Anyone with a younger brother or sister knows it's not always an easy ride (unless you happen to be very fortunate), but a new study suggests there could be real, tangible health benefits to having a younger sibling. Researchers have linked having a little brother or sister to a lower risk of obesity in a comparison of the body mass indexes (BMIs) of 697 children in the US.

The University of Michigan team found that the birth of a sibling between the ages of two and four was associated with a healthy BMI, while those without a sibling by the first grade (age six) were almost three times more likely to be obese at that age. Those are quite dramatic findings, but the researchers behind them aren't entirely sure why such a link exists.

"This study is believed to be the first to track subsequent increases in BMI after a child becomes a big brother or sister," said senior author Julie Lumeng. "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."

The researchers suggest that the birth of another child might cause more active play time and less time stuck in front of a TV or computer screen. Another hypothesis is that parents change the way they feed their first child once a second one arrives. We already know that children develop long-lasting dietary habits around the age of three, so that fits the time window.

One of the main aims of the study was to look at the growing problem of childhood obesity in the United States. The number of affected kids has more than doubled in the last three decades, according to data from the Centres for Disease Control and Prevention, and in 2012, more than one-third of children and adolescents were overweight or obese.

That in turn leads to a higher risk of cardiovascular disease, bone and joint problems, sleep apnoea, and other related health issues.

"Childhood obesity rates continue to be a great cause of concern," says Lumeng. "If the birth of a sibling changes behaviours 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."

Until researchers can come up with an explanation for the apparent link between younger sibling and weight, whether it's psychological, biological, or a combination of the two, we can't jump to conclusions about what's going on here, but it's certainly cause for more investigation.

The researchers now want to take a closer look at how the birth of a sibling might impact mealtime patterns and physical activity, but next time your little brother or sister annoys you, give them a break - turns out they're probably good for your health.

The study has been published in the journal Pediatrics.