Wilderness medical providers are well-versed in the pathophysiology of the various disease processes that occur at high altitude (generally, above 2500 m., 8200 ft). Acute mountain sickness (AMS), high-altitude pulmonary edema (HAPE), and high-altitude cerebral edema (HACE) are well-studied pathologies that afflict many patients in high altitude settings, and wilderness medicine physicians must be equipped to rapidly identify and treat these conditions. Much is known in the literature about the steps outdoor enthusiasts can take to minimize their risk of developing these conditions in terms of acclimatization and prophylactic medications, yet we also know that one’s risk for developing these conditions can be due to factors outside our immediate control, such as genetic factors.
Hiking in Summit County, Colorado, elevation 11,000 ft. (Jason Cinti)
But what exactly is known about the role of exercise at altitude in terms of overall health? This topic has been mostly under the purview of exercise physiologists, athletic trainers, and endurance athletes. I would argue that wilderness medicine physicians should also seek to understand how certain types of exercise at altitude can confer benefits not only in terms of physical fitness and aerobic capacity, but to overall health and longevity. And I would argue that such an understanding should be within the wilderness medicine physician’s fund of knowledge, given that the active, outdoor-recreating patient may turn to a wilderness medicine physician for guidance in this area. Below I provide a brief overview of the different exercise training zones that providers should be familiar with, and what the data suggests in terms of which training zones individuals should target specifically at altitude in order to maximize fitness and overall health.
The body utilizes different energy systems depending on the type of exercise being performed. The two major energy systems that have come to be understood are the aerobic and anaerobic systems. More recently, physiologists have developed a more refined view of these systems, delineated by zones 1 – 5. There are various ways to think about these zones; percentage of one’s maximal heart rate (HR), lactate threshold, or functional threshold potential (FTP), amongst others. The higher the zone, the higher the exercise intensity.
Zone 1 can be thought of as exercise at the lowest intensity, such as light walking. In Zone 1, you are typically at 50-60% of your maximal HR, and you are still utilizing fat stores for energy, thus allowing liver and muscle glycogen stores to be replenished and muscles to recover.
Zone 2 has garnered the most interest. When you are in Zone 2, you can carry out a conversation, but it is slightly more uncomfortable than having a conversation at rest. Cyclists think about Zone 2 as the highest metabolic output or amount of work that can be sustained for a 7-8 hour ride, all while keeping one’s lactate level below 2 mmol / liter. It is approximately 60-70% of your maximal HR, and is known to be most suited for building one’s aerobic capacity. It is clear that the longer you spend training in Zone 2, the more aerobically fit you will be.
Zone 3 is the space between one’s aerobic threshold and anaerobic threshold. In Zone 3, you can only really manage quick bursts of words or a short sentence at a time before catching your breath. It is about 70-80% of the max HR.
Zone 4 is the lactate threshold potential or functional threshold potential (FTP). You have tipped into anaerobic metabolism at this point. It is approximately 80-90% of the max HR and is an effort that can be sustained for about one hour - that is, if a gun was pointed to your head, you could sustain it for one hour. Zone 5 is of highest intensity at 90-100% of the max HR, an effort that can only be sustained for 10-15 seconds.
Which training zones should be targeted while at high altitude to really maximize the benefits of the hypoxic conditions? One study that is worth diving into is by Wisniewska et al. in 2020, published in The Journal of Sports Medicine and Physical Fitness. The authors investigated changes in the levels of erythropoietin (EPO) and vascular endothelial growth factor (VEGF) in the blood of athletes undergoing three different altitude/hypoxic training conditions. The authors used EPO and VEGF because increased production of these factors indicate greater capillarization and improved blood oxygen-carrying capacity, which ultimately allows for better delivery of oxygen to skeletal muscle and thus improved exercise capacity.
The first group of athletes was the live high - train high (LHTH). These athletes would go about daily living, including sleeping, at altitude (1800 – 2300m), while also training at altitude. Notably, the type of training was high intensity training (Zone 4). The next group was live high – train low (LHTL), meaning daily living at altitude and then training (again, high intensity training) at sea level. Another group was the live high - base train high – interval train low (HiHiLo), which is essentially the LHTL group with the added feature of low intensity (Zone 2) at altitude. The last group was termed the intermittent hypoxic training (IHT) group, meaning daily living at sea level, and then intermittently training (again, high intensity training) at altitude. So, in all of the groups, participants were training at 100% of their FTP (Zone 4 training) for 30-45 minutes three times a week (except the HiHiLo group, which added Zone 2 training).
Their conclusions? EPO increased in the HiHiLo and LHTL groups, but not in the IHT group, whereas VEGF-A increased in the IHT and HiHiLo groups, but not in the LH-TL group. What this means is that the HiHiLo seems to be the only condition that results in an increase in both EPO and VEGF. This finding supports the notion that the added low-intensity (Zone 2) training led to improved EPO and VEGF, and thus may be the best type of training at high altitude to optimize aerobic capacity.
Is there more support for the finding that Zone 2 training at high altitude confers the most benefits? While not hard data, there are some prominent experts in the field of longevity and lifespan medicine that advocate for Zone 2 training at high altitude. Peter Attia MD, is a physician with a large platform who frequently podcasts/blogs about the benefits of Zone 2 training. Dr. Attia hosted Ryan Hall, a two-time Olympian with the fastest American marathon (2:04:58) time on his podcast in March 2022, where they discussed this topic. According to Attia, research over the past 20 years has increasingly shown the efficacy of living high and training at lower intensities, and even living high and then training at sea level at lower intensities. Attia claims that higher intensity training at high altitude can in fact be counterproductive. Hall mentioned that he primarily utilized a living high - training low model while training for his record-breaking marathon time.
This is an area of ongoing research, and more data is needed to flush out certain details such as the optimal duration of training and how long one should expect physiologic changes to be sustained after concluding training. However, for the wilderness medicine physician interested in altitude medicine who may counsel their patients about the health benefits of altitude training, the recommendation of living high and training low seems to already be on solid ground.
Putting aside longevity and the overall health benefits to be gained from altitude training aside, what are some things to consider for the athlete travelling to high altitude to specifically compete in an athletic competition? There are a few important considerations. For one, bike events or foot races often involve rapid rates of ascent, which further increases one’s risk for developing acute mountain sickness. High altitude also exposes athletes to higher levels of UV radiation, which in the acute setting may cause UV keratitis (particularly in snowy environments), and in the long-term can increase one’s risk of skin cancer and cataract formation. Thus, it is critical to ensure adequate protection with proper clothing, wrap-around sunglasses, and sunscreen with a sun protection factor of at least 30.
Athletes should also be aware of the disruption of sleep at altitude; hypoxic conditions can lead to periods of irregular respirations during sleep in which ventilation can be slowed, accelerated, or altogether interrupted, which has a significant effect on athletic performance. Furthermore, from a nutritional standpoint, studies have shown that time spent at altitude can initially result in weight loss – this is thought to be due to dysregulated protein metabolism caused by hypoxia, irregular sleep, cold exposure, and nutritional deficiencies in diet.
Whether you are advising an athlete on how to compete at high altitude or offering insight into which mode of training is optimal for improving one’s health, your patients can benefit tremendously from a basic understanding of some of the core concepts in altitude medicine and exercise physiology discussed above.