Why those oh-so-healthy diet foods make us eat even more

On a diet but struggling to shed the pounds, or - horror of horrors - actually gaining weight?

Well it could be because you're on a diet, according to scientists.

A study has shown that when faced with a healthy, low-calorie dish, we instinctively increase the portion on our plate or feel justified in going back for second helpings.

Healthy: But scientists claim dieting on healthy food may not make you lose weight

Researchers at the University of Bristol discovered those on low-calorie diets believe you can't have too much of a good thing and end up consuming just as many calories as if they were eating regular dishes.

'A person's perception of how full a meal will make them feel will no doubt affect portion size,' said Lisa Miles, of the British Nutrition Foundation. 'It's so important to be aware of behavioural triggers for overeating.'

The Bristol team, led by Dr Jeff Brun-strom, looked at the responses of 76 adults to 18 foods and found they quickly learnt their calorie values and over-compensated accordingly.

The findings back up a 2007 Canadian research paper on the causes of childhood obesity, which found that rats given low-calorie food also tended to over-eat.

In a second study, Dr Brunstrom found children whose parents regulated sugary snacks, such as chocolate or crisps, ended up bingeing on them when given a chance.

The researchers tested 70 children aged between ten and 12 years old, presenting them with six unhealthy treats.

Cakes: A child rarely allowed to snack is more likely to over-estimate how much they should eat

A child who was rarely allowed the snack was more likely to over-estimate how much they should eat, miscalculating a 250kcal portion as a 120kcal one.

Meanwhile, a youngster who had eaten the foods previously would be able to assess accurately how calorific it was, on average guessing that a 250kcal portion contained 230kcal.

Dr Brunstrom, a lecturer in experimental psychology, will present his work at a BNF conference this week. He said: '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.'

This could be bad news for parents who believe they are doing their children a favour by placing sweet treats off-limits.

Tam Fry, chairman of the Child Growth Foundation and a member of the National Obesity Forum, 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.'