Study: Could Bro Or Sis Affect Weight More Than Mom Or Dad?

Veronica Thomas
CommonHealth Intern

My adolescence was a blur of rushing from school to dance classes with my older sister. After hours of practice, we couldn't wait to get home and make berry smoothies that we’d slurp from the blender. My sister and I did almost everything together.

A new study suggests this relationship may have played a key role in keeping me healthy and fit.

The study, released online by the Journal of Preventive Medicine, found that siblings may have a greater influence on a child’s risk of obesity than parents do. Specifically, having an obese older sibling is associated with more than double the risk of being obese compared to having an obese parent. The association is even greater among siblings of the same gender.

It may seem obvious that family members influence a child’s chances of being obese, but the importance of the type of family relationship has been less clear. This new study, led by Dr. Mark Pachucki at the Mongan Institute for Health Policy at Massachusetts General Hospital, is the first to compare the influence of sibling obesity and parent obesity on a child's obesity risk.

Dr. Pachucki and his team surveyed almost 2,000 only-child and two-child families from the larger Family Health Habits Survey. One parent from each family reported on the food environment, physical activity, weight and height for themselves and their children. The researchers also considered and analyzed the parents’ socioeconomic status, demographic background and overall health.

From the press release:

In families with only one child, having an obese parent was associated with more than double the risk that the child would be obese, although that risk was reduced if high levels of physical activity were reported for the child.  In two-child families, having an obese sibling was associated with risk that was more than five times greater than if the sibling were not obese.  While the impact of parental obesity on an older sibling was the same as on an only child – approximately doubling the risk – among younger siblings there was no association with parental obesity.

In addition, if the two children in a family were of the same gender, an older sibling’s obesity had an even stronger association with obesity in the younger sibling – 8.6 times greater for girls and 11.4 times greater for boys. Of other demographic, socioeconomic and behavioral factors analyzed – including the number of fast food meals consumed weekly – the only one that significantly changed the impact of a sibling’s obesity was the level of physical activity.

Because this study is merely a snapshot of family characteristics, it's not clear that one sibling’s obesity directly causes the other’s obesity. Dr. Pachucki noted that the survey was unable to delve deeper into questions about the behaviors of individual family members and how the members feel about each other.

However, the well-studied influence of role models and imitation on health behaviors should not be underestimated. “Younger siblings automatically look up to older siblings in a lot of cases,” says Dr. Pachucki.

Older siblings should be aware of that responsibility.

Like my sister and me, siblings often eat and participate in physical activities together, which creates opportunities for siblings to model health behaviors separately from their parents. “If a child is making efforts to eat healthy food and get enough physical activity, that can have positive effects on his or her brother or sister,” Dr. Pachucki says. “Older siblings should be aware of that responsibility.”

Parents, this doesn’t mean you’re off the hook. According to Dr. Pachucki, "Prevention-wise, it’s always good for parents to be aware of the possibility that their children shape one another’s health behaviors. As their children grow up, not only will this mindset help them think more tangibly about the health behaviors they model for their children, but they can also help enforce their children’s health with one another."

While the study's findings are interesting for families to consider, more research is needed before they can be integrated into any formal family interventions, Dr. Pachucki says. The analyses will need to be replicated with data and studies that interview and follow families over time, including families with varying configurations, like step-siblings.

It will also be important to closely examine the impact of peers on a child's obesity risk, he says. As many parents can likely attest, there is a critical turning point in adolescence when peer influence reigns supreme.