Researchers noted the link but could not explain the cause and effect.
Children who welcome a younger brother or sister between age 2 and 5 are less likely to be obese than similar peers, although researchers aren’t sure why.
The findings, released today by the journal Pediatrics, also showed that children who did not have a sibling by the time they reached first grade were 3 times as likely to be obese.
The study tracked 697 children across 10 states; researchers checked every 3 months to see if a new sibling had arrived in the household. Weight status and body mass index were checked starting at 15 months, with checks again at 24 months, 36 months, 54 months, and in first grade.
Children were grouped based on how old they were when a new sibling arrived. Only 5% of those who welcomed a younger sibling between age 3 and 4 were obese in the first grade. By contrast, 13% of those who did not have a younger sibling by first grade were obese.
While the authors note the link, they do not have the data to determine whether there is a cause and effect. Julie Lumeng, MD, senior study author and a researcher in pediatrics and public health at the University of Michigan, told Reuters that when a sibling arrives, “a child might become more active—for example running around more with their toddler sibling.”
The full study will be published in the April print edition of Pediatrics.
Mosli RH, Kaciroti, N, Corwyn RF, Bradley RH, Lumeng JC. Effect of sibling birth on BMI trajectory in the first 6 years of life [published online March 11, 2016]. Pediatrics. DOI: 10.1542/2015-2456.