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 an ounce. 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 no doubt affect portion size," said Lisa Miles, a nutritionist at the British Nutrition Foundation. "It's so important to be aware of behavioural triggers for over-eating."
The researchers, who studied the responses of 76 people to 18 different foods, found that people quickly learnt if 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," Ms Miles said.
Dr Brunstrom, senior lecturer at the University of Bristol who led the study, will present these findings at a British Nutrition Foundation conference, entitled "Satiation, satiety and their effects on eating behaviour", on Thursday.
"This is quite a controversial idea, which goes against the perceived wisdom that you just eat what is put in front of you," said Tam Fry, chairman for the Child Growth Foundation and a member of the National Obesity Forum.
In a study to be published later this month, Dr Brunstrom 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 much larger portions when they were presented with them.
Researchers tested 70 children aged between 10 and 12 years old, presenting them with six snack foods. They found that a child who is unfamiliar with snack food was more likely to over-estimate how much they would need, regarding a 250kcal portion as only containing 120kcal, whereas a child who had eaten the foods previously would be able to assess accurately how calorific it was, guessing that a 250kcal portion contained 230kcal.
"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," Dr Brunstrom said.
This could be bad news for parents who believe they are doing their children a favour by placing treats such as sweets and cakes off-limits.
"The object lesson here is that obesity is all to do with education, education, education," Mr 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 affected by obesity, it goes across all social classes."
Snacks banned: 'His problem is portions'
Jane Galley, 43, Sheffield, is mother to David, 10, and Jonathan, 12
"The younger boy has always been as thin as a rake, but Jonathan, the older, has been heavier. When they were little I never gave them sweets and rubbish, I always gave them homemade food, but other people would give them it when they were out of the house. I suppose that I was quite restrictive, but nothing was ever completely out of bounds. When I was swimming with my older son he saw a poster for an organisation called Mend, which helps overweight children. He learnt about portion control, and balancing energy intake and exercise. His problem was never excessive snacking, more just portion control."
Snacks allowed: 'He knows he should eat sensibly'
Tracy Rogers, 35, a civil servant from south Wales, is relaxed about letting her children, Lewis, nine, and Jack, 11, eat snacks
"The only rule, really, is that they have to have eaten their dinner before they can have snacks. They always ask first: they are not allowed to just help themselves. They are allowed to have fruit, crisps, biscuits and chocolate. Jack is at secondary school now and so has access to whatever he wants at school. The snacks I give him for lunchboxes are often portion-controlled, like cake bars. He knows that he should eat sensibly, which is good because it is difficult to monitor what he is eating. Both boys are slim, and I've never worried about their weight. They are very active. Jack plays rugby and cricket, while Lewis plays football three or four times a week."