Health-related quality of life, or functioning, begins to decline as soon as a child is above average weight, say researchers, but the effects may not be obvious to parents.
The new study found less dramatic impact on health in an overweight child than in previous research, although the effects increase more markedly as a child becomes obese.
"Our findings may explain why so few parents of overweight children express concern about their child's weight, yet with a quarter of all children now overweight or obese, even a minor reduction in health-related quality of life at an individual level is still likely to have a major effect at a population level," the researchers write in yesterday's issue of JAMA (293(1), pp70-6).
The study is important given the current debate around who is responsible for increasing numbers of overweight children around the world.
While governments in Europe, especially in the UK, are increasingly placing pressure on food companies to offer healthier foods for children, a number of studies suggest that parents do not recognise the problem and are therefore unlikely to change their children's diets.
This increases the challenges involved in positioning healthy foods for children.
Dr Joanne Williams of the Royal Children's Hospital and Murdoch Children's Research Institute, Melbourne, Australia, and colleagues, used data collected in 2000 from more than 1,500 children with an average age of 10 years old.
The researchers measured health-related quality of life, as defined by the World Health Organization. It includes an individual's physical, mental, and social well-being. The survey was completed by a parent and by child self-report.
Summary scores for children's total, physical, and psychosocial health and subscale scores for emotional, social, and school functioning were compared with weight categories based on International Obesity Task Force criteria.
"The decrease was small for overweight children but more marked for those who were obese. These new observations are less dramatic than the much lower scores reported for children attending tertiary clinics, but are consistent with those observed for adults," said the authors.
They added that the findings have both positive and negative implications.
"As for any chronic condition, a relatively small effect of overweight on children's functioning across multiple domains is welcome."
"However, if neither children nor parents perceive a health effect, it seems unlikely that they will seek health care or initiate behavioural change that might lead to a healthier BMI, and consequently lessen the long-term health risk for the current generation of children," write the researchers.
The findings support research published in the British Medical Journal last November.
Researchers surveyed the parents of 277 children and found that only a quarter recognised when their offspring were overweight. Where children were obese, a third of mothers and 57 per cent of fathers thought their sons and daughters were 'about right'.
Some 30 per cent of British five-to-nine-year-olds are overweight or obese and this is expected to rise to 36 per cent by 2008, according to figures from Datamonitor.