If diet snacks and drinks fill up your shopping basket, the chances are you already have an eye on your waistline, which is a polite way of repeating Paris Hilton's famous observation that "Diet Coke is just for fat people."
But earlier this week, scientists in Alberta, Canada, reported that low-calorie foods might actively trigger overeating in children, putting them at risk of obesity. The warning followed experiments on rats that suggested only older animals could rely on their taste buds to tell them when they had eaten enough. Younger animals kept eating for much longer.
The picture for humans is different though. Children up to the age of five are exceptional at controlling their calorie intake, and know when they have had enough. The knack diminishes as we get older. That said, countless studies have shown that if you give people diet snacks they are likely to eat more than they would have done otherwise - probably because they feel less guilty.
Last year, researchers at Cornell University in New York dropped into cinemas, holiday camps and people's homes and gave them boxes of chocolates. Half the boxes were marked low fat, although the contents were identical to the others. They found people scoffed a third more of the chocolates they thought were low fat.
Eating more diet foods might not be such a problem if it wasn't for a pitfall in labelling. Food labelled as "low fat" is not necessarily low in calories, because manufacturers often just swap fat for sugar as a marketing trick. In the Cornell study, the researchers worked out that if the chocolates had really been low fat, people would still have eaten 28% more calories. "People feel they can have more if it's labelled low fat," says Dr Beckie Lang at the Medical Research Council's Nutrition Research Centre in Cambridge.
