MAGGIE VALENTINE, a 12-year-old in Oklahoma City, swims four days a week. She hops into the pool cheerfully and uses the butterfly stroke to propel herself forward. Despite exercising, as well as attempting to diet, she weighs 212lb. In a new documentary, “Fed Up”, Maggie is occasionally overwhelmed, tears trickling down her cheeks: “My weight is not really going the way it’s supposed to go.”

America’s obesity problem is hardly news. Indeed there are some hints that, among certain groups, it might be abating. But more than one in three American adults and one in six children are fat. Who is to blame? “Fed Up” is directed by Stephanie Soechtig and produced by Katie Couric, a prominent American newscaster, and Laurie David, who made “An Inconvenient Truth”. That film helped to popularise the fight against climate change. Ms David and Ms Couric hope to do the same for fighting flab.

Obesity is a global problem (as “Fed Up” mentions in passing). But on its surface it would seem the most personal of failures: an individual eats too much and exercises too little. This ignores the work of behavioural economists and biologists. Humans have evolved over millennia to ensure that weight is hard to lose. It is too simplistic to blame obesity merely on lack of willpower. That would let both food companies and politicians off the hook.

“Fed Up” is determined to hold them to account. Food companies are keen for children to eat their junk foods, which are available not just in shops but in schools. America’s politicians, “Fed Up” argues, are complicit. Notoriously, America’s school-lunch law counts the tomato paste on pizza as a vegetable.

This film lambasts many, from Coca-Cola to a less obvious target, Michelle Obama, for waging too feeble a war against food companies. However, the main foe in “Fed Up” is sugar. Food companies have pumped sugar into bread and peanut butter, as well as puddings and fizzy drinks. The film explains how insulin helps store sugar as fat, how sugar acts on the brain much like a drug and how sugar helps interfere with signals that a person is full. Ms Soechtig interviews Robert Lustig, of the University of California, San Francisco, whose YouTube video on sugar has been viewed more than 4.6m times.

The film occasionally feels almost militantly one-sided, threatening to undermine its argument. There are serious scientists who counter that sugar is one of many culprits, rather than the starring villain. It might have been worth including their work. The film would also have benefited from interviews with food companies; the film-makers say that they declined their invitations. “Fed Up” therefore relies on clips. One McDonald’s executive says of the restaurant’s omnipresent smiling clown, “Ronald McDonald never sells to children—he informs and inspires through magic and fun”. This is ridiculous. But so is the fact that the clip is ten years old.

Despite these shortcomings, “Fed Up” makes a powerful case for action. Joe Lopez, a 14-year-old featured in the film, weighs nearly 400 pounds. So he undergoes bariatric surgery. That it is easier to reroute children’s intestines than keep them from becoming fat would seem a grave political failure. The film’s mantra is summarised on its poster: two round sweets stand side by side, one with an “F”, the other with a “U”.