By Julie Steenhuysen

CHICAGO (Reuters) - Genes that helped early humans adapt to cold climates may be driving metabolism-related diseases such as obesity or diabetes in many countries, U.S. researchers said on Thursday.

They found a strong correlation between climate and genetic adaptations that influence the risk of metabolic syndrome, a group of related disorders such as obesity, high cholesterol, heart disease and diabetes.

"Climate over a long period of time has shaped the distribution of genetic variants that may be associated with the risk of these common metabolic disorders," said Anna Di Rienzo, a professor of human genetics at the University of Chicago.

Anthropologists have long made the case that certain traits such as differences in skin pigmentation reflect early human migration from equatorial Africa to cooler climates -- for instance, the link between paler skin and an ability to synthesize vitamin D from sunlight.

"There are all of these traits, body mass or skin pigmentation, that we know are strongly correlated with environmental variables," Di Rienzo said in a telephone interview.

Di Rienzo and colleagues wanted to see if genes that were once useful for tolerating cold climates were playing a role in diseases of the metabolism.

"To survive in these climates, they had to adapt," said Di Rienzo, whose study appears in PLoS Genetics, a journal published by the Public Library of Science. (http://genetics.plosjournals.org/perlserv/?request=get-document &doi=10.1371/journal.pgen.0040032).

ENERGY BOOSTER

"They had to develop genetic variants that made them more efficient in terms of energy metabolism and that made them more able to cope with cold climates by increasing their rate of thermogenesis -- the ability to generate and maintain heat," she said.

The research team picked 82 genes associated with energy metabolism and looked to see if there were any correlations with climate. They studied variations in 1,034 people from 54 populations.

They saw several clusters of different genetic variations related to metabolic syndrome in colder climates.

One gene, the leptin receptor, is increasingly common in areas with colder winters. Leptin is important to appetite and weight gain -- something people need no help with in modern times.

"We eat a lot more, we don't exercise nearly as much as our ancestors used to do, and these adaptations that made us cope well to a cold climate now make us prone to a number of metabolic disorders," Di Rienzo said.

She said the study sheds light on why some of these variants are more common in certain populations. "It helps explain the evolutionary origins of these diseases," she said.

(Editing by Maggie Fox and Xavier Briand)