The ability of sure-footed Sherpas to scale the oxygen-rare slopes of the Himalayas without losing puff has enthralled the world since Tenzing Norgay summited Everest with Edmund Hillary in 1953.
Now research led by Mike Stembridge, an exercise physiologist at Cardiff Metropolitan University in the UK, has upended the traditional wisdom on how these high-altitude dwellers pull off their remarkable feat.
As you go higher oxygen levels plummet, which means there is a clear cut-off for human habitation. It sits just above 5000 metres, the altitude of the world’s highest permanent settlement, the gold mining town La Rinconada in the Andes of southern Peru.
Humans adapt at these dizzying heights by making more red blood cells, which contain oxygen-carrying haemoglobin. It’s an adaptation that comes, however, with a cost.
More red blood cells means thicker, stickier blood that clots more easily causing strokes and heart attacks.
In a twist, the Sherpas of the Tibetan plateau were thought to have bucked that blood-making trend. They have lower haemoglobin levels compared to folk from the Andes and, consistent with the survival advantage that should offer, better reproductive stats.
They would need to compensate, of course, by extracting more oxygen via the lungs and offloading it better when red cells arrive at muscles and organs.
It’s a theory that fits very nicely with findings that Tibetans are endowed with genes, including one called EPAS1, that lower haemoglobin and bump up the efficiency of oxygen use.
But for Stembridge and colleagues that theory faced a stumbling block.
It came in the form of a 2018 study of 1000 Tibetan women from the high Himalayan valleys. Despite an exhaustive search, it failed to find clear evidence that genes for lower haemoglobin improved survival, measured in terms of live births and better infant mortality.
Stembridge’s team came up with a deceptively simple explanation for why that might be so.
Perhaps the total haemoglobin mass of the Tibetans wasn’t actually low after all. Instead, could the levels have merely been diluted down by a bigger blood volume?
They decided to put that theory to the test in the current study.
They recruited Sherpas from Tenzing Norgay’s home in the Khumbu valley of Nepal, Quechua natives from the Peruvian Andes, and a group of lowlanders. The volunteers were subjected to a battery of tests, including blood draws.
The Andeans, it turned out, had a bigger haemoglobin mass than any other group and also a higher blood concentration of haemoglobin. But, like the Andeans, the Sherpas also had a bigger total haemoglobin mass compared to the lowlanders. The concentration of haemoglobin in their blood, however, did not go up.
There was only one conclusion.
The Sherpas’ blood volume had increased, lowering the haemoglobin concentration. Their total amount of haemoglobin was, as orthodox physiology predicts, indeed up compared to sea level residents.
“This difference is critically important, as it means Sherpa are able to benefit from the increased oxygen carrying capacity that comes from an expansion of their haemoglobin mass but are not restricted by an increase in blood viscosity that hinders microcirculatory blood flow,” the authors write.
The physiological feat, they believe, results from adaptations in the kidney, which holds on to fluid to increase blood volume.
“Collectively, our findings demonstrate a unique adaptation in Sherpa that reorientates attention away from haemoglobin concentration and toward a paradigm where haemoglobin mass and plasma volume may represent phenotypes with adaptive significance at high altitude,” they conclude.
The study appears in Proceedings of the National Academy of Sciences.