It's clear that obesity is a major public health issue. Treating weight issues costs healthcare systems around the world trillions of dollars. 

But what really causes people to become obese? And what can be done to prevent it?

Jane Ogden, Professor of Health Psychology at the University of Surrey, explains that obesity is a highly complex problem, created by a mix of biological, environmental and psychological factors.

She says it's vital that we change the very world surrounding us if we are to solve the obesity crisis. The food industry, she says. must stop the supply of cheap, high-calorie foods, and governments must provide more opportunity and space for exercise.

"I think blaming people for making the wrong choices is missing the point." She adds: "We have to change the world and create an environment where those choices are easier to make in the right direction."

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