By looking at almost 700 children nationwide, a longitudinal study found that kids who had no younger brothers or sisters by the time they entered first grade were more often obese compared to children who had a younger sibling when they were between the age of three and four.
ABC News' Julie Barzilay says the research showed that children who gained a sibling during the first few years of their lives were likely to have a healthier body mass index over time.
Dr. Julie Lumeng, a pediatrician at the C.S. Mott Hospital at the University of Michigan and one of the authors of the study, said the research is not claiming that the birth of another child in the family will be a direct cause of weight loss, but, she says, there is an association. The findings of the study require further research through which the scientists can discover what factors are involved in causing the results.
She added that the most compelling possibility at this time seems to be that having a younger sibling means the child is more likely to run around and play. Having a sister or brother is like having a full-time play partner and suggests that the two would be inclined to engage in active play more frequently.
Though a bit trickier to prove, another theory proposes that after the first child, parents can sometimes become less strict, which results in less confining feeding practices for kids. Prior study has found that the more restrictive parents are about their children's eating habits, the higher the chance of obesity.
The possibility is that excessive control keeps children from following their hunger cues, thus developing eating habits that could be detrimental to their health.
"There's a tendency for parents to constantly feed, whether the child is hungry or not," Keith Ayoob, a nutrition expert and associate clinical professor in pediatrics at the Albert Einstein College of Medicine 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. I think technology has convinced parents, and everybody, that solutions come instantly, and with kids they just don't."
Both Lumeng and Ayoob explained that neither the study nor members of the medical community have implied that parents have a second child solely to regulate the first child's weight.
The report was published in the journal Pediatrics on March 11.
Research leader Rana H. Mosli, Ph.D., from King Abdulaziz University, Jeddah, Kingdom of Saudi Arabia, stated that plenty of work had already been done on the association between parenting and children's obesity. However, the idea that siblings can play a role in causing an obesity risk has not been well researched, writes Jennifer Garcia of Medscape.
The impact of the effect on the BMI of the first child varied by the age of the eldest child when the next sibling was born. Children who were in the 9- to 24-month-old age-group experienced the least increase in BMI.
Reuters' Lisa Rapaport points out that the study did not include measured birth weights and other information such as divorce, moving, job loss, parental weight, breastfeeding, time of introduction to solid foods, and other factors that could have influenced the study's findings. Dr. Sandra Hassink, medical director of the American Academy of Pediatrics Institute for a Healthy Childhood Weight, who was not involved in the study, said the lack of this information causes the study to be incomplete.