Hypoxia refers to the reduced oxygen content of the air. The average person may wonder why exercising or even living under that kind of self-imposed stress would be desirable….but endurance athletes are a different breed. There is a widespread popularity for athletes looking to gain an edge in cardiovascular fitness to travel to places of higher altitude for training, some even relocating. It is not unknown that the world’s best East African runners live and train at altitudes of 1500 to 2000m. Much [DS1] of the literature up to this point has gone back and forth on whether or not training at altitude really enhances performance. The physiological adaptations occur at essentially four steps in the oxygen transport system starting with alveolar ventilation and moving down the chain through lung diffusion, circulatory oxygen transport, and ending with tissue oxygenation. Hypoxic environments can be both beneficial and detrimental on the body’s adaptive mechanisms, depending on the mode and duration of exposure. This article serves to weigh the risks versus the benefits when considering altitude training as a means of enhancing cardiovascular and all-around fitness.
The latest research in the Journal of Strength and Conditioning from August 2014 examined an eight-week program of intermittent hypoxic training (IHT) at submaximal versus maximal workout intensities. The IHT scheme consisted of two, weekly 40-minute runs under normobaric hypoxia interspersed with normoxic (normal oxygen) training on the other days. The experimental hypothesis stated that IHT would elicit greater gains in both the submaximal and maximal categories, with a resulting enhancement of exercise tolerance when compared with the control group. The single-blind study results proved IHT to be effective in enhancing cardiovascular fitness during submaximal intensity, but that same was not true for maximal exercise. The enhancement in cardiovascular fitness was attributed to the submaximal heart rate reductions observed post-IHT. (2)
One concern may be the effect of hypoxic environments on the immune system. This stems from the fact that after acute bouts of low-intensity exercise the body’s immune system is boosted, but after high-intensity workouts, the opposite is true, at least temporarily. Since altitude training provides a seemingly “more intense” workout environment, the concern for the immune system takes shape. One study examined the immunologic and metabolic markers as they vary under hypoxic stress (1). After measuring these markers under normoxic and hypoxic conditions and differing workout intensities, the study concluded that acute hypoxic environments do not adversely affect the immune system or metabolic response, and may be of benefit to the athlete (1).
Under hypoxic stress, the body signals a number of red flags that danger lurks around the corner. One of the most obvious ones is the stress that hypoxia puts on the heart. One cardiac marker in particular, NT-proBNP, is the usual measure for heart-related pathologies such as heart wall stress and heart failure. A 2010 study by Banfi and colleagues evaluated the NT-proBNP levels in mountain marathoners pre- and post-race. They found that the cardiac marker levels in the professional marathon group increase from 39.7pg/mL to 97.6pg/mL on average, with any number above 125pg/mL being pathological. In the nonprofessional marathoners however, the levels started at 106pg/mL pre-race and jumped to 182pg/mL, putting this category of recreational athletes at a greater risk for exercise-induced heart damage. Both of these groups had lower levels of NT-proBNP at rest than nonathletes, which creates more of a buffer zone so the spike in cardiac stress during the race does not reach the pathologic threshold. Interestingly, the levels in altitude runners were never higher than those observed in athletes at sea level. The study concluded that exercise in general is the correlate to increased levels of NT-proBNP and thus cardiac stress, and running at altitude did not appear to be the culprit. With that being said, NT-proBNP provides a reliable prediction of heart-injury risk in both the professional and recreational runner and may be used as a screening tool for prevention. (3)
Another study by Robertson et al (2010), evaluating swimmers, found that altitude training produced negligible performance advancement when compared with the non-altitude trained swimmers at the level of National Competition. There were measurable gains observed such as hemoglobin mass and physiologic improvement, but the correlation to performance was nonexistent. Training at altitude did however, proved to be individually successful for certain athletes and hindering to others. (4)
On the psychological side of the coin, the rate of perceived exertion at altitude may negatively prevent the reaping of cardiovascular benefits. Buchheit et al (2012), confirmed this theory, stating that hypoxia itself, rather than exercise-related stress, increases perceived exertion. The hypoxia-induced central fatigue is the culprit, owing to its resulting reduced oxygenation to the brain. Overall the study negated the expected benefits of altitude training, due to reduced cardiovascular, neuromuscular, and motor control responses and the substantial increase in perceived exertion in the athletes being tested after training at 2400m. (5)
One of the most common adverse effects experienced at altitude in the general population is acute mountain sickness (AMS). The array of nonspecific symptoms including headache, nausea, loss of appetite, insomnia, dizziness, and peripheral edema, usually affect 25% of the population within six to eight hours of exposure to moderate altitude (2000 to 3000m) or hypoxia (6). Strenuous exercise is thought to intensify AMS symptoms. Classic high altitude training is done at 2000 to 2500m. A study by Schommer et al (2012), investigated the health risk for AMS in athletes with prolonged exposure to this moderate altitude range (6). Data from the study suggests that more intense physical activity does not increase the prevalence or severity of AMS, meaning the risk is the same whether travelling to altitude as a tourist or a competitive athlete (6). Moderate altitudes can however, exacerbate present cardiovascular and/or pulmonary conditions or even lead to initial manifestations of previously undiagnosed conditions. Another concern to be aware of for those with sickle cell trait is the risk of splenic infarction under hypoxic conditions (6).
Hypoxia increases the demand for and mobilization of iron within the body, making endurance athletes particularly prone to iron deficiency (7). Iron supplementation is worth considering, especially in vegetarian athletes who may have trouble meeting daily iron requirements normally. There have been a few minor study findings of oxidative free radical tissue damage under hypoxia (7). To avoid this adverse effect, tocopherol (Vitamin E) supplementation can limit tissue injury via its antioxidant properties (7). Sustained exposure to severe hypoxia has detrimental effects on muscle structure as implicated by Hoppeler and Vogt’s study on muscle tissue adaptations to hypoxia (8). Muscle tissue oxidative capacity is found to be moderately reduced by acute exposure to altitude (8). It was expected that with two months of exposure, there would be a larger capacity for oxygen use and delivery, but this was not the case (8).
Another adverse response to altitude training is a phenomenon known as detraining. Since hypoxic conditions make sea-level workout paces difficult to match, the intensity at altitude tends to be lower in order to sustain endurance and avoid over-fatiguing. The resulting fitness gains from each environment are seemingly equivalent based on effort level, however, upon returning to sea level for training, the athlete may observe a detraining response from persistent training at a lower intensity. This can be physiologically explained by the reduction in VO2 max, which decreases proportionally in a linear fashion above 1,500m, at a rate of 10% per 100m (9). During the acute phase of exposure, the low oxygen content in arterial (systemic) circulation is most responsible for the early reduction in maximal aerobic power, but chronic impairment results from an adaptive decrease in cardiac output due to reduced plasma volume (9).
These findings provoked the “live high-train low” principle, in which the athletes sleeps or spends 8-20hrs reaping the hematological benefits [such as increases in red blood cell volume] at altitude and training at low levels to keep the intensity high (10). Another study involved training competitive cyclists with one leg in a hypobaric chamber (2300m) and the other under sea level conditions for 4 weeks with 3-4 sessions per week (8). The work capacity of the hypobaric-trained leg was increased and time to fatigue was significantly improved (8).
One can infer from the above presented evidence, that the risks involved with altitude training greatly outweigh the benefits. I was personally in disbelief of the surmounting data stacked against my prior conviction that altitude training was the secret to getting on the elite level of endurance sports, the classic advantage that only the best in the world seek. I am happy to share the other side to this debate and hope to raise some concern for the lack of performance enhancement typically expected of this training lifestyle. The “live high, train low” model seems to be the most promising and noteworthy of evidence in favor of altitude, and definitely worth considering if you have the means to live in such a way. This article should invoke some rethinking of the importance of workout intensity in sea-level training and a greater appreciation for the ability to sustain such break-neck pacing with normal amounts of oxygen. Happy running!
- Blegen, M, Cheatham, C, Caine-Bish, N, Woolverton, C, Marcinkiewicz, J, & Glickman, E. “ The immunological and metabolic responses to exercise of varying intensities in normoxic and hypoxic environments.” Journal of Strength and Conditioning Research 22:5 (Sept 2008). Pgs 1639-44.
- Holliss, BA, Burden, RJ, Jones, AM, and Pedlar, CR. “Eight weeks of intermittent hypoxic training improves submaximal physiological variables in highly trained runners.” Journal of Strength and Conditioning Research 28:8 (Aug 2014). Pgs 2195–2203.
- Banfi, G, Lippi, G, Susta, D, Barassi, A, Melzi d’Eril, G, Dogliotti, G. & Corsi, MM. “NT-proBNP concentrations in mountain marathoners.” Journal of Strength and Conditioning Research 24:5 (May 2010). Pgs 1369-72.
- Robertson, EY, Aughey, RJ, Anson, JM, Hopkins, WG, & Pyne, DB. “Effects of simulated and real altitude exposure in elite swimmers.” Journal of Strength and Conditioning Research 24:2 (Feb 2010). Pgs 487-93.
- Buchheit, M, Kuituen, S, Voss, SC, Williams, BK, Mendez-Villanueva, A, & Bourdon, PC. “Physiological strain associated with high-intensity hypoxic intervals in highly trained young runners.” Journal of Strength and Conditioning Research 26:1 (Jan 2012). Pgs 94-105.
- Schommer, K, Menold, E, Subudhi, AW, & Bartsch, P. “Health risks for athletes at moderate altitude and normobaric hypoxia.” British Journal of Sports Med 46:11 (Sept 2012). Pgs 828-32.
- Baily, DM and Davies, B. “Physiological implications of altitude training for endurance performance at sea level: a review.” Journal of Sports Medicine 31:1 (1997). Pgs 183-90.
- Hoppeler, H, & Vogt, M. “Muscle tissue adaptations to hypoxia.” Journal of Experimental Biology 204 (July 2001). Pgs 3133-39.
- Fulco, CS. “Human acclimatization and physical performance at high altitude.” Journal of Applied Sport Science Research 2:4 (1988). Pgs 79-84.
- Lundby, C, Millet, GP, Calbet, JA, Bartsch, P, & Subudhi, AW. “Does ‘altitude training’ increase exercise performance in elite athletes.” British J of Sports Med 46:11 (Sept 2012). Pgs 792-5.