Washington, Feb 17 (IANS): Increasing urbanisation is cutting off people from adequate sunlight, which helps the skin produce vitamin D, facilitating the absorption of calcium, says a study.

About two million years ago, permanent dark skin colour imparted by the pigment -- melanin -- began to evolve in humans to regulate the body's reaction to ultraviolet rays from the sun, said Nina Jablonski, professor of anthropology at Penn State University.

Melanin helped humans maintain the delicate balance between too much sunlight and not enough sunlight.

The pigment allowed enough ultraviolet radiation to produce vitamin D, while protecting the skin from the intense ultraviolet radiation in the equator. Too much sunlight can cause the destruction of folate, which is also critical to cell division.

Unlike their ancestors, modern humans are more mobile. "We move around a lot now," said Jablonski.

"People can move across 90 degrees of latitude in a single day whereas early humans generally only went a few kilometres in the same time."

Most people now live in cities with limited exposure to the sun. Nearly 60 percent of world's people live in cities now, she said.

Most of these people indoors, further reducing their ability to make enough vitamin D in their skin, according to Penn statement.

"Think about a farmer who lived in northern England and worked outside," said Jablonski.

"In the past, that farmer had the right amount of light pigmentation to make it possible for him to produce enough vitamin D in his skin in the summer to satisfy most of his yearly needs."

However, Jablonski said a typical worker who lives in England today is rarely exposed to that amount of sun, which only compounds health problems.

"This can lead to a vitamin D catastrophe for many people," Jablonski added.

These finding were presented at the at the annual meeting of the American Association for the Advancement of Science in Boston.