It is already known that children are at an increased risk of becoming obese if their parents are obese. Now a new study has found that this risk may be even greater if their siblings are obese.

In fact, having an obese brother or sister may be a more revealing indicator of child obesity than having an obese parent.

US researchers surveyed adults in over 10,200 households. They found that the risk of childhood obesity varied depending on the number of children in a household. Where someone was an only child, they were 2.2 times more likely to be obese if their parent was also obese.

However, if there was more than one child in a household, there appeared to be a stronger link with sibling obesity than parental obesity. For example, in a household with two children, older children were 2.3 times more likely to be obese if their parent was obese also. But if they had a younger sibling who was overweight, their risk of obesity was 5.4 times higher.

The researchers also found that if a child is the younger of two siblings, parental obesity did not increase their risk of becoming obese. However, if they had an older sibling who was obese, they had a 5.6-fold increased risk of also being obese.

"The family environment is known to exert a strong influence on the trajectory of children's health, and prior research has done a great deal to illuminate connections between parent and offspring obesity. Others have also found that obesity is also often correlated between siblings. Our study extends these findings by integrating data on both parent-child, and sibling relationships," commented lead investigator, Dr Mark Pachucki, of Massachusetts General Hospital in Boston.

He said that these findings show that the obesity status of a younger child's older sibling ‘is more strongly associated with a child's obesity than is the parent's obesity status'.

"This association is independent of a host of socioeconomic and demographic attributes, health behaviors, and overall health status," he pointed out.

The study also found a link between obesity risk and gender. In homes with one child, girls were less likely to become obese than boys.

In homes with two children, younger children appeared to be very susceptible to the influence of an older sibling, particularly if they were the same gender.

"For youngest boys in two-child families, obesity is 11.4 times more likely with a male older sibling. If that younger boy's elder sibling is a girl, the boy is 6.6 times more likely to be obese. In two-child families, youngest-girl obesity is 8.6 times more likely with a female older sibling and is not significantly more likely if the older sibling is male.

"Thus, for younger children, there is a discernible gender correlation in sibling obesity status - having an obese elder same-gender sibling is associated with an increased likelihood of the younger child being obese," Dr Pachucki said.

The researchers emphasised that aside from these risks, food intake and exercise obviously play a key role in the development of obesity. They noted that children with no siblings were more likely to eat fast food and less likely to exercise than children with siblings.

"Because this study is cross-sectional, a snapshot of one point in time, we cannot claim that these particular siblings are causing one another's weight status. And we were only able to look at one-child and two-child families. Still, our findings are consistent with research showing that siblings tend to eat alike and have similar levels of physical activity.

"In seeking to reduce the prevalence of childhood obesity, it may be productive to consider prevention and treatment models that meaningfully recognise siblings as interconnected," Dr Pachucki added.

Details of these findings are published in the American Journal of Preventive Medicine.