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.
The relative balance of overeating to lack of exercise is a matter of debate, however.
Some experts have proposed that our need for calories has dropped drastically since the industrial revolution, and this is a bigger risk factor for obesity than changes in diet.
A study published in the <link> <caption>PLoS ONE </caption> <url href="http://www.plosone.org/home.action" platform="highweb"/> </link> journal tested the theory, by looking at energy expenditure in the Hadza tribe of Tanzania.
The Hadza people, who still live as hunter gatherers, were used as a model of the ancient human lifestyle.
Members of the 1,000-strong population hunt animals and forage for berries, roots and fruit on foot, using bows, small axes, and digging sticks. They don't use modern tools or guns.