The idea that exercise is more important than diet in the fight against obesity has been contradicted by new research.
A study of the Hadza tribe, who still exist as hunter gatherers, suggests the amount of calories we need is a fixed human characteristic.
This suggests Westerners are growing obese through over-eating rather than having inactive lifestyles, say scientists.
One in 10 people will be obese by 2015.
And, nearly one in three of the worldwide population is expected to be overweight, according to figures from the World Health Organization.
The Western lifestyle is thought to be largely to blame for the obesity "epidemic".
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Daily energy expenditure might be an evolved trait that has been shaped by evolution and is common among all people and not some simple reflection of our diverse lifestyles” Dr Herman Pontzer Department of Anthropology, Hunter College, New York
Various factors are involved, including processed foods high in sugar and fat, large portion sizes, and a sedentary lifestyle where cars and machines do most of the daily physical work.