NEW YORK (Reuters Health) - Brothers or sisters might have a greater influence than parents on a child's likelihood of being obese, suggests new U.S. research.
The study, based on data from the larger national Family Health Habits Survey, found that kids with obese parents were about twice as likely to be obese themselves, but having an obese sibling raised a child's risk of obesity five-fold or more in some cases.
"When you look at a two-child family, a child's obesity status was more strongly related with their sibling than with their parent," said Mark Pachucki, a researcher with the Mongan Institute for Health Policy at Massachusetts General Hospital in Boston, who led the analysis.
Past research has shown that parents' obesity, health habits and the environment they create in the household can all influence whether their kids will be obese, Pachuki and his colleagues note in the American Journal of Preventive Medicine.
But little is known about how siblings affect one another. If brothers and sisters do have more influence on obesity than parents, then they need to be factored into obesity prevention efforts, the authors write.
"This is really the first project to come out the bigger study, in which we are actually merging about 10 years of families' food purchases with information about their health," Pachucki said.
For this part of the study, Pachucki and his colleagues focused on participating families with one or two children that had provided information about height and weight for all family members.
A total of 1,948 families were included in the analysis: 1,141 with a single child and 807 families with two children.
The researchers looked at eating and exercise patterns in all the families and used height and weight information to categorize participants as either obese or not obese.
About 12 percent of only children were obese, according to the results, almost a fifth were not physically active and more than a third ate fast food at least two times per week.
The patterns were similar in two-child households, but only 8 percent of the older children were obese compared to 12 percent of their younger siblings.
"We looked at one-child and two-child households, and in one-child households having an obese parent made a child more than twice as likely to be obese themselves," Pachucki said.
Higher intake of fast food by parents was also associated with greater odds that an only child would be obese, but the risk of obesity was reduced when those kids participated in vigorous physical activities.
In two-child families, having an obese brother or sister was associated with a risk that was more than five times greater than if the sibling was not obese.
In two-child families, the impact of parental obesity on an older sibling was the same as on an only child - approximately doubling the risk - but among younger siblings there was no association between the parent's and child's risk of obesity.
The researchers also found that if the two children were of the same gender, the older sibling's obesity was even more strongly linked to obesity in the younger child.
Younger girls with an obese older sister were more than 8 times more likely to be obese than girls whose older sister was not obese, and younger boys with an obese older brother were 11 times more likely to be obese.
"Parents are often very explicit models of behavior; they do the food purchasing, and they control a lot of aspects of their children's lives," Pachucki said. "I was expecting there to be a stronger correlation with parents' obesity, but I was surprised that the siblings were stronger."
Among other limitations, the study looks at a snapshot in time and cannot prove that the siblings are influencing each other, or how that effect might be transmitted.
However, Pachucki said, "Older siblings especially can be very persuasive. I think younger siblings look up to their older siblings - and older children generally - as different kinds of role models and I think that's probably the best mechanism that we have to explain what we found."
"Examining sibling influences on weight-related behaviors is important because sibling relationships are the longest lasting relationships, in terms of life expectancy, and thus they may have the most sustained influence on a child's weight and weight-related behaviors over the lifespan," said Jerica Berge, who was not involved in the study.
"While it's important to take into account the limitations of this study, the findings are interesting and support the need for more family-based childhood obesity interventions," said Berge, a researcher at the University of Minnesota Medical School, who studies the influence of family members on obesity.said.
As an example, she said, it may be useful to include siblings in setting family goals around health behaviors, or at least including the influence of the sibling in the conversation when setting health behavior goals.
"Using a true family-based approach in childhood obesity interventions will increase the likelihood that child health behavior change will be sustainable, given the child spends a large amount of time with their sibling and are highly influenced by them," Berge said.