Christmas is a time for harmless lies, the chocolatey indulgences of the thought world. We know when to stop, because if we all acted on our belief in Santa there would be no presents: and then Christmas would be meaningless.

My favourite Christmas traditions are the "red wine is good for you" and "chocolate is good for you" stories, which have become a festive science tradition of their own. Red Heart Wine, from Sainsbury's, with extra antioxidants, is "a red wine that is actually good for your heart".

Drink it down with new Choxi+, milk chocolate with "extra antioxidants". "Guilt free," says the Daily Mail, it's "the chocolate bar that's 'healthier' than 5lbs of apples". "Too good to be true," says the Mirror. "Chocolate that is good for you, as well as seductive," says the Telegraph. The Choxi+ manufacturers recommend two pieces of their chocolate every day. It's almost as good as Fruitella Plus, with added vitamins A, C, E and calcium.

These are jokes which have gone too far, fat and spotty on wishful thinking. Antioxidants are like an endlessly repeated Christmas movie that you've never quite watched from start to finish: let's recap.

Firstly there's the theoretical plausibility, from biochemistry textbooks. Sainsbury's tells this story in the style of a children's story. "Exposure to UV rays, pollution and smoking produce free radicals," they say. Oh modern woes! "Free radicals are compounds that cause cell damage, which in the long term can damage health." It's a simple tale of right and wrong. "Antioxidants help counteract the harmful effects of free radicals."

It's an attractive idea. But if you're going to pore over the flowcharts in a biochemistry textbook, and pick molecules out at random on the basis of their function in the body, then you can prove anything you like. When you have a bacterial infection white cells build a wall around invading bacteria and then use free radicals - amongst other things - to kill them off, like tipping bleach down the toilet. Should we be selling wine with extra free radicals, instead, to help people fight bacterial infections, on the grounds of theoretical plausibility?

Anyway. In the 1970s men who looked like Father Christmas made amazing discoveries about smoking and health: buoyed with the enthusiasm of it all, they decided that all other cancers must have lifestyle causes, such as diet perhaps. They started looking for data, and this is what they found: people who choose to eat antioxidant pills seem to live longer; people who choose to eat fruit and vegetables seem to live longer; fruits and vegetables contain lots of antioxidants.

Are antioxidants the key to that link? Possibly. But people who choose to eat fruit and vegetables are getting a lot of good stuff into them, and they're also like me: they're a bit posh, they get plenty of exercise, they work, they have strong social supports, and more.

So trials were done, in huge numbers, giving one group extra antioxidants, in pills, and the other group our old friend the placebo sugar pill. Some of these trials were stopped early because the people getting the antioxidants were dying faster. Overall, if you look at all the results on a big spreadsheet (a technique called meta-analysis) it seems that antioxidant supplement pills either do nothing, or worse, kill you quicker. There might be something in the antioxidant story, but they might be rubbish. You don't read that everyday in press releases on wine and chocolate.

So what does this do for our Christmas fable? Well fruit and veg are definitely still good for you. But you like chocolate. I'm not your mother. Eat it. Enjoy it. Believe in Santa. Chocolate is healthier than 5lbs of apples. And in the new year you can perform a symbolic purification ritual, involving five days of abstinence. You can dress that up in crap science too.

· Please send your bad science to bad.science@theguardian.com