Monday, November 12, 2007

Tibetans born & raised in plains grow taller

New Delhi, Nov. 11: Tibetans who were born and raised in India are taller and display other physiological changes that are not observed among original Tibetan highlanders, Calcutta-based scientists have found.
The changes are among the first signals that Tibetans — the world’s longest-surviving high altitude residents — are adapting to the low altitude environment in India, anthropologists at the Indian Statistical Institute (ISI), Calcutta, said.
Their studies show that Tibetan men and women between the ages of 18 and 40 who were raised at low altitudes in India are on an average about 4 to 5cm taller than their counterparts in high altitude regions.
“Among all human populations, Tibetans have adapted best to the high altitude environment,” said Ranjan Gupta, head of the ISI’s biological anthropology unit. “It’s possible we’re now looking at a reversal of this adaptation,” Gupta said.
Gupta and researcher Vikal Tripathy studied body features of Tibetans in three settlements at different altitudes in India — Choglamsar in Leh at 3500m above sea level where the environment is similar to the high altitude regions of Tibet, a settlement in Chandragiri (Orissa) and another at Bylakuppe (Karnataka), both within 1000m above sea level.
The lower height of Tibetans at Choglamsar might be explained through low oxygen in the air and relatively poor nutrition in high altitude regions, the researchers said, presenting their findings in the American Journal of Human Biology. Populations at low altitudes have access to a greater diversity of vegetables and fruits in winter than people living at high altitudes.
“Everyone has genetic potential... when the different stresses associated with high altitude are removed, the genetic potential gets activated,” said Gupta.
“This is very significant work. It’s the first time anyone has systematically looked at Tibetans at different altitudes,” said Charles Weitz, chairman of anthropology at Temple University, Philadelphia, in the US. Weitz is an authority on adaptation to high altitude environments, having studied populations in South America and China.
Archaeological evidence suggests that humans had settled in Tibet around 20,000 years ago, long before any other population had settled in high altitude zones around the world.
Scientists believe both the genetic make-up and the environment contribute to human growth and development. “This study is part of a broader effort to determine how much of human adaptation is driven by the genes and how much by the environment,” Weitz said.
Gupta and Weitz are now writing a proposal for a joint study to pursue this line of research in finer detail. “India offers a unique natural environment for such a study,” Gupta said. Some 80,000 Tibetans fled their native land in 1959 and have settled in India where the environment is very different from that found on the Tibetan plateau.
The ISI study also found that Tibetans living at low altitudes had slightly higher values of weight, skin-fold thickness at the triceps, and upper arm circumference. The researchers believe greater physical activity may be causing Tibetans at low altitudes to gain more weight than Tibetans living at high altitudes.
“Some changes may also relate to body surface area. In extreme cold, a low surface area helps the body retain heat, but this is not necessary at low altitudes,” Gupta said. “Better nutrition and the absence of high altitude stress — both might be driving the changes we’re seeing,” he said.