Some people who live at high altitudes suffer breathlessness, palpitations and dizziness, while others have no health problems, and now a new study reveals which genes may explain the difference.
The genetic changes, described today (Aug. 15) in the American Journal of Human Genetics, allow people to take in enough oxygen from the thin mountain air without developing the heart attacks and strokes of chronic mountain sickness.
"We have ascertained there is a major genetic component that allows populations at high altitude to live better," said study co-author Dr. Gabriel Haddad, a pediatric pulmonologist at the University of California at San Diego.
When people who live at low-altitude lowlands go to the highlands, the short-term lack of oxygen can cause acute mountain sickness, which brings headaches, nausea and brain swelling.
Some people, however, live all their lives at higher altitudes, yet still face chronic mountain sickness. To adapt to the lower oxygen content of the air, their bodies have increased the fraction of red blood cells, making their blood more viscous, which in turn makes it more likely that the cells will block blood vessels.
As a result, these people are more prone to heart attacks and strokes, Haddad said. They also suffer from fatigue, depression and headaches.
However, in populations where people's ancestors have lived for thousands of years at altitude, some people are able to get enough oxygen from the air without developing the increased risk of heart attacks and strokes.
Haddad and his colleagues analyzed the genes of 20 people who lived at least 14,000 feet (4,300 meters) above sea level in the Andes Mountains, and whose ancestors had done so for generations. Half the people had signs of chronic mountain sickness. [High and Dry: Images of the Himalayas and the Tibetan Plateau]
Those people who had chronic problems tended to have differences in 11 gene regions, compared with those who did not have health problems.
The researchers inserted the genes from the well-adapted people into fruit flies, and placed the flies in low-oxygen chambers.
Fruit flies that had two of these mutations survived longer in the low-oxygen condition, suggesting those genes were responsible for the human adaptation to altitude. Still, the researchers said it remains unclear exactly how those genes work.
And while these two genes may have evolved to help people live at altitude, there are probably other mutations that also help them, and different populations around the world — for example people in the highlands of Ethiopia, or in the Himalayas — may have different mutations still, Haddad told LiveScience.