Peter York On Ads: Make sure you wear your halo when you sit down to eat breakfast

For years, I used to think the normal glass of house red at the Groucho was a unit, as in "drink no more than 21 units of alcohol a week". To me, 21 of those seemed quite reasonable, even generous. It was only recently I realised "unit" meant practically a thimbleful. The Groucho glass, full to the rim, is probably the thick end of three units, certainly well over two. Meaning five glasses is 12 or 13 units and two nights in Dean Street would have you well over the limit... and, and, and.

I'm not alone in this heroic bit of self-deception. I've got halfway responsible friends who believed in the one-glass-equals-one-unit ready reckoner for years too. If that straw poll is remotely representative, then large chunks of the respectable, middle-class wine-drinking world have been fooling themselves. And big alcohol business, which does tens of millions worth of market research every year, will have known exactly how their customers paced themselves.

Or take "low fat". People used to believe fat made you fat. It made sense: any glorious, pork cracklingy visible fat stuff – the animal's own spare tyre – would end up yours. And there was worse: it was so thick and claggy, it'd make your blood thick and greasy and clog up your arteries. The worst kind was saturated animal fat – butter, the stuff on the meat, cheese, all those "natural", expensive to produce, transport and store things.

The "low fat" statement added profit to all kinds of processed food. You charged more for it, but it cost less to produce, because instead of expensive real fats, you got bulked-up glop, floury thickeners and gelatines cooked up by chemists.

The history of food advertising with health claims is a tale of massive misinformation based on what producers know sounds broadly credible.

Everyone used to be cheerfully cynical about breakfast cereals. They were an easy way of fuelling up children with basic carbs, and we all knew that kids ran perfectly well on low-octane, low-cost stuff. That was before the whole complex carbohydrate shtick and the Glycaemic Index slow- release palaver. Now a new generation of cereals says it's wholegrain and 100% natural and nothing added and completely saint-like.

The new Kellogg's Bran Flakes commercial can't exactly do this. Bran Flakes are a hybrid between Corn Flakes – orange flakes of refined carbo – and All Bran – heavily sugared and textured up wheat bran.

So instead, the Bran Flakes commercial spoofs "healthy lifestyle" advertising – quite amusingly in a rather American campus fun kind of way.

But the laughs will wear out after about four showings. Then you'll be left with the idea that Bran Flakes are a healthy food group in themselves, the common sense of intelligent eating, like an apple and a glass of water.