Birth of a Sibling Could Mean a Healthier Body Weight for the First-Born

— -- Younger siblings can be annoying, but a new study suggests they may be good for your health.

In the longitudinal study that tracked nearly 700 children across the U.S., researchers found that kids who did not have a sibling by the time they were in first grade were more often obese at that age compared to children who gained a sibling between ages three and four.

Essentially, the birth of a sibling a few years into a child’s life was associated with a healthier body mass index trajectory for that first child. The study was published today in the journal Pediatrics.

Researchers emphasized they are not claiming the birth of a sibling directly causes weight loss but that there is an association, and these findings need to be studied further. The surprisingly robust association led the study's authors to wonder what factors might be at play.

“The possibility that seems most compelling,” said Dr. Julie Lumeng, a pediatrician at the C.S. Mott Hospital at the University of Michigan and an author on the study, “is that if you have a younger sibling, you’re more likely to run around.”

Simply put, having a younger sibling is like having a built-in playmate: at any given time, the siblings are more likely to engage in some kind of active play.

Another theory that is a little harder to prove is that once a second child arrives, parents tend to loosen up, which means less restrictive feeding practices for children. Somewhat counter-intuitively, previous research has shown that the more a parent restricts a child’s eating, the higher the risk of obesity. It may be that parental control prevents kids from learning to listen to their own hunger cues, thus promoting unhealthy eating habits.

Keith Ayoob, a nutrition expert and associate clinical professor in pediatrics at the Albert Einstein College of Medicine, said family dynamics may play an important role in determining whether a child develops sound eating habits and a healthy BMI.

“There’s a tendency for parents to constantly feed, whether the child is hungry or not,” he said. “Children can be silenced with food -- and that really ends up leading to a dysfunctional relationship with food. It’s a very quick fix.”

While reflecting on his 30-plus years working with children and families, Ayoob noted that parents often lack patience.

“I think technology has convinced parents, and everybody, that solutions come instantly, and with kids they just don’t,” he said.

Parents must practice consistency and discipline, and never reward tantrums, he said. But be sure to make it clear to your little one that it’s the bad behavior, not the child, you don’t like.

Both physicians emphasized that no one is recommending having a second child purely for the sake of affecting the first child’s weight.

Instead, Lumeng encourages parents to consider setting up a play date this weekend, or enjoying a day out in the park, to promote healthy habits.

“This study might be a trigger for people to reflect on their family rhythms and what the family dynamic is,” she said. "If there were a younger sibling in the family, how might the rhythms change in a way that might be protective against obesity?”