimuli, an increase in the size, or, in number of active sweat glands.

Acclimatization↗
PubMed1997

VO2max and haemoglobin mass of trained athletes during high intensity training.

The correlation between relative haemoglobin mass (Hb mass, g x kg[-1]) and relative maximal oxygen consumption (VO2max, ml x kg(-1) x min[-1]) in 62 trained athletes (33 male runners, 12 male rowers and 17 female rowers) with national and/ or international competitive experience was examined. The correlation between Hb mass and VO2max was highest for the female rowers (n=17, r=0.92, p<0.0001), lower for the male rowers (n = 12, r=0.79, p < 0.005) and lowest for the male runners (n=33, r=0.48, p = 0.005). These results suggest that, within an athletic sample, Hb mass may be used to estimate potential aerobic power. In a second series of experiments, Hb mass was measured before and after three different training programs in sub-sets of the subjects used in the earlier study. Hb mass did not change following 12 weeks of intense rowing training, 4 weeks of heat training (32 degrees C), or 4 weeks of medium-altitude training (1740 m). The corresponding increases in VO2max were 7.8%, no change and 2.1 %, respectively. These results suggest that heat or altitude training does not increase Hb mass in trained athletes. Previous studies that demonstrate increases in total red cell volume following altitude acclimatization used subjects with only modest aerobic power, whereas the present study used trained subjects. It is concluded that trained athletes with erythrocythemic hypervolemia have limited capability to increase further either total red cell volume or Hb mass.

Acclimatization↗
PubMed1991

Does the glycogen synthase (EC 2.4.1.21) of brown adipose tissue play a regulatory role in glucose homeostasis?

Glycogen synthase (GS) activity was characterized in rat brown adipose tissue (BAT) and the activity was found to be much higher than that in white adipose tissue. Prolonged starvation had no effect on the active form of GS, as found in the liver and muscle. The GS activity was similar in BAT of rats housed in an animal room (21 +/- 1 degree) whether they were fed on high-carbohydrate, high-fat, or stock diets. Acclimatization of rats to cold (4 +/- 1 degree) for 2 weeks significantly increased GS activity. This increase in the cold was fivefold greater when rats were fed on high-carbohydrate diets than in control rats at room temperature fed on an identical diet. The increase was accompanied by a large accumulation of glycogen in BAT. It was concluded that GS may play an important role in BAT and may contribute to the control of blood glucose in a cold environment. Its relevance to thermogenesis requires further elucidation.

Acclimatization↗
PubMed1975

Heat wave mortality in New York City, 1949 to 1970.

Epidemiological studies have been carried out to search for an influence of air conditioning on patterns of heat wave mortality in New York City. Two models were used to predict total daily summer mortality as a function of temperature; one of the models included heat acclimatization effects. Ratios of the models' predictions of heat wave mortality to actual mortality were calculated for heat waves occurring during 12 summers in the period 1949 to 1970. Trends in these ratios as a function of time were sought. The analysis suggests that the relative magnitudes of excess mortality during initial summer heat waves may have decreased over the 21-year period. No trend was observed for excess mortality during late summer heat waves.

Acclimatization↗
PubMed1975

High altitude stress and retinal hemorrhage: relation to vascular headache mechanisms.

Retinal hemorrhage occurred in 36% of 39 subjects exposed to altitudes at or above 14,200 feet. In subjects with a history of vascular headaches at sea level, there was a higher incidence of and more severe altitude headache, as well as a higher incidence of retinal hemorrhage than among those previously headache-free. In subjects without altitude headache, none had retinal hemorrhage. In subjects with altitude headache, 42% had retinal hemorrhage. A progressive rise in the incidence of retinal hemorrhage was correlated with progressively greater intensity of altitude headache. Factors that intensified the rate or degree of exposure, including rapid ascent and strenuous exertion, appeared to increase the likelihood of hemorrhage. An optimal balance between acclimatization and subsequent altitude stress appeared to prevent retinal hemorrhage. Increased retinal blood flow, retinal vessel engorgement, increased retinal vein and prevenous capillary pressure, and possibly decreased intraocular pressure may contribute to the pathogenesis of retinal hemorrhage.

Acclimatization↗
PubMed1989

Improvements in heat tolerance induced by interval running training in the heat and in sweat clothing in cool conditions.

To compare the effectiveness of training in heat and in sweat clothing in cool conditions on improving heat tolerance, two groups of active subjects (n = 6 in each) performed an interval running heat-tolerance test before and after a 7-day experimental treatment. On each treatment day the subjects attempted to complete 4 x 15 min interval treadmill running periods (a 7.5 s effort every 30 s, on 15 km h-1, 15% grade; the same exercise format as the heat-tolerance test), which were interspersed with 5-min recovery periods (total time each day = 80 min). Group 1 (heat) ran in shorts, socks and shoes in hot humid conditions, and Group 2 (sweat clothing) ran in cool conditions dressed in shorts, socks and T-shirt covered by a polyester-cotton tracksuit, over which was worn 100% nylon spray-proof pants and jacket (cotton lined) with an acrylic cloth bobble hat (beanie) on the head. Both groups displayed changes typical of heat acclimatization over the 7-day period, with significant decreases in final rectal temperature (Tr) and heart rate (HR) being evident, but no change in sweat loss. Mean skin temperature (Tsk) was similar in both groups during the training sessions (heat group: 34.8-35.7 degrees C; sweat clothing group 34.9-35.5 degrees C). After the heat-tolerance test, both groups had significantly lower Tr, Tsk and HR values than before, and sweating sensitivity (g m-2 h-1 degrees C rise in Tr) was significantly increased. There was only one significant difference between the two groups (Tsk, 20th min value). It was concluded that training in sweat clothing in cool conditions can provide the same improvements in heat tolerance as training in hot humid conditions where a fixed exercise intensity and duration are used.

Acclimatization↗
PubMed1996

Exercise at low altitude (Jordan Valley) causes changes in serum levels of ACTH, insulin, cortisol and lactate.

This study was designed to examine the effects of exercise on adrenocorticotropic hormone (ACTH) and cortisol at low altitude (350 meters below sea level) and to compare these effects with those at a moderate level altitude (620 meters above sea level). Ten male trained athletes participated in a 21-K(m) non-competitive race. Serum levels of ACTH, luteinizing hormone (LH), growth hormone and cortisol were measured before and after the race at each of the altitudes. A significant increase in serum levels of ACTH was observed in response to this exercise only at low altitude. Serum levels of growth hormone were increased at both altitudes. Those of LH were not affected. Serum cortisol levels were increased following exercise at both altitudes. It is proposed here that ACTH may play a role in acclimatization to exercise at low altitudes. The role of growth hormone and LH in this conditioning process seems to be insignificant. Additionally, serum levels of insulin and lactate were also measured in these experiments. Exercise caused a decrease in serum insulin levels at both altitudes. Serum levels of lactate were decreased only at low altitude. These changes of serum levels of insulin and lactate suggest a type of metabolic adjustment to meet energy requirements. Changes in energy metabolism can be correlated by changes in the ratio of insulin to serum cortisol levels and those of other counter-regulatory hormones in response to exercise at both altitudes.

Acclimatization↗
PubMed2002

Limiting metabolic rate (thermal work limit) as an index of thermal stress.

The development of a rational heat stress index called thermal work limit (TWL) is presented. TWL is defined as the limiting (or maximum) sustainable metabolic rate that euhydrated, acclimatized individuals can maintain in a specific thermal environment, within a safe deep body core temperature (< 38.20 degrees C) and sweat rate (< 1.2 kg/hr(-1)). The index has been developed using published experimental studies of human heat transfer, and established heat and moisture transfer equations through clothing. Clothing parameters can be varied and the protocol can be extended to unacclimatized workers. The index is designed specifically for self-paced workers and does not rely on estimation of actual metabolic rates, a process that is difficult and subject to considerable error. The index has been introduced into several large industrial operations located well inside the tropics, resulting in a substantial and sustained fall in the incidence of heat illness. Guidelines for TWL are proposed along with recommended interventions. TWL has application to professionals from both the human and engineering sciences, as it allows not only thermal strain to be evaluated,. but also the productivity decrement due to heat (seen as a reduced sustainable metabolic rate) and the impact of various strategies such as improved local ventilation or refrigeration to be quantitatively assessed.

Acclimatization↗
PubMed2005

Seasonality and seasons out of time--the thermoregulatory effects of light interference.

The change in photoperiod is the main environmental cue for seasonal function of the reproductive, thermoregulatory, and immune systems in rodents existing outside of the tropics. In Israel, the social vole Microtus socialis breeds mainly under short photoperiod (SP) conditions. Previous studies showed that exposing voles to light interference (LI) in the field during the winter resulted in death. The aim of the current study was to determine the thermoregulatory response of SP-acclimated voles to LI. Therefore, heat production (VO2) at different ambient temperatures (Ta) and nonshivering thermogenesis (NST) were measured. Results show that LI of 15 min every 4h during the dark period significantly (p < 0.02) decreased VO2 at Ta = 15 degrees C and significantly (p < 0.05) decreased NST-capacity. These results can at least partly explain why LI-voles died during the winter under field conditions, through eliminating winter acclimatization of the thermoregulatory system, or what is considered as "seasons out of time."

Acclimatization↗
PubMedDate not supplied

Seasonal thermogenic acclimation of diurnally and nocturnally active desert spiny mice.

Diurnally active golden spiny mice (Acomys russatus) and nocturnal common spiny mice (Acomys cahirinus) coexist in hot rocky deserts of Israel. Diurnal and nocturnal activities expose these species to different climatic conditions. Nonshivering thermogenesis (NST) capacity of individuals of both species immediately upon removal from the field exhibited seasonal changes, with no significant interspecific difference. Colony-reared mice of either species transferred in the laboratory from long to short photoperiod increased NST capacity, though to a lesser extent than observed in the seasonal acclimatization. The underlying biochemical mechanisms of short photoperiod acclimation differed between the species. In both Cytochrome-c oxidase (Cox) activity was higher in short as compared to long photoperiod. In short-photoperiod-acclimated A. cahirinus uncoupling protein (UCP) content in brown adipose tissue (BAT) was significantly higher than in long photoperiod, while in A. russatus there was no significant change. In A. russatus there was a significant increase in lipoprotein lipase (LPL) activity in BAT in short-photoperiod-acclimated individuals, while in A. cahirinus LPL activity was high under both acclimations. The low LPL activity in brown adipose tissue of desert-adapted A. russatus may facilitate lipid uptake in white adipose tissue, an advantage in desert conditions where food is scarce and irregularly distributed in space and time.

Acclimatization↗
PubMed2000

Hematological parameters in high altitude residents living at 4,355, 4,660, and 5,500 meters above sea level.

There have been a number of reports describing the hematological indicators of Andean residents living at altitudes above 4,000 m, but several confounding factors have made the published results difficult to interpret. To clear up the effect of hypoxia on hemoglobin concentration (Hb, g/dL), hematocrit (Hct, %) and red blood cell concentration (RBC, cells/microL), this publication describes and analyzes these variables in children, men, and women from three large and homogeneous populations living at 4,355 m (n = 151), 4,660 m (n = 400), and 5,500 m (n = 273) in the Southern Peruvian Andes. Hb, Hct, and RBC increase with age in men (p < 0.001), as well as in women (p < 0.001) at the three altitudes of the study. In children (boys and girls) living at 5,500, Hb increases 11% when compared with children living at 4,355 m, and in adults, Hb increases 9.6% when comparing the same altitudes. The maximum percentage increase in Hb with age was 5.6% at 5,500 m, in men and 3.2% at 4,355 m, in women. The average percentage of difference for the Hb concentration between adult men and women is 6.6% at 4,355 m, 9.8% at 4,660 m, and 11.6% at 5,500 m. The differences in Hb concentration between men and women can only be seen after puberty. Finally, Hb is higher in older than younger women, which confirms the role of menopause in the development of erythremia. The result of this analysis reinforces the notion that Hb and Hct seem to be stable and useful parameters for acclimatization only at moderate altitudes; with aging or with increasing altitude, they may become excessive and lose their efficiency to protect the venous oxygen pressure.

Acclimatization↗