Birth of a Sibling May Decrease Obesity Risk

Jennifer Garcia

March 14, 2016

Having a younger sibling before first grade may decrease the risk of a child becoming overweight or obese, according to a new study published online March 11 in Pediatrics.

Researchers led by Rana H. Mosli, PhD, from King Abdulaziz University, Jeddah, Kingdom of Saudi Arabia, found that children who experienced the birth of a sibling between ages 24 and 36 months or between ages 36 and 54 months had a lower body mass index z-score (BMIz) trajectory and a lower BMIz at first grade compared with children of the same age who did not have a younger sibling (0.27 vs 0.51 [P = 04] and 0.26 vs 0.51 [P = .03], respectively).

"[A]lthough much work has focused on the association between parenting and obesity, the potential role of siblings in shaping obesity risk is not fully understood," write Dr Mosli and colleagues.

The researchers note that, with respect to obesity prevalence, children with no younger siblings were nearly three times more likely to be obese by the time they reached first grade compared with children who became older siblings between 36 and 54 months of age (odds ratio, 2.94; P = .046).

The researchers evaluated data from 697 children from across the United States and calculated z-scores and BMIz based on national data sets. Weight and height (or length) were documented for each child at 15, 24, 36, and 54 months of age, as well as during the child's first grade year at school. During this time, data were also stratified as follows, based on the birth of a sibling: no siblings born by the time the child entered first grade (mean age, 72 months), sibling born between age 9 and 24 months, sibling born between age 24 and 36 months, and sibling born between age 36 and 54 months.

The authors found that the magnitude of the effect on BMIz varied by the age of the child when a sibling was born, with those in the 9- to 24-month category experiencing the lowest subsequent increase in BMIz overall.

"To our knowledge, this study is the first to document a longitudinal association between the birth of a sibling and a lesser subsequent increase in BMIz," write Dr Mosli and colleagues.

When discussing potential mechanisms of association, the researchers suggest that changes in mealtime behaviors after the birth of a sibling, as well as the possibility of the older sibling assuming a more active caregiver or playmate role, may have an effect on calorie intake and expenditure. Although they note that measures to test these associations were not available in the data set used, identifying the underlying mechanism of association, "could help inform interventions and improve children's outcomes," they write.

Given that the information evaluated in this study was collected between 1991 and 1998, the researchers acknowledge that more contemporaneous data may alter the results. Further, racial/ethnic information was limited, which may limit the generalizability of the data to certain groups. Finally, the researchers also note that the transition from weight-for-length z-scores to BMIz based on developmental period may affect interpretation of the data.

Funding for this study was provided by a grant from the National Institutes of Health. The authors have disclosed no relevant financial relationships.

Pediatrics. Published online March 11, 2016. Abstract

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