Scientists routinely perturb physiological systems away from homeostasis and study the ensuing responses to answer basic questions about how those systems are regulated. Hypoxia has been particularly useful in this regard because the stress of reduced oxygen availability induces both immediate and more gradual adaptations in every major (e.g., metabolic, cardiovascular, respiratory, hemodynamic, sympathoadrenal, etc.) physiological system. In addition to its value as a stressor that aids in understanding basic physiological processes, the study of hypoxia has considerable practical relevance. A better understanding of how hypobaric hypoxia impacts physiological function can be applied to improve the heath and well-being of high-altitude residents, soldiers, South American gold miners, mountain climbers, skiers, and ultra endurance athletes. The effect of hypoxia on substrate use has received considerable attention because it is germane to basic knowledge about metabolic regulation and has implications for nutritional requirements for humans living, working, or playing at high altitude.
HYPOXIA GENERALLY INCREASES DEPENDENCE ON BLOOD GLUCOSE
In response to hypoxia, a shift in substrate use to favor greater dependency on glucose and less use of lipid has been reported in a variety of experimental models (1,12,19,35,43). Glucose uptake by isolated rectus abdominus muscle strips was elevated two- to threefold when exposed to anoxic medium (0% O2) relative to a high- oxygen (95% O2) condition (1). Glucose transport and the concentration of muscle membrane GLUT-4 glucose transporters were both elevated when rat hind limb muscles were perfused with anoxic medium compared with normoxia. (12). Dogs exercising under hypoxic conditions used more blood glucose than when exposed to normoxia (43). In humans, both whole-body and leg glucose uptake rose sharply on acute exposure to hypobaric hypoxia (4300 m on Pikes Peak) and remained elevated above sea level values even after 18 d of acclimatization (8,35) (Fig. 1). Holden et al. (19) used positron emission tomography to show that glucose use by the heart was elevated in high-altitude-adapted Quechua and Sherpa subjects as compared with their lowlander counterparts.
In critiquing potential mechanisms that underlie the effect of hypoxia to increase the use of glucose as an energy substrate, several questions are relevant. First, are there situations in which exposure to hypoxia does not cause a shift to increased glucose use and what is "different" about those counterexamples? Second, are the changes in glucose metabolism observed at rest and during exercise consistent with the effects of hypoxia on metabolism in the fed state? Third, is there a plausible mechanism(s) to explain all (or at least the majority) of the observed patterns?
Counterexamples to greater glucose dependency with hypoxia.
In several studies, exposure to hypobaric hypoxia has not resulted in a greater dependency on glucose. In one of the first studies to systematically evaluate substrate use at high altitude, Young et al. (42) reported the opposite, i.e., reduced dependence on carbohydrate after several weeks' acclimatization. Because exercise at high altitude was done at the same relative intensity (same percent of environment-specific V˙O2peak), there is no way to determine whether economy at the same absolute workload was enhanced. The discrepancy between these results and those from the series of studies by Brooks et al. (7,8,35) is likely a result of different energy states. In the study by Young et al., the combination of hypophagia and elevated basal metabolism (10,11), both common responses to hypobaric hypoxia, resulted in energy deficit and weight loss. A shift to more fat use and muscle glycogen conservation are typical responses to energy deficit. In contrast, energy balance was maintained and weight loss was prevented in the studies by Brooks et al. to deliberately isolate the impact of hypoxia from energy deficit. The fact that energy deficit can obscure the effects of "true" hypoxia suggests that it is a more potent metabolic stimulus. Taken together, these data have led to novel research directly testing the independent effects of hypoxia and energy deficit (2) and also provided key insights that inform practical recommendations for nutrition at altitude.
All of the results noted above, irrespective of the hypoxic model, were obtained in men or male animals. There is no a priori reason to expect that the shift to glucose dependency should be limited to the male gender. However, as shown in Figure 2, when women were exposed to both acute and acclimatized (10 d) hypoxia at 4300 m on Pikes Peak, a shift to greater glucose dependency did not occur (4). At rest or during submaximal exercise, the contribution of plasma glucose to energy expenditure did not change at high altitude. Similarly, McClelland et al. (32) reported no change in the proportion of energy derived from glucose or glycogen in female rats after being acclimated to hypoxia. Instead, at sea level, substrate selection in altitude-acclimated rats varied with relative exercise intensity. Unlike men, women (or female rats) do not seem to shift substrate use toward greater glucose dependency in response to hypobaric hypoxia. Although the counterexample to greater glucose dependency at altitude observed by Young et al. can be explained by the accompanying weight loss, the sex difference noted above occurred despite maintenance of energy balance. Therefore, it seems that there is a more fundamental sex difference in the effects of hypoxia on substrate selection, at least during fasting conditions (i.e., rest and exercise).
Is increased glucose dependency tied to greater insulin sensitivity?
In several studies (e.g., Roberts et al. (35)), resting rate of glucose disapprearance from the blood was higher after acute and acclimatized altitude. Beacause circulating plasma insulin concentrations generally do not change with altitude acclimatization, does hypoxia increase glucose dependency by raising tissue sensitivity to insulin? Data from as early as 1936 suggest that glucose tolerance is enhanced after acclimatization to high-altitude (13,34). Using the more sensitive glucose clamp technique, however, Larsen et al. (24) reported that 2 d of high-altitude exposure reduced insulin sensitivity in young healthy men but returned toward the sea-level baseline after 7 d of acclimatization at 4550 m on Monte Rosa. Similarly, insulin action (estimated from insulin responses to a standardized high-carbohydrate meal) was lower compared with sea-level values in young healthy women after 2 d at a simulated altitude of 4300 m in a hypobaric chamber (5). After 10 d of acclimatization, however, estimated insulin action was actually higher than values obtained at sea level, indicating greater insulin sensitivity (3) (Fig. 3A and B).
The temporal mismatch between changes to insulin sensitivity (i.e., transient insulin resistance acute exposure with enhanced sensitivity after acclimatization) and the changes in resting glucose uptake (dramatic rise on acute exposure with return toward, but not to, sea level values after acclimatization) suggests that the two phenomena are not related. It is plausible though that increased use of glucose in the first few days of exposure leads to greater insulin-stimulated glucose uptake in the fed state to help preserve muscle glycogen stores. Green et al. (14) found that muscle glycogen stores in men were significantly lowered after acute exposure to 4300 m but were not significantly different from sea level after 3 wk of acclimatization. The similarities between men and women in how insulin sensitivity changes during altitude acclimatization contrast markedly with the clear sex difference in glucose metabolism in the nonfed (i.e., rest and exercise) states.
There is recent evidence that hypoxia up-regulates glucose transport pathways that are independent of insulin signaling (e.g., mediated via AMPK α2). These pathways are modulated by hypoxia and exercise intensity (39,41); both of which up-regulate the sympathoadrenal system and stimulate catecholamine secretion. There is clearly a relationship between plasma catecholamine concentrations and up-regulation of these noninsulin-dependent pathways for blood glucose uptake (41), although the causal mechanisms are still not clearly understood (39). Given the marked changes to the sympathoadrenal system in response to hypoxic exposure, it is reasonable to look for a downstream link to alterations in glucose metabolism.
ROLE OF THE SYMPATHOADRENAL SYSTEM
On acute altitude exposure, stimulation of the sympathoadrenal system (as evidenced by an immediate and sharp rise in circulating epinephrine) is associated with dramatically increased glucose uptake, a shift to more carbohydrate oxidation and reduced fatty acid clearance, at least in males (7,8,29,30,35). With acclimatization, there is a gradual decline in plasma epinephrine back to the sea-level baseline (29-31). In contrast, circulating norepinephrine rises gradually during acclimatization (29-31). In men, the increase in glucose dependency generally tracks these changes in blood concentrations of epinephrine; the immediate spike in glucose uptake wanes during of acclimatization but remains significantly elevated compared with sea-level baseline values (35). With the combination of high-altitude exposure and beta-blockade, the epinephrine response was potentiated, and concomitantly, glucose dependency was exaggerated relative to a hypoxia + placebo condition (7,35). In women, however, a qualitatively similar pattern of catecholamine response to acute and acclimatized altitude exposure had no significant impact on substrate use in the fasting or in the exercise condition (4,29,31). These results suggest that in men, but not in women, there is at least a crude dose-response relationship between circulating epinephrine concentrations and enhanced reliance on glucose for energy. Such a link might explain the "how" of increased glucose dependency in males but not the "why." Is there a potential advantage gained by shifting to a more glucose-based metabolism?
Glucose oxidation is more "economical" than lipid use.
More than 15 yr ago, Brooks (7) and Hochachka (17) independently proposed that, under hypoxic conditions, greater dependency on glucose was explainable as an adaptation to optimize metabolic economy (7). Brooks succinctly encapsulated the theory as follows: "Discovery of enhanced glucose dependency in hypoxia comes as no surprise because the advantage of increased energy yield per unit oxygen consumed has been suspected for some time." The biochemical basis for the increased energy yield per unit oxygen consumed is sound. Depending on the accounting systems used (9,22), complete oxidation of glucose:
and that of the most commonly oxidized fatty acid, palmitate:
indicating that a shift from pure lipid to pure carbohydrate oxidation could increase metabolic economy by 8-10%. There is even some evidence from studies of cardiac metabolism that the actual difference in energy consumption may be twice the theoretical value (19), although there are no equivalent whole-body data. Even an 8-10% increase in metabolic economy could be an advantage when oxygen availability, especially during exercise, may be limiting. At high altitude, humans and other animals often struggle to acquire sufficient energy to meet elevated (because hypoxia raises basal metabolic rate) energy requirements (10,11). Adding to the burden, hypoxia commonly inhibits appetite and energy intake declines, further exacerbating the energy deficit (10,11). An 8-10% rise in the energy output per unit input could "conserve" as much as 300-350 kcal · d−1 in a physically active man or woman. Is there experimental evidence-that high-altitude exposure increase metabolic economy?
Does increased glucose dependency confer greater metabolic economy?
Whether there is greater metabolic economy after exposure to hypoxia is a matter of some debate. In 1991, Wolfel et al. (40) found no differences in whole-body V˙O2 during steady-state exercise after 3 wk of residence at 4300 m. Matheson et al. (27) reported greater metabolic economy in high altitude natives relative to lowlander controls as measured by 31P-MRS. Levine and Stray-Gunderson (25) reported that runners exposed to moderate altitude using the "live high (approximately 8000 ft), train low" paradigm did not have better running economy after acclimatization. Saunders et al. (36), using a similar model, reported a subtle but statistically significant 3.3% decrease in oxygen consumption measured at a single speed in well-trained runners. Recognizing that the increased cost of ventilation at very high altitude could obscure any change in muscular efficiency, Green et al. (15,16) looked at whole-body V˙O2 during submaximal exercise in men immediately after returning from a 3-wk trek to the summit of Mt. Denali (6194 m). They found that whole-body V˙O2 was reduced by approximately 20% at the same absolute exercise intensity during cycle ergometry (16) and by more than 10% during leg kicking at 50 W (26). After expedition, respiratory exchange ratio was higher, indicating increased reliance on carbohydrate.
In addition to the cost of ventilation, increased resting metabolic rate at altitude is a potentially confounding variable to the measure of metabolic economy. Investigators have generally reported gross measures of V˙O2 at a given submaximal workload without factoring out resting metabolic rate or the costs of moving the limbs. Removing those parameters from the calculations and reporting "net" or "delta" efficiency (9) could impact any differences in metabolic economy because resting metabolic rate is generally higher at altitude (10,11), but the cost of moving the limbs against lower air resistance would be reduced. In that respect, the studies in which metabolic economy was measured after acclimatization to altitude, but under sea-level conditions, have some inherent advantages in terms of experimental control of potentially extraneous variables. The disadvantage is that, depending on the length of time between descent and testing, deacclimatization processes are likely to obscure the effects of altitude. A recent study by Schmitt et al. (37), however, suggests that athletes acclimatized to moderate altitude using the "live high, train low" paradigm retained a higher metabolic economy for at least 15 d under sea-level conditions. Taken together, there is support, although it is not universal, for the idea that hypoxic exposure raises metabolic economy, a result that is consistent with the shift to greater glucose dependency.
However, is the increased metabolic economy actually caused by a greater reliance on glucose as an oxidizable fuel? Green et al. (16) concluded that a shift to greater glucose dependency was insufficient to explain the observed rise in metabolic economy with altitude acclimation. Although a rise of 8-10% is possible with a shift from pure lipid to pure carbohydrate oxidation, the actual changes in substrate use are far more subtle. Given that, even at sea level, exercise at more than 45% V˙O2peak requires that the majority of the energy is derived from carbohydrate (9), the actual advantage potentially gained by greater glucose dependency is likely to be 2% or less. On the basis of an analysis of the likely sites where a 10-20% increase in metabolic economy could be generated, Green et al. (15) postulated that more efficient excitation-contraction coupling was a likely explanation (17). More efficient excitation-contraction coupling was also invoked as a potential explanation for more economical use of oxygen to produce ATP in high-altitude natives compared with lowlanders (18,27).
There is strong evidence that acclimation to hypoxia reduces mitochondrial mass and muscle oxidative capacity (20). Transcription of genes related to membrane glucose transport and glycolysis is up-regulated (38). After acclimatization to hypoxia, there is greater density of lactate transporters and increased activity of lactate dehydrogenase, both clearly related to the metabolism of carbohydrate for energy (33). Hochachka and Lutz (18) suggested that, under hypoxic conditions, muscle energy-yielding pathways were more sensitive to regulation by energy charge (ATP/ADP) than to oxygen delivery. In a comprehensive review by Hoppeler et al. (20), the authors summarized these data to propose a role for a tighter coupling between the supply and demand for ATP.
On the basis of several lines of evidence 1) the potential to increase metabolic economy by only a few percent with a shift to more glucose use, 2) the lack of consistent association between increasing glucose dependency and greater metabolic economy, and 3) the likelihood that other adaptations to hypoxia, involving tighter coupling between energy use and energy provision, are more promising mechanistic explanations; the shift to a more "glucose-based metabolism" in response to hypoxia does not seem to be responsible for greater metabolic economy. The lack of a comparable shift in substrate metabolism in women, despite a qualitatively similar sympathoadrenal response, suggests that the sex differences are mediated by the differential impact of circulating catecholamines on carbohydrate and lipid use. Such a difference has been postulated before (6,21) and is consistent with sex differences observed in response to other metabolic stressors (e.g., hypoglycemia, exercise) (6).
Greater dependence on glucose for energy does not seem to be the cause of the increased metabolic economy often observed in response to high-altitude exposure and acclimatization. The greater glucose dependency may be an obligatory response to stimulation of the sympathoadrenal system, at least in men and male rodents (Fig. 4). Although the relationship may not be causal, increased use of glucose does serve to maximize ATP yield per unit oxygen, albeit with the downside of more rapidly depleting the limited reserves of stored carbohydrate. The balance between those 2 competing forces was succinctly captured by McClelland et al. (32): "when exercising in hypoxic environments, animals and humans must strike a metabolic compromise to deal simultaneously with low O2 availability and small CHO reserves… The strategy observed here suggests that the energetic constraint imposed by limited CHO reserves outweighs the O2-saving advantage of this critical substrate."
A debt of gratitude is owed to pivotal figures in hypoxia and metabolism who continue to serve as role models: the late Peter Hochachka, Ph.D., George Brooks, Ph.D., and the late Jack Reeves, M.D. I have also been fortunate to work with an array of impressive colleagues: Lorna Moore, Ph.D., the late Gail Butterfield, Ph.D., Paul Rock, M.D., Ph.D., Gene Wolfel, M.D., Anne Friedlander, Ph.D., Bob Mazzeo, Ph.D., Steve Muza, Ph.D., Charles Fulco, Ph.D., and Al Cymerman, Ph.D.
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