Ascent to Altitude as a Weight Loss Method: The Good and Bad of Hypoxia Inducible Factor Activation

Abstract

Objective

Given the epidemic of obesity worldwide there is a need for more novel and effective weight loss methods. Altitude is well known to be associated with weight loss and has actually been used as a method of weight reduction in obese subjects. This review demonstrates the critical role of hypoxia inducible factor (HIF) in bringing about the reduction in appetite and increase in energy expenditure characteristic of hypobaric hypoxia

Design and methods

A MEDLINE search of English language articles through February 2013 identified publications associating altitude or hypobaric hypoxia with key words to include hypoxia inducible factor, weight loss, appetite, basal metabolic rate, leptin, cellular energetics, and obesity. The data from these articles were synthesized to formulate a unique and novel mechanism by which HIF activation leads to alterations in appetite, basal metabolic rate, and reductions in body adiposity.

Results

A synthesis of previously published literature revealed mechanisms by which altitude induces activation of HIF, thereby suggesting this transcription factor regulates changes in cellular metabolism/energetics, activation of the central nervous system, as well as peripheral pathways leading to reductions in food intake and increases in energy expenditure.

Conclusions

Here we present a unifying hypothesis suggesting that activation of HIF under conditions of altitude potentially leads to metabolic benefits that are dose dependent, gender and genetic specific, and results in adverse effects if the exposure is extreme.

Introduction

The drive and requirement to eat food is a major barrier limiting the success of any weight loss initiative. Hunger is invariably associated with caloric restriction. In addition, weight loss is accompanied by a decrease in energy expenditure. Reductions in energy expenditure mirror the decline in body weight following any given reduction in caloric intake adding to the difficulty obese subjects face when initiating or maintaining a diet. The most effective weight loss strategy is to suppress appetite and at the same time maintain or even increase energy expenditure.

Disease states such as chronic obstructive pulmonary disease and cancer are conditions where there is a persistent loss of appetite with an increase in energy expenditure. Another situation in which weight loss is accompanied by appetite suppression and increased energy expenditure is when otherwise healthy subjects are taken to altitude. While the cause of weight loss in chronic obstructive pulmonary disease and cancer is complex and multifactorial, a feature these diseases share in common with normal subjects exposed to altitude is hypoxia (1). At altitude hypoxia is secondary to the reduction in barometric pressure causing a decrease in the inspired partial pressure of oxygen whereas hypoxia is due to parenchymal lung injury in chronic obstructive pulmonary disease. Hypoxia in cancer patients is more localized and results from tumor growth outstripping its vascular supply (2).

This review will examine the physiologic response to hypoxia and focus on those mechanisms which may contribute to weight loss in these otherwise disparate conditions. Several points will be emphasized. First, hypobaric hypoxia facilitates weight reduction not only by suppressing appetite but also by increasing energy expenditure. Second, up-regulation of hypoxia inducible factor (HIF) mediates the loss in weight, change in energetics, and alterations in metabolism which accompany ascent to altitude. Third, the degree to which altitude leads to weight reduction is dose dependent; with modest altitude, loss of weight is comprised of fat with sparing of lean muscle mass; while exposure to extreme altitudes results in muscle wasting. Fourth, increases in HIF activity causes reductions in exercise performance. Fifth, genetics and gender account for variable differences in weight reduction and exercise performance observed. While sojourns to altitude may not be a feasible strategy to lose weight for the vast majority of obese subjects, understanding the mechanisms by which altitude exposure causes weight loss in the face of reduced appetite and increased energy expenditure may lead to novel therapeutic tools for treating obesity.

Hypoxia Inducible Factor (HIF)

HIF is a transcription factor which transactivates genes facilitating an adaptive response to conditions of hypoxia (3,4). There are three heterodimers (HIF-1, HIF-2 and HIF-3) consisting of an oxygen-sensitive α-subunit and a constitutively expressed β-subunit. In the presence of oxygen, HIFα is unstable due to an oxygen-dependent hydroxylation mediated by three prolyl hydroxylases (PHD1–3) which target HIF for proteosomal degradation. Prolyl-hydroxylated HIFα is bound by the von Hippel-Lindau (VHL) tumor suppressor protein and ubiquinated, marking HIFα for proteosomal degradation. In the presence of low oxygen, HIFα is not hydroxylated and accumulates in the nucleus where it dimerizes with HIFβ and binds to hypoxia response elements in target genes.

Activation of HIF increases red cell mass by increasing erythropoietin, increases angiogenesis through stimulation of vascular endothelial growth factor (VEGF), and causes a shift in metabolism away from oxidative phosphorylation to a less oxygen requiring production of ATP via the glycolytic pathway (5) (Figure 1). Since glycolysis only produces 2 ATP’s for every mol of glucose, upregulation in HIF leads to greater dependency on glucose uptake in order to generate adequate amounts of ATP.

Figure 1.

Under condition of limited oxygen supply upregulation of hypoxia inducible factor leads to a shift in cell metabolism favoring glycolysis so as to limit generation of potentially harmful reactive oxygen species via oxidative phosphorylation in the mitochondria. A critical step in this shift is HIF-mediated activation of pyruvate dehydrogenase kinase-1 (PDK-1). This enzyme inactivates pyruvate kinase which is the mitochondrial enzyme responsible for converting pyruvate to acetyl-CoA. In combination with activation of lactate dehydrogenase A (LDHA) which converts pyruvate to lactate, there is less delivery of acetyl-CoA into the Krebs cycle and therefore a reduction of flavin adenine dinucleotide (FADH2) and nicotinamide adenine dinucleotide (NADH) delivered to the electron transport chain. HIF up regulates the expression of GLUT1 and GLUT3 on the cell membrane so as to facilitate glucose entry into the cell. HIF also up regulates monocarboxylate transporter 4 which is a proton-lactate symporter. This transporter creates an avenue for lactate exit from the cell. The cotransport of the proton contributes to the maintenance of intracellular pH. Cell pH is also defended by HIF-induced expression of carbonic anhydrase-9 and increased expression of the Na⁺/H⁺ antiporter, NHE1.

Both HIF-1α and HIF-2α are increased several fold in skeletal muscle biopsies taken after 7–9 days following ascent to 4559 m (6). Glucose kinetic studies using radiolabeled tracers show high altitude is associated with increased rates of glucose appearance, disappearance, and oxidation both at rest and exercise compared to sea level consistent with a HIF mediated change in body metabolism (7–9). Since metabolism of glucose via glycolysis requires less oxygen as compared to oxidative phosphorylation, this metabolic shift represents an adaptive response to limited oxygen availability.

Activation of the Cori cycle provides the necessary substrate to support the increased dependency on glucose uptake. Under conditions of hypoxia cell lactate is exported and taken up by the liver in a more efficient manner where it can then serve as a substrate for increased gluconeogenesis (10). HIF directly binds to the promoter region of phosphoenolpyruvate carboxykinase (PEPCK), the rate limiting enzyme for gluconeogenesis providing a molecular mechanism by which hypoxia can stimulate hepatic as well as renal gluconeogenesis (11,12). Taken together, increased gluconeogenesis under conditions of hypoxia serves to minimize accumulation of lactate in the setting of increased cellular production and provide adequate amounts of glucose to facilitate the switch in fuel utilization.

This metabolic shift creates an energy wasting cycle and may play a role in the increased basal metabolic rate (BMR) characteristic of altitude exposure (Figure 2). In a recent study 20 male obese subjects went to an environmentally controlled research station at an altitude of 2650 meters (m) (13). Subjects were allowed to eat and drink without restriction and physical activity was limited to slow walks throughout the station for 7 days. Body weight decreased from 105.1 kg to 103.6 kg, caloric intake decreased by ~734 kcal/day, and BMR increased from 23.7 to 27.3 EE/kg/day at the end of the seven day period. Sustained body weight reductions were present after returning to low altitude (530 m) 4 weeks later.

Figure 2.

An increase in Cori cycle activity under conditions of hypoxia can supply the necessary substrate to fuel the greater dependency on blood glucose brought about by increased HIF activity. Under conditions of hypoxia both hepatic lactate uptake and glucose production are increased. HIF up-regulates the gene expression of phosphoenolpyruvate carboxykinase which is the rate limiting enzyme for gluconeogenesis. This change in metabolism is energy inefficient since only 2 ATP’s are formed for each mol of glucose metabolized and 6 ATP’s are consumed for every 2 molecules of lactate converted to glucose. This energy wasting may play a role in the increase in basal metabolic rate which occurs at altitude. This energy inefficiency may also be detrimental to exercise performance at altitude do to the shift away from mitochondrial respiration where up to 34 ATP’s are formed for each mol of glucose metabolized.

BMR increases in lowlanders by 6–27% over the first several days after arrival at high altitude which is directly proportional to altitude gain. At 3650 m BMR increases by 6%, whereas increases of 10% and 27% occur at 3800 and 4300 m respectively (14–16). With acclimatization there is a variable decline in BMR to some lower steady state, but BMR generally remains above sea level values. In one study acute altitude exposure to 4300 m increased basal metabolic rate by 27% over that of sea level and remained elevated by 17% after 3 weeks of acclimatization (16).

Upregulation in HIF activity has been implicated in energy wasting and progressive weight loss in cancer patients (17,18). Expansion of proliferating cells in tumors outstrips the vascular supply leading to zones of hypoxia and stabilization of HIF resulting in a shift to glycolysis as the primary means for ATP synthesis. This phenomenon, known as the Warburg effect, leads to increased lactic acid production and glucose uptake in cancer cells which are indicators of tumor aggressiveness. Cori cycle activity and glucose production and turnover rates are significantly greater in cancer patients with progressive weight loss in comparison to those whose weight is stable (19,20). The increase in Cori Cycle activity has been estimated to account for 300 kcal/day of additional energy loss in cancer patients (21). All of these effects are similar to what happens following altitude exposure and increases in HIF activity.

Decreased Appetite with Altitude

Reductions in appetite are maximal in the first several days upon arrival to altitude when protein and caloric intake decrease 30 and 40% respectively. Below 5000 meters food intake tends to return towards normal following several days of acclimatization (22,23). At extreme altitude anorexia is more pronounced and becomes persistent.

Leptin may be a critical factor influenced by altitude and responsible for changes in energy expenditure and food intake. Leptin is an adipocyte derived hormone which circulates in the plasma at concentrations proportional to fat mass. Leptin crosses the blood brain barrier where it activates its receptors in the hypothalamus to control body weight by reducing food intake and increasing energy expenditure (Figure 3) (24). The leptin gene contains HIF response elements providing a molecular mechanism for increased leptin gene expression under conditions of hypoxia (25).

Figure 3.

Circulating leptin is transported across the blood brain barrier where it binds to key hypothalamic neuronal sites such as in the arcuate nucleus and specifically on the proopiomelanocortin (POMC) neuron. This interaction leads to release of the POMC-derived neuropeptide, α-melanocyte stimulating hormone (α-MSH) which in turns binds to melanocortin 4 receptors in the paraventricular nucleus. The POMC/ MC4-R system mediates leptin signaling to induce anorectic effects. MC4R activation also increases peripheral sympathetic nerve activity, probably by both direct and indirect signaling processes, leading to increased expression and activity of mitochondrial uncoupling protein 1 (UCP-1) in brown adipose tissue. This protein uncouples oxidative phosphorylation causing an increase in thermogenesis and increased energy expenditure. Sympathetic nerve activity stimulated by leptin is subject to negative feedback as catecholamines inhibit leptin gene expression in fat through β-adrenergic receptors.

The role of leptin in mediating weight loss upon altitude has provided conflicting results. (26). Some of this discrepancy may be methodologic in nature, since leptin is secreted in a diurnal fashion and is subject to feedback regulation; therefore, values are likely to vary due to timing of the sample collection and whether the subject was at altitude acutely or chronically. In two independent studies ascent to altitude was associated with increased leptin levels with more pronounced changes in subjects with the greatest weight loss (27). Importantly, weight loss per se’ reduces leptin levels since leptin is secreted by fat cells; therefore, loss of adiposity can mask the stimulatory effect of hypoxia on leptin production. Failure to account for the effect of weight on leptin production may account for the failure of some studies to detect elevated levels. Importantly, leptin levels are greater in subjects following weight loss at altitude when compared to comparable weight loss at sea level (28).

Leptin signaling increases sympathetic nerve activity contributing to increases in BMR at altitude (29). Upon arrival to altitude, sympathetic nerve activity is increased and remains so even in well acclimatized subjects (30,31).

Reduced appetite following hypobaric hypoxia exposure results from increased HIF activity in the hypothalamus. The HIF-2α isoform is expressed in the arcuate nucleus of the hypothalamus where it functions as a nutrient sensor. The POMC gene contains HIF response elements providing a molecular mechanism for the ability of HIF to regulate POMC neuronal activity (32). Selective deletion of HIF-2α in the hypothalamus increases body weight and reduces energy expenditure. By contrast, over expression of HIF-2α in the arcuate nucleus results in a hypermetabolic state with resistance to diet-induced obesity.

Acclimatization Attenuates HIF Activity

The maximal increase in BMR and suppression of appetite soon after arrival to altitude followed by a return toward baseline levels with acclimatization mirrors the pattern of HIF activity upon exposure to hypoxia. Mice exposed to a simulated altitude of 4300 m had muscle protein levels of HIF-1α increase by 70% returning to baseline after one week of exposure (33). Blood and muscle lactate levels with exercise are significantly greater following 24 hours of hypoxia, but return towards normoxic levels after one week of hypoxic exposure. Down regulation of HIF activity and a secondary reduction of PDK1 would allow exercising muscle to assume a phenotype more reminiscent of muscle at normoxia where pyruvate conversion to acetyl-CoA is preferred and conversion to lactate is reduced. Although controversial, this phenomenon has been referred to as the lactate paradox (34). PHD2 and PHD3 are transcriptionally regulated by HIF and retain modest levels of activity under conditions of hypoxia providing a mechanism for downregulation of HIF (35).

Changes in erythropoietin levels acutely and chronically at altitude also reflect changes in HIF activity. Erythropoietin increases rapidly with peak levels occurring 48–72 hours upon altitude exposure; however, levels return toward baseline following acclimatization (36,37). Upon further ascent, erythropoietin increases and, depending on the degree of elevation, may remain significantly elevated. Using erythropoietin levels as a biomarker of HIF activity, this pattern suggests feedback inhibition of HIF activity can be overridden by more extreme hypobaric hypoxia.

Adverse Effects of Persistent HIF Signaling

At moderate altitude loss of body fat accounts for 70% of weight reduction; whereas, with extreme elevations, muscle catabolism becomes dominant (38). For this reason base camps are placed at altitudes no higher than 5000–5500 meters since body weight homeostasis can be maintained (Table 1). Operation Everest II placed normal subjects in a decompression chamber and exposed them to progressive lowering of inspired O₂ pressure to simulate a 40 day ascent of Mt. Everest (39). Changes in weight and appetite were attributed to hypobaric hypoxia since participants were not subjected to cold, overexertion, or other rigors of climbing high mountains. Weight was reduced by 7.44 Kg; however in addition to weight loss, total muscle area calculated in 6 subjects from CT scans of the thigh and upper arms was reduced by 13 and 15% respectively.

Table 1.

Beneficial and Adverse Effects of HIF Activation: Influence of Altitude

Parameter	<5000 meters	>5000 meters
HIF activity	Initial increase followed by decline to lower steady state level (33)	Persistent increase
Basal metabolic rate	Initial increased with decline toward lower steady state level (14–16)	Sustained increase
Appetite	Reduced followed by improvement with acclimatization (22,23)	Persistent reduction
Body weight	Reduced with preferential loss of fat	Reduced with loss of fat and lean muscle mass
Muscle mass	Stable	Reduced with decrease in mitochondrial volume
Leptin	Increased (26,27)	Increased when factored for weight loss (28)
ATP production	Mild reduction	Severe reduction

References where indicated are in parenthesis.

Peroxisome proliferator-activated receptor (PPAR)-γ coactivator (PGC)-1α and its homolog PGC-1β are transcriptional co-activators abundant in skeletal muscle which stimulate mitochondrial biogenesis. HIF induces mitochondrial autophagy and downregulates PGC-1α accounting for reductions in mitochondrial volume noted on muscle biopsies taken from climbers at high altitude (40,41). PGC-1α levels are decreased by 35% in climbers after extended stay at altitude with ascent beyond 6400 meters (42).

HIF-induced changes in cellular energetics come at the expense of less ATP production resulting in rapid fatigue in exercising muscle. Chuvash polycythemia is an autosomal recessive disorder due to a mutation in the VHL gene resulting in an inability of VHL to bind to HIF-α causing stabilization of HIF and increased expression of HIF-target genes under normoxic conditions (43). A marked reduction in exercise capacity along with increased mRNA expression for PDK1 and other glycolytic enzymes on muscle biopsy demonstrate these patients are limited in the capacity to utilize oxygen due to a shift in metabolism away from oxidative phosphorylation. By contrast, experimental mice with selective deletion of HIF in skeletal muscle exhibit increased exercise endurance when compared to controls (44). Glycolytic and mitochondrial enzyme activity and reductions in lactate with exercise indicate these mice have increased activity of oxidative pathways in muscle.

Genetic variability in HIF activation at altitude

Lowlanders ascending to altitude have increases in hemoglobin and hematocrit which are higher than basal levels of native highlanders. Tibetans living at high altitude have hemoglobin levels similar to lowlanders, and upon ascent to higher altitudes have less increase when compared to those residing at lower elevations. Genomic and candidate gene comparisons of Tibetan highlanders and lowland Han Chinese indicate a divergence between Tibetans and lowlanders in the allelic frequency of single-nucleotide polymorphism (SNP) alleles located in or near the HIF-2α/EPAS1 and PHD2/EGLN1 genes which correlates with lower hemoglobin levels in the Tibetans (45,46). Activation and stabilization of HIF-2α mediates hypoxia-induced increases in erythropoietin production, therefore, HIF-2α/EPAS1 and PHD2/EGLN1 allelic differences reflect the reductions in hemoglobin found in Tibetans conferring a survival advantage at high altitude by facilitating a blunted hematologic response to hypobaric hypoxia.

In a manner analogous to the effect on hemoglobin, shifts toward anaerobic metabolism under the dictates of HIF is attenuated and aerobic metabolism via oxidative phosphorylation is better preserved in Tibetans. This difference allows for greater ATP production for exercising muscle accounting for the enhanced climbing performance Sherpas exhibit on high altitude expeditions. Enzyme activity measured in Sherpa muscle biopsy specimens demonstrate low activity of LDH relative to pyruvate kinase, indicative of muscle metabolism shifting to burning carbohydrate to CO₂ and H₂O as opposed to lactate (47). Furthermore, the intensity of exercise where lactate begins to accumulate in the blood (lactate threshold) is greater in Sherpas when compared to lowlander populations.

Weight changes in Sherpas following prolonged altitude exposure are also reduced indicative of an attenuation of HIF effects on metabolism. In the American Medical Research Expedition to Everest (AMREE) study, Sherpas who arrived at base camp with half as much body fat as Western counterparts were able to maintain body weight during prolonged durations above 5400 meters (38). Limb circumference remained the same in the Sherpas compared to reductions in the Westerners indicating preservation of muscle mass in the Sherpas.

Sexual dimorphism in HIF activation at altitude

There is a sexual dimorphism in metabolism at altitude suggestive of hormonal influences on HIF activity. As discussed previously, HIF induces a shift in metabolism favoring increased glucose utilization in men taken to altitude. By contrast in women taken to 4300 meters, blood glucose utilization rates are lower when compared to values measured at sea level (48). Measurement of the respiratory exchange ratio suggests women exposed to altitude demonstrate a greater reliance on fatty acids for metabolism.

Shifts in metabolism towards fatty acid oxidation provide greater amounts of ATP translating into improved exercise performance when compared to men under hypoxic conditions. Comparisons of men and women undergoing high-intensity intermittent static contractions of the adductor pollicis muscle at sea level, and shortly after arrival to 4300 meters, demonstrates reductions in altitude-induced impairments in women (49). In particular, women have reduced fatigue rates and enhanced endurance times when compared to men. Furthermore, women have less altitude-induced changes in body weight when compared to men. Eight college women residing at 4300 meters for 2.5 months had reductions in skin fold thickness and limb circumference, but modest changes in body weight (50). The authors concluded women had similar energy intake requirements at high altitude and at sea level. Additionally, 12 women taken to 5050 meters for 21 days, had no change in body mass or fat free mass compared to baseline values (51). Basal metabolic rate was increased by 6.9% above sea level in these women by day 3 upon exposure to 4300 meters; however BMR returned to sea level values by day six (52). This transient rise in BMR is in contrast to the greater and more prolonged increase in BMR reported in men at a similar altitude.

Attenuation in HIF activation in response to hypobaric hypoxia in woman may account for gender differences in glucose utilization, exercise performance, body weight change, and changes in BMR noted above. Specifically, estrogens have been demonstrated to downregulate HIF activity. In ovariectomized (OVX) female rats there is increased protein expression of HIF-1α in periaortic fat when compared to non-OVX controls (53). Western blot and immunohistochemical staining demonstrate reductions in HIF-1α to basal levels following estrogen treatment. In rodent models of obstructive sleep apnea where animals are exposed to intermittent bouts of hypoxemia, estrogen administration significantly attenuates genioglossus muscle fatigue (54). Both mRNA and protein levels of HIF-1α were increased in intermittent hypoxia animals as compared to controls; however, estradiol administration decreased both gene expression and protein levels of HIF-1α dose dependently. Physiologic doses of 17-β-estradiol attenuates hypoxia-induced erythropoietin gene expression by interfering with hypoxia-induced increases in HIF-1α levels and activity (55). Lastly, chronic mountain sickness is rare in premenopausal women; whereas it occurs frequently in post-menopausal women not on estrogen supplementation suggestive of estrogen-induced reductions in HIF-mediated erythrocytosis (56).

In summary, hypobaric hypoxia through activation of HIF brings about the ideal ingredients to effect significant weight loss; namely, a decrease in appetite and increase in metabolic rate. This benefit is dose dependent, gender and genetic specific, and results in adverse effects if the exposure is extreme. To be sure, other mechanisms may influence weight loss at altitude such as altered intestinal function, increased inflammatory cytokines such as interleukin-6, and the physical rigors associated with mountaineering to include physical exertion, exposure to cold, and reduced availability of food (57). Understanding the mechanisms by which altitude exposure causes weight loss in the face of reduced appetite and increased energy expenditure can lead to novel therapeutic tools for treating obesity. These might include drugs designed to activate HIF so as to increase energy expenditure and reduce appetite. Such agents would likely have to be administered on an intermittent basis in order to maximize their benefit since acclimatization attenuates the weight reducing effects of HIF activation. Intermittent dosing would also serve to minimize the potential of adverse effects of persistent HIF signaling.

What is already known about this subject?

1. Altitude is associated with weight loss and decreased appetite

2. Altitude is associated with an increase in basal metabolic rate

3. Hypoxia leads to activation of hypoxia inducible factor (HIF)

What does this study add?

1. Activation of Hypoxia Inducible Factor (HIF), which is a transcription factor, provides a unifying mechanism by which exposure to altitude induces weight loss, increases energy expenditure, and shifts in metabolic flux.

2. Activation of HIF upon altitude exposure may transcriptionally upregulate leptin levels, enhance leptin sensitivity, which in turn may suppress appetite and facilitate weight loss through increased energy expenditure.

3. There are gender and genetic differences in metabolism following exposure to altitude which may be a result of differing levels and sensitivity to activation of HIF.