HEALTH HALO:Er, does my soul look big in this?

You probably think you've been very saintly this Christmas, eating healthy organic food and staying away from fatty, processed products. You've gorged yourself on corn-fed turkey, organic vegetables and low-sugar cranberry sauce. You've scarfed down the low-cholesterol Christmas pud and finished off with a few hydrogenated-fat-free cookies and a box of 70 per cent cocoa choccies.

Mmm, yes, and thoroughly enjoyable - and healthy - it was, too.

And yet, afterwards, when you sat down to watch the queen's speech, you still felt completely bloated, and when you weighed yourself this morning you had gained a stone over the Christmas period. Why do you think that is?

I didn't use free-range eggs in the egg-nog?

No, what's happened is that you've been fooled into thinking that it's all right to eat extra large portions of food, as long as it's "healthy" food. You've been taken in by the health halo that hovers over your dinner. The health halo is that aura of nutritional goodness that surrounds certain foods. It confers an air of divinity on an otherwise unremarkable grocery item, exaggerating its healing powers, and allowing the purchaser to adopt a smug healthier-than-thou attitude.

Sort of an onion-ring of confidence? Advertisers have become adept at bigging up the health qualities of food brands, and attributing all sorts of benefits to, say, a yogurt drink or a breakfast cereal. They might portray, say, two angels enjoying cream cheese in heaven, to underline the "light" nature of the product. If the marketing campaign works well, then a health halo begins to form around the product, and people start to believe that the food in question is nothing less than the ambrosia of the gods.

So the health halo is actually a shop-tical illusion?

Scientists say that people have become dazzled by the health halo, and are blind to the reality that so-called healthy foods are not actually imbued with magical powers to reduce weight, lower cholesterol and generally cure all your ailments.

What, you mean that spinach doesn't really make me as strong as Popeye?

A French nutritionist, Dr Pierre Chandon, has been studying the eating habits of people in the US, and trying to determine why many Americans, despite being obsessed with healthy eating, are still gaining weight. His theory is that they've been hypnotised by the health halo associated with some foods - when they see a product emblazoned with such slogans as "low-fat", "low-cholesterol" and "healthy heart", that's the signal to shovel up unhealthy amounts of the stuff.

So, it's a case of too much of a good thing, eh?

The health halo also tricks people into eating foods that are bad for them, in the mistaken belief that eating a similar amount of healthy food will kind of cancel out the calories. So, if someone sees that a cracker is fat-free, they're more likely to slather more full-fat cheese on top of it. And if someone thinks their burger is low-fat, they're more likely to order an extra-large fries and a sugary drink on the side, which is why fast-food restaurants are feverishly conjuring up health halos in order to keep their customers going, "Supersize me".

Try at home: "I couldn't help myself - I took one look at those alfalfa sprouts and ate the whole banoffi pie instead."

Try at work: "It's worked a treat, boss - our new range of Halo Hoops are flying off the shelves!"