For many adventurers, scaling Mount Everest is an endurance test like no other. For the Sherpas, the ethnic Nepalese who help them ascend to its summit, it's all in a day's work. After all, Sherpas routinely set the record for the fastest and most ascents – in fact, two of them have made it to the top of Everest 21 times. So how do they do it? According to a new study led by Cambridge University scientists, the answer could literally be in their blood.

For those attempting to make their way up the world's tallest mountain, the key challenge is overcoming the high altitudes and low-oxygen atmosphere. That's why an Everest expedition takes around two months – climbers need to acclimatize to their surroundings and give their bodies time to adapt to the changes so as to prevent altitude sickness, which can result in headaches and nausea in mild cases, and death due to a build-up of fluid in the brain and/or lungs at the other end of the scale. Oftentimes, climbers have to rely on supplementary supplies of oxygen.

For the Sherpas, who migrated to Nepal from Tibet around 500 years ago, their ability to go about life as normal at such high altitudes might seem like some kind of superhuman ability. Nature, however, has a simpler explanation: evolution. Previous studies have suggested that the Sherpas have a gene that enables them to survive in this extreme environment, which isn't all that far-fetched given that the first humans arrived on the Tibetan plateau 30,000 years ago.

"Sherpas have spent thousands of years living at high altitudes, so it should be unsurprising that they have adapted to become more efficient at using oxygen and generating energy," says senior author Andrew Murray from the University of Cambridge.

But how exactly did their bodies adapt to this low-oxygen environment? The researchers suspected that the Sherpa cells were somehow wired to make better use of whatever oxygen was available to them and they suspected that this probably had something to do with their mitochondria, the cell's power generators.

To test their hypothesis and understand the metabolic differences between the Sherpas and people from low-lying regions, they embarked on a scientific expedition called Xtreme Everest 2 to the Mount Everest basecamp, located at an elevation of 5, 300 meters (17, 388 feet). The subjects of the study were 10 Caucasian researchers and 15 Sherpas, who were chosen from relatively low-lying areas. Samples of the scientists, including blood and muscle biopsies, were taken in London before they left for the expedition to give a baseline sample. A second batch was taken when they arrived at Base Camp and a third after they had spent two months there. These were compared to the Sherpas' samples, whose baseline measurements were taken at Kathmandu in Nepal.

As expected, the researchers found the Sherpas' mitochondria were much better at using oxygen, regardless of where they were located. Even though the scientists' bodies eventually showed signs of adapting to the high altitude to become more "Sherpa-like," they were still no match for their efficiency, says Murray.

A key difference between the Sherpas' and researchers' mitochondria lies in the way they produce energy. There are basically two ways to do this: one is to burn sugars, such as glucose, the other is to burn or oxidize fat. We usually get our energy via the latter; however, the problem with this is that it requires more oxygen than the first method, making it an inefficient process. For the Sherpas' mitochondria, glucose oxidation is a biological hack that lets them do more with less.

Another striking discovery is the way the researchers' phosphocreatine levels crashed after spending two months at camp, whereas energy reserves in the Sherpas' muscle increased at altitude despite the lack of oxygen. Found in muscle cells, phosphocreatine, which is also known as creatine phosphate, plays an important role in muscle contraction during high energy exercise. If someone scaling Mount Everest has more of it in their muscles, it follows that their muscles will be able to undergo a longer period of intense contraction.

While this is yet another example of how Sherpa metabolism has adapted to high altitude conditions, what it also suggests is that the amount of oxygen we have in our blood might not play as critical a role in the body's functions as we might think.

"We've seen that Sherpas and other related groups in Nepal have lower red blood cell counts than Europeans or other lowlanders, which means they have less oxygen in their blood at altitude, so it's not simply a case of how much oxygen you have but how you use it," says Murray.

The goal now is to use these findings to help patients in intensive care units who have hypoxia, or a lack of oxygen in the body's tissues, say the researchers. When faced with the condition, the body tries to compensate for the low levels of oxygen by making more blood cells. This, unfortunately, only makes the situation worse for the patient as it makes the blood thicker, causing it to move slowly and clog up the blood vessels. Interestingly, previous studies have shown that though Sherpas have fewer red blood cells at altitude, they also have higher levels of nitric oxide, a compound that helps open up the blood vessels and keep the blood flowing.

"One in five people admitted to intensive care in the UK each year die and even those that survive might never regain their previous quality of life," says Mike Grocott, chair of Xtreme Everest from the University of Southampton. "By understanding how Sherpas are able to survive with low levels of oxygen, we can get clues to help us identify those at greatest risk in ICUs and inform the development of better treatments to help in their recovery."

The study was published in the Proceedings of National Academy of Sciences and Andrew Murray discusses the study in the video below.

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