A joint study conducted by researchers at Massachusetts General Hospital, Cornell University, and Duke University and published in the American Journal of Preventive Medicine has found that having an obese sibling can also increase the obesity risk of other siblings.
By surveying adults in 10,244 American households, investigators found that the likelihood of childhood obesity varies with the number of children in a household, as well as their gender. According to the study, in a single child household, a child is 2.2 times more likely to be obese if a parent is also obese.
However, in families with two children, the data showed a stronger relationship with sibling obesity than with parental obesity. Older children in a two-child household with an obese parent are 2.3 times more likely to be obese, but that number jumps to 5.4 times for those with overweight younger siblings. If the child is the younger sibling in a two-child household, parental obesity is not relevant to risk, but having an obese older sibling is associated with a 5.6-fold higher risk.
The data also uncovered a link between gender and obesity risk. In homes with only one child, girls were less likely to be obese than their male counterparts. Gender was also an important factor for siblings in two-child households. The study suggests that younger children may be particularly susceptible to the influence of an older sibling, especially if that sibling is of the same sex.
"For youngest boys in two-child families, obesity is 11.4 times more likely with a male older sibling," write the authors. "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."
Of course, exercise and food consumption play key roles in the prevention of obesity, and investigators found that only children were less likely to engage in physical activity and more likely to eat fast food than their counterparts with brothers and sisters. Curiously, however, the data also showed that having a highly active older sibling raised the obesity risk for the younger sibling. "Younger siblings with more vigorous physical activity are significantly less likely to be obese, though having an elder sibling who is extremely active is associated with a higher risk of younger-sibling obesity," clarifies Dr. Pachucki.
While the authors stress that more within-family obesity research is needed, this new study offers key data that will be beneficial in the fight against childhood obesity. "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," concludes Dr. Pachucki. "In seeking to reduce the prevalence of childhood obesity, it may be productive to consider prevention and treatment models that meaningfully recognize siblings as interconnected."