People who think they are overweight or obese are more likely to pile on the pounds than those who are unaware that they may be heavier than doctors would advise, according to research.
The findings of the study raise a difficult issue for public health experts involved in the fight against obesity. It is well-known that many people who are overweight do not realise it because so many others around them look the same size and shape. But it has been thought, until now, that people with a weight problem need to recognise the fact in order to do something about it.
Eric Robinson of the Institute of Psychology at Liverpool University and colleagues in California and Stirling who carried out the research says feelings of stigmatisation may be causing overweight people to stress-eat, leading their weight rising further.
"There is quite a substantial body of research showing it is not really very much fun being an overweight person in this climate," said Robinson. "It is a stigmatised condition. Realising you are an overweight individual is in itself likely to be quite stressful and make making healthy choices in your lifestyle more difficult.
"It is a tricky finding for public health intervention work. You would hope that making a person aware they are overweight would result in them being more likely to change and lose some weight."
The paper, published in the International Journal of Obesity, looks at the lives of 14,000 adults in the US and the UK through data captured in three studies: the US National Longitudinal Study of Adolescent to Adult Health, the UK National Child Development Study and Midlife in the United States.
The researchers investigated the data from time periods after the children had reached adulthood to find out their perception of their own weight " whether or not it was correct " and their subsequent weight gain over time. The UK study followed participants from 23 until 45, but the other two studies had shorter followup periods, of seven years and nine to 10 years.
The paper says all three studies "found consistent evidence that perceiving oneself as being overweight was associated with increased weight gain". Evidence that feeling overweight " whether or not it is true " can cause people to comfort-eat to relieve stress came from the third study.
"Individuals who identified themselves as being "overweight' were more likely to report overeating in response to stress and this predicted subsequent weight gain," the authors write. "These findings are in line with recent suggestions that the stress associated with being part of a stigmatised group may be detrimental to health."
Robinson said he and the fellow researchers could not know exactly what people were doing " whether habitually overeating or embarking on crash diets which lead to rapid weight loss followed by even greater weight gain. "The widely accepted finding is that these types of diets don't work in the long run and the debate is over how much of a harmful effect they have," he said. "Weight regain is going to happen."
What is important is to tackle stigma in society, he said. "People with a heavier body weight have body image challenges. That is not surprising given the way we talk about weight and fat and obesity as a society.
"But the way we talk about body weight and the way we portray overweight and obesity in society is something we can think about and reconsider. There are ways of talking about it and encouraging people to make healthy changes to their lifestyle that don't portray adiposity as a terribly deviant thing."
Angelina Sutin, assistant professor at the department of behavioural sciences and social medicine at Florida State University College of Medicine, said it was "a really impressive study" for several reasons. It had previously been shown that adolescents who are weight-conscious tend to gain more weight but "this paper shows this is a life course phenomenon that has consequences across the life span", she said. "The effect are there over a very long period of time."
It followed that experts should be promoting healthy behaviours like eating well and getting more physical activity, rather than focusing on the weight and body mass index numbers, she said. "We should not be classifying people as overweight and giving them a label that has a stigma attached to it, but enabling people to make healthier choices."
Traci Mann, a psychologist at the University of Minnesota who has been studying eating habits and diets for decades, pointed out that it didn't matter whether those who thought they were overweight were correct or not. "It's about people who perceive they are overweight. What that says to me that I find interesting is that there is some psychological thing going on here.
"It certainly goes against all the common wisdom that you must find all the overweight people and tell them. This suggests that is not a good strategy."