Scientists link obesity to 'thrifty gene' of our ancestors

Fat people should not be blamed for a lack of willpower but helped to overcome a genetic predisposition that produces a desire as strong as needing a glass of water in a desert, researchers in the United States say today.

The global epidemic of obesity – which now affects 300 million people – is a legacy of evolution and cannot be tackled unless we recognise that the disease is a fundamental consequence of our innate biology, they argue.

People from Africa, South-east Asia and Polynesia are especially prone to obesity because they are more likely to have inherited the genes that encourage the storage of fat, Jeffrey Friedman, an obesity specialist at the Rockefeller University in New York, writes in the journal Science.

There is growing evidence for believing that genes strongly influence whether someone is likely to become overweight in a society where calories are easily and cheaply obtained, says Professor Friedman, who led the research team that discovered the obesity hormone leptin.

"One might ponder why, in our current environment where almost everyone has essentially free access to calories, anyone is thin. The answer appears to reside in our genes and the way in which they interact with environmental factors," he says.

He adds: "Indeed, the heritability of obesity is equivalent to that of height and exceeds that of many disorders for which a genetic basis is generally accepted."

A growing amount of evidence suggests that people from some ethnic groups are more likely than others to suffer obesity when they adopt a Western lifestyle and diet. They range from the Pima Indians of Arizona to the Polynesian islanders.

Obesity now affects almost one in three Americans – a 10 per cent increase in the past decade – but it is markedly higher among black and native Americans than in Americans descended from Europeans.

Professor Friedman says that the difference in obesity rates between ethnic groups could have something to do with their respective genetic histories. "For people who lived in times of privation, such as hunter-gatherers, food was only sporadically available and the risk of famine was ever- present.

"In such an environment, genes that predispose to obesity increase energy stores and provide a survival advantage in times of famine. This is the so-called thrifty gene hypothesis," he says.

"For people descended from the inhabitants of the Fertile Crescent [in the Middle East] or, more recently, Western societies, the risk of starvation was markedly reduced by the domestication of plants and animals and the ability to store food.

"But these developments also exposed those who became obese to significant health problems. In this environment, selection against obesity might be expected," Professor Friedman says.

Humans today carry the genetic legacy of both type of environments. The reality might be that the obese carry the "hunter-gatherer" genes and the lean carry the Fertile Crescent genes.

"If true, this means that the root of the problem is the interaction of our genes with our environment. The lean carry genes that protect them from the consequences of obesity, whereas the obese carry genes that are atavisms of a time of nutritional privation in which they no longer live," he says.

James Hill of the University of Colorado in Denver says in a separate study published in Science that even though obesity is in our genes, a minor change in lifestyle can still affect the chances of a person becoming overweight.

By walking an extra mile or eating about 100 calories fewer than usual each day, an overweight person could typically prevent an extra 2lb (1kg) increase in weight each year, Dr Hill says.

"When we look at what's causing the obesity epidemic, it doesn't look like it's our biology gone bad, it's the environment acting on our ancient biology," he says. "We need to look at what's happening to changes in energy intake and physical activity, and then determine the extent of the problem."