Most Tibetans are genetically adapted to life on the "roof of the world," according to a new study.
The Tibetan Plateau (map) rises more than 13,000 feet (4,000 meters) above sea level. At such heights, most people are susceptible to hypoxia, in which too little oxygen reaches body tissues, potentially leading to fatal lung or brain inflammation.
To survive the high life, many Tibetans carry unique versions of two genes associated with low blood hemoglobin levels, the researchers found.
Since hemoglobin is the oxygen-carrying component of red blood cells, the find might seem "really counterintuitive," said study leader Tatum Simonson at the University of Utah's Eccles Institute of Human Genetics in Salt Lake City.
"Usually, if you or I or any nonadapted person went to high altitude, we would increase our hemoglobin levels to compensate for the low amount of oxygen."
But high hemoglobin levels have been linked to complications such as hypertension and chronic mountain sickness, Simonson said.
These negative effects could have led to a genetic mutation among Tibetans that "prevented them from making as much" hemoglobin, she noted.
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Previous research had found that Tibetans compensate for low oxygen levels by taking more breaths per minute than people living at sea level. In addition, Tibetans' blood vessels are wider, making them more efficient at delivering oxygen to body tissues.
Simonson and her colleagues searched for the genetic basis of high-altitude adaptations by collecting blood samples from villagers in Tibet living at 14,720 feet (4,486 meters) above sea level. (Get insider's tips on life in Lhasa, capital of China's Tibet Autonomous Region.)
The team then looked for patterns of genetic variation in the Tibetans' DNA and compared their findings to existing data on gene variation in lowland Chinese and Japanese populations, which are closely related to Tibetans.
Several variants of genes associated with high-altitude living, such as those that process oxygen, were found in Tibetans but not in their low-living neighbors. That includes the two genes that are strongly associated with low hemoglobin production.
Future research is aimed at teasing out more details about what exactly the altered genes do, which could help scientists find ways to "prevent people from getting sick" at high altitudes, Simonson said.
Findings were published online May 13 in the journal Science.