Obesity link for siblings

“Children are five times more likely to become obese if their older brother or sister is overweight,” reports the Daily Mail.

There is a widespread assumption that a significant risk factor for child obesity is if they have one or both parents who are obese.

A new US study suggests that a more influential risk factor may be if a child has a brother or sister (or both) who are obese.

A study of US families has found that among those with two children, if one child was obese then there was a relatively large chance that the other child would also be obese.

This “obese sibling” effect was particularly pronounced if the children are of the same gender. This study has several limitations, including its reliance on one parent self-reporting the height and weight of both themselves and their children.

The study also relied on data taken at one point in time, so it cannot prove a direct cause and effect.

It does arguably highlight the fact that a family environment can play an important part in influencing individual family member’s health outcomes.

Often, the family that exercises and eats healthily together, achieves a healthy weight together.

Where did the story come from?

The study was carried out by researchers from Massachusetts General Hospital, MassGeneral Hospital for Children, Harvard Medical School, the National Bureau of Economic Research, Cornell University and Duke University, all in the US. It was funded by the Robert Wood Johnson Foundation.

The study was published in the peer-reviewed American Journal of Preventive Medicine.

The study is open-access, so is available online free of charge.

The Mail Online’s reporting that children are five times more likely to become obese if their older brother or sister is overweight was a little skewed. It implied that older siblings influence younger children’s eating and exercise behaviour.

But this study had a cross-sectional design, which means all the data was gathered at the same time, so we cannot be sure whether one factor (such as an older sibling’s obesity) follows another (a younger sibling's obesity).

In some cases, the younger sibling may have be the first to develop obesity, followed by the older sibling.

As the study makes clear, older children were five times as likely to be overweight if their younger sibling was also obese.

What kind of research was this?

This was a cross-sectional study looking at how the obesity status of different children within the same family is related to parental or other sibling obesity.

The researchers point out that while the parent-child obesity link is well established, little is known about any association in obesity status between siblings.

They also say that unhealthy behaviours of children are shaped by the family and peer environment, school and neighbourhood – factors which together may influence sibling health differently than parental health.

Cross-sectional studies look at all data at the same time, so they cannot be used to see if one thing follows another, but are useful for showing up patterns or links in the data.

What did the research involve?

In 2011 the researchers contacted adults in 14,400 US households in a web-based survey about family health habits, using the resources of a national market research company. Of the 14,400, 71% (10,244 households) responded.

To take part in this aspect of the larger study, adults had to have either one or two children under 18 years of age who were living at home. Participants completed a survey on family health habits via the internet, as well as data on socioeconomic status, physical activity and overall health and “food environment”.

The questions were adapted from a variety of validated sources.

The responding adult also had to report height and weight for themselves and their children. Of the households asked, 1,948 adults had the required one or two children; provided the information needed and reported on adult and child height and weight.

From the information on height, weight and gender, researchers classified adults and their children as obese or not obese.

For children they also used information on age and growth chart data, classifying them according to internationally accepted criteria to measure obesity in children.

For adults they calculated the body mass index (BMI) from self-reported height and weight.

They analysed the association between obesity in a child and obesity in his or her siblings and parents. They adjusted the results for a range of possible factors that might influence results (called confounders).

What were the basic results?

The researchers found that:

In one-child households, a child was 2.2 times more likely to be obese if a parent was obese (standard deviation 0.5).
In households with two children, having an obese younger sibling was more strongly associated with the elder child’s obesity (odds ratio [OR] 5.4, SE.9) than a parent’s obesity status (OR 2.3, SE 0.8).
Having an obese elder sibling was associated with a younger child’s obesity (OR 5.6, SE 1.9), and the parent’s obesity status was no longer significant.
The link between siblings and obesity was stronger when they were the same gender.
Child physical activity was significantly associated with obesity status.

Surprisingly, they also found that having an extremely active older sibling was associated with a higher risk of younger sibling obesity.

How did the researchers interpret the results?

The researchers say that siblings may have greater influence on informal behaviour than parents and that older siblings may influence their younger siblings’ attitude and behaviours around food and physical activity. Taking sibling information into account may be of benefit in efforts to prevent childhood obesity, they argue.

Conclusion

This study found that while parent obesity made it more likely that a child would be obese, in two-child families, sibling obesity had an even stronger association.

However, as the authors point out, the study has several limitations.

It was based on self-reported data on height and weight and proxy reports for children, which limits its reliability.
Its cross-sectional design means all the data was gathered at the same time, so we cannot be sure whether one factor (such as an older sibling’s obesity) follows another (a younger sibling's obesity).
It was not a representative sample of the US population, which has a higher prevalence of childhood obesity.
It was restricted to families with only one or two children. Different results may be found in larger families.
Importantly, it included only the obesity status of one adult in each household – the one who responded to the survey.

Obesity is a major global health problem in which many factors play a part, including social and food environment, family lifestyle and shared genetic background.

It is likely that siblings have a common exposure to many such factors.

It is also possible that siblings influence each other’s behaviour around food and physical activity, but this study is not strong enough to prove it.

Read more advice about what to do if your child (or children) is overweight, or very overweight.

Your GP should also be able to provide advice.