Are Your Brother or Sister Obese?

Siblings have a greater influence on obesity than parents

Posted Nov 22, 2012

Whether it’s a result of heredity or environment, we’ve long known that an obese parent is more likely to have an obese child.  There are numerous studies demonstrating the parent-child link but a new study suggests that siblings may exert more influence than parents in determining a child’s obesity.

Researchers at the University of California, San Francisco looked at data from a survey of 3663 adults with at least one child. As expected, kids with no siblings were twice as likely to be obese if their parent was overweight or obese. In families with just one child the parents influenced the child’s weight but the findings were different when they looked at families with two children. Surprisingly, there was NO significant relationship between parent and child obesity when there were siblings. Although not conclusive, these findings suggest that the parents have less influence on the child’s weight when the child has a brother or sister.

Another analysis clearly demonstrated the effects of sibling obesity. When an older sibling was obese the younger child was six times as likely to be overweight or obese. The results were even more pronounced when the data for each sex was examined separately. A younger boy with an obese older brother was 13 times more likely to be obese or overweight while a younger girl with an obese older sister was 16 times more likely to be obese or overweight.

Even when the siblings were of the opposite sex there was still a strong influence. For example, a younger boy with an obese older sister was more than nine times as likely to be obese. Clearly having an obese older brother or sister increased the chances that the younger child would be obese or overweight.

Dr. Mark Pachucki, the lead researcher in the study, speculated that the obese older sibling is a role model regardless of the age difference between the older and younger siblings. The influence of the parent declines as more siblings enter the family. This would be consistent with earlier research with adults which showed that obesity could be “contagious.” For adults as well as kids, you’re more likely to be obese if others in your social network are obese. Older siblings are an important part of any child’s social network. It’s likely that kids spend more time with their siblings than they do with their parents so the siblings may be more important as weight role models.

The findings again point the significant role that people in the immediate environment play in determining weight. For parents, the findings suggest that helping your first-born maintain a healthy weight makes it more likely that the child’s brothers and sisters will also have a healthy weight. If you are concerned about your child’s weight there are several helpful books including It’s NOT Just Baby Fat!: 10 Steps to Help Your Child to a Healthy Weight available on Amazon.