NO Way Up
High altitude dwellers exhale more nitric oxide
WEDNESDAY, Nov. 21, 2001 (HealthDayNews) -- In the foothills of the Himalayas or on the steps of Machu Picchu, the calmly inhaling locals stand out next to the gasping, oxygen-gulping tourists. Now, a new study may explain why the locals breathe so easily.
An international team of researchers has discovered a previously unknown mechanism by which populations living at high altitudes cope with reduced oxygen levels. The discovery could point to a better way to treat disabling hypoxia, a condition which develops when the brain and muscles get too little oxygen.
The study, published in the Nov. 22 issue of Nature, suggests that populations living at high altitudes exhale more nitric oxide (NO), a gas that triggers blood vessel dilation, allowing more blood to reach oxygen-starved tissues.
At sea level, atmospheric oxygen makes up about 21 percent of the air we breathe, but as altitude increases, the atmosphere thins out, reducing the number of oxygen molecules in each breath. At 12,000 feet, for example, each breath contains roughly 40 percent fewer oxygen molecules.
To get enough oxygen to run most basic body's systems, people unaccustomed to high altitudes need to supplement their oxygen, increase their breathing rate or reduce their activity. Most importantly, their bodies need time to acclimatize to the new altitude.
But some populations live their entire lives at high altitudes in a chronic state of low oxygen.
The new study found that two geographically separate groups of mountain dwellers -- Tibetans and the Bolivian Aymara -- have adapted to low oxygen conditions by altering their use of NO to help regulate blood flow.
Led by Cynthia Beall, professor of anthropology at Case Western Reserve University, in Cleveland, Ohio, researchers studied 105 healthy Tibetans living at 13,860 feet and 144 Bolivian Aymara, who live at 12,870 feet. The two groups were compared to a similar group of 33 Americans living at low-altitude.
Beall found the low-altitude group exhaled 7.4 parts per billion (ppb) of NO, while the Aymara exhaled an average of 9.5 ppb and the Tibetans exhaled 18.6 ppb.
Beall says further testing suggested the two populations used different physiological mechanisms to maintain their high NO exhalation levels.
"The difference between the Tibetan and the Andean highlanders was striking," says Beall. Evidence suggests that people have been living at high altitudes in the Andes for about 11,000 years, while the data from Tibet, though less clear, suggest high-altitude living for only 7,000 years, she says.
"Presumably, the ancestors of Tibetans who went to Tibet and ancestors of these Andean highlanders who went to the Andes were low-altitude natives just like us. Yet, in many traits associated with oxygen delivery, not just nitric oxide, there are big quantitative differences," she says.
For example, the Aymara had high levels of oxygen-carrying hemoglobin cells in their blood, while Tibetans had levels only slightly higher than people living at low altitude, Beall says.
The study also found that inhaling air mixtures with different NO concentrations could improve oxygen absorption in the lungs for healthy people living at sea level. "The effect was larger in a hypoxic situation," Beall says.
"Inhaled nitric oxide has been used therapeutically for conditions such as pulmonary hypertension because the NO in the air can cause the relaxation of the blood vessels," decreasing blood pressure, says Beall.
She says, "It's also been used at high altitude as a therapy for a condition called high altitude pulmonary edema," a rare but potentially deadly complication that leads to fluid buildup in the lungs.
Mark Weiss, program director of physical anthropology at the National Science Foundation, says the study opens up a new avenue in understanding how different groups of humans deal with low-oxygen environments.
Weiss says the apparent difference in how the Tibetans and Aymara have adapted using NO also is important. "To somebody interested in human evolution, it illustrates the fact that people, or any organism, can try to deal with a stressor via different routes," he says.
"This is a very important lesson about human variability and its causes, and it carries an important message for clinicians. What is 'normal' can vary depending on one's ancestry," says Weiss.
What To Do
To learn more about the effect of high altitude on the body, check the High Altitude Medicine Guide, Health Canada's statement on high altitude illness and the Outdoor Action Program guide to high altitude acclimatization and illnesses.