By Rachel Shields
It is every dieter's nightmare: hopping on to the scales after weeks of forgoing steak for salad, only to find they haven't lost a gram.
Now researchers at the University of Bristol claim to have found a simple explanation for this phenomenon: when people choose lower-calorie dishes, they just compensate by eating bigger portions.
These findings are sure to come as a blow to the diet industry, which makes millions selling low-calorie foods, but should make cheering reading for any dieters sworn off their favourite fatty foods.
The study also showed that when faced with foods they liked, participants did not pick bigger portions of them than of any other food.
"A person's perception of how full a meal will make them feel will ... affect portion size," said Lisa Miles, a nutritionist at the British Nutrition Foundation. "It's so important to be aware of behavioural triggers for overeating."
The researchers, who studied the responses of 76 people to 18 different foods, found that people quickly learnt when food offered fewer calories per serving and upped their portion size to compensate.
"We know from experimental studies that eating large portions does not necessarily mean that you eat less at a subsequent meal, so this can lead to an increase in calorie intake overall," Miles said.
Tam Fry, chairperson for the Child Growth Foundation and a member of the National Obesity Forum, said: "This is quite a controversial idea, which goes against the perceived wisdom that you just eat what is put in front of you."
In another study, Dr Jeff Brunstrom, a senior lecturer at the University of Bristol, also found that children whose parents restricted their consumption of high-calorie snack foods such as crisps and chocolate were more likely to eat them in larger portions when they were presented with them.
Researchers tested 70 children aged between 10 and 12, presenting them with six snack foods. They found that a child who is unfamiliar with snack food was more likely to overestimate how much they would need, regarding a 250 kilocalorie (kcal) portion as only containing 120kcal, whereas a child who had eaten the foods previously would be able to assess accurately how calorific it was.
"These findings suggest that limiting access to certain snack foods limits learning about their properties. Thus, when snack foods are eventually encountered they might tend to be selected in larger portions," Brunstrom said.
This could be bad news for parents who believe they are doing their children a favour by making treats such as sweets and cakes off-limits.
"Obesity is all to do with education," Fry said. "Early in a child's life they need to be introduced to portion size as a positive measure, otherwise it becomes forbidden fruit.
"It isn't just the ignorant (who are) affected by obesity; it goes across all social classes." - The Independent