Sugar v fat? A question I always answer by enjoying a Coca-Cola with my pizza, just to be on the safe side.

I may or may not be joking. But in terms of contributing to the debate, it’s as useful as Horizon (BBC2, 9pm, 29 January), which hired two twins and filled one with sugar and one with fat for a month to see which one is worse. Seriously, that’s what they did.

One twin ate an Atkins-style diet of steak, eggs and cheese, the other loaded up on sugary carbs, while tests determined the effect on their minds and bodies. We discovered that lots of fatty food slows you down and loads of sugar makes you hyper. I don’t want to jinx it, but I hear whispers of Nobel.

Both lost weight. Fat twin dropped eight pounds! But the majority was muscle mass, which is bad news for his life expectancy, according to a doctor, who also warned fat twin he was approaching “pre-diabetic” levels. “In a month I’ve done myself some proper damage,” he bleated.

After 45 minutes, some sense started to filter into the show. “It’s about the overall balance of the diet,” explained a nutritionist.

“We’ve heard that before, but now we know our enemy,” said a twin. “It’s not fat. It’s not sugar. It’s the deadly, addictive, delicious mixture of fat and sugar combined!”

Sigh. So a decade after Supersize Me warned us a junk food diet would make us fat, Horizon warns us a junk food diet will make us fat. With progress like that, perhaps it’s no surprise that fixing the UK’s obesity problem is moving at a similarly lethargic rate.