Becoming a older brother or sister, particularly between the ages of two and four, may lead to a healthier BMI and lower the child’s risk of becoming obese, according to a new study at the University of Michigan which involved 697 children across the US.

In fact, the findings show that preschoolers who don’t have a younger sibling are nearly three times more likely to be obese by first grade.

“Research suggests that having younger siblings — compared with having older or no siblings — is associated with a lower risk of being overweight. However, we have very little information about how the birth of a sibling may shape obesity risk during childhood,” said senior author Julie Lumeng, M.D., a developmental and behavioral pediatrician at University of Michigan’s C.S. Mott Children’s Hospital.

“This study is believed to be the first to track subsequent increases in BMI after a child becomes a big brother or sister.”

While the exact reasons for the healthier weight of an older sibling are still unknown, the researchers suggest a few explanations. First, it could be that parents may change the way they feed their older child once a new baby is born. Because children develop long-lasting eating habits at around three years old, changing dietary habits may have a significant impact.

Another guess is that children may engage in more “active play” and less sedentary time in front of screens once a younger sibling is born, contributing to healthier BMIs.

“We need to further study how having a sibling may impact even subtle changes such as mealtime behaviors and physical activity,” said Lumeng, who is also with University of Michigan’s School of Public Health and Center for Human Growth and Development.

“Childhood obesity rates continue to be a great cause of concern. If the birth of a sibling changes behaviors within a family in ways that protect against obesity, these may be patterns other families can try to create in their own homes.”

“Better understanding the potential connection between a sibling and weight may help health providers and families create new strategies for helping children grow up healthy,” says Lumeng.

Childhood obesity has more than doubled in children and quadrupled in teens within the last 30 years, according to the Centers for Disease Control and Prevention. In 2012, more than one-third of children and adolescents were overweight or obese.

The findings are published in the journal Pediatrics.

Source: University of Michigan Health System

Pre-K Kids With No Younger Sib 3 Times More Likely to Become Obese