The result is close to a massacre, as Peretti goes through the history of diet and exercise from 1945. Right at the start US researchers, who persuaded willing volunteers to live on a starvation diet, found that their guinea pigs put the weight back on and more besides when the experiment was over. Every reputable scientific study since has observed the same phenomenon. With 27 million people dieting in the UK, many of them bullied women who are not overweight in the slightest, that fact deserves to be better known, and Peretti hammers it home with gusto.

He gives us a former senior manager at Weight Watchers, who boasts that the dieting business is the perfect business. Any other corporation that gave its customers faulty goods would go bankrupt. When the customers of Weight Watchers and its rivals find that their diets fail, they blame themselves and come back for more. The documentaries were informative and enjoyable. But any obese viewer, including the 17st 8lbs Cohen, must have concluded that, if they watched what they ate and exercised, they would be the dupes of corporate conmen. Wised-up fatties should not waste their time and money, but carry on as before. Or as Peretti said, "While there are people for whom simply eating less and moving more will work, for most of us it doesn't."

That is not true, but Peretti cannot see it because he is in the grip of a euphoria that afflicts polemicists from Noam Chomsky on the far Left to Pamela Geller on the far Right. You have a sensational argument, and you convince yourself that you can prove it because every fact you quote is right. I speak from experience when I say that the elation you feel as you contemplate your watertight case is close to ecstasy. You only come down when you realise — if you realise at all — that while all your facts stand up, your conclusion when you string them together can still make no sense at all.

Peretti unconsciously admits his failure. In the second episode, he denounces the Olympics for associating with McDonald's, Coca-Cola and Cadbury. But if dieting is a waste of time, why should we not eat Big Macs and chocolate and wash them down with a fizzy drink? By the final episode, he is describing sugar as the tobacco of the 21st century, and saying, rightly in my view, that governments will have to tackle food manufacturers with the same earnestness they deployed against cigarette manufacturers. But governments might watch Peretti and reply: if it is all a con, why should we bother?

For only one minute in four hours of programming does he allow the scientists who had exposed the lies of the diet industry to give the viewers their recommendations. Guess what? They say you should eat a balanced diet and exercise. This brief moment of common sense passes in a flash, and is never repeated. What your mother told you will not make profits for food corporations, nor will it make for exciting television. And so Peretti, like everyone else with a product to sell, ignores it.