Obesity is a middle-class issue

Children with wealthy, middle-class, working parents were more likely to be overweight than those from poor households, The Independent on Sunday said on July 22 2007. "The risk of childhood obesity soars in direct correlation with family income," the paper reported.

The stories are based on the findings of a large study looking at the health of children born into the new millennium and appear to show a link between maternal employment and childhood obesity. Women who return to work after the birth of their children "increase the risk of their offspring being overweight or obese", the newspaper stated.

To look for any links between income, working hours and children’s weight, the study statistically adjusted for more than 15 potential confounding or mediating factors and, as such, these conclusions should be treated with caution.

Where did the story come from?

The study was conducted by Professor Summer Hawkins and colleagues of the Millennium Cohort Study Child Health Group based at the Institute of Child Health in London and was published in a peer-reviewed medical journal, International Journal of Obesity .

What kind of scientific study was this? 

The study was a large prospective cohort study of more than 13,100 children in the UK.

Children born between September 2000 and January 2002 were enrolled in the study when the child was aged 9 months. Families were sampled in a way that meant that those from disadvantaged areas and ethnic minority groups were not under-represented.

Parents (with the mother as the main respondent) were interviewed twice; first when their child was 9 months old and second when their child was 3 years old. Families were only included in the study if information was available on child height and weight and parental employment.

What were the results of the study?

The study found that children were more likely to be overweight at birth if their mother had been in “any kind of employment”. The likelihood of being overweight increased with longer working hours per week. Children where the income is £33,000 or more per year are 15% more likely to be overweight than those from lower income households.

What interpretations did the researchers draw from these results?

The researchers conclude that “long hours of maternal employment, rather than lack of money may impede young children’s access to healthy foods and physical activity”. They say that “policies supporting work-life balance may help parents reduce potential barriers”.

What does the NHS Knowledge Service make of this study?

This was a well-conducted, large prospective study that took into account many potential risk factors for being overweight in children. It provides preliminary results on the chances of pre-school children being overweight, depending on a variety of familial factors. There are some limitations to the interpretation of the study results:

• The study was looking at the outcome of being "overweight", not specifically obesity. To conclude that the study was solely looking for causes of obesity would be a misinterpretation of the study’s intentions.
• Odds ratios (the way the researchers have reported their results) should not be interpreted as a risk ratios (i.e. "decreases risk by" or "increases risk by"). The researchers do not present their risk figures, though they do say that when they calculated them "the rate ratios tended to be closer to one" – which represents no effect of the characteristic on the risk of being overweight in children. 
• The study used complex statistical methods to adjust for various factors and the relationship between them. The link between income and working hours is not simple and needs further study. 
• There are likely to be other factors related to income and hours worked that may affect the likelihood of being overweight in pre-school children. As a result, the policy implications of such a finding are unclear.

There are many social, environmental and individual behavioural factors that contribute to increasing rates of obesity; studies, such as this, which look for associations among the data and find them when adjusting for more than 15 potential confounding or mediating factors, should be treated with caution.

Sir Muir Gray adds…

The weight of children is influenced by many factors and in any single child the interplay of these factors, and the influence of genetics, means that it is not possible to identify a single factor that explains why this particular child is overweight or not.

Mothers and fathers need to think about what their children are doing when they are not there, and try to influence the choices and behaviour of children when they are not present. The longer the hours the parents have to work, the greater this challenge; all children need more exercise; some need a change in diet too.