Contrary to popular belief that a child’s obesity risk is more determined by their parent’s weight, it could be true that siblings have more of an impact on their brother’s or sister’s chance of becoming obese. This is according to the findings of a new study carried out in the US. The research analysed information extracted from the national Family Habits Survey, finding that kids with obese siblings were five times more likely to be obese themselves, whereas children with obese parents were only twice as likely to also be obese. “When you look at a two-child family, a child's obesity status was more strongly related with their sibling than with their parent,” commented Mark Pachucki, a researcher with the Mongan Institute for Health Policy at Massachusetts General Hospital in Boston, and leader of the research. According to the NHS, you are considered obese if you have a body mass index (BMI) reading of between 30 and 39.9. Becoming obese is basically the result of a calorie imbalance; gaining a large amount of weight from taking in more calories through your diet than you are burning from physical activity. However, the risk can increase through a combination of different factors such as genetics, the environment and lifestyle choices. Previous research has found links between parent’s obesity and their children’s weight, but there has been little in the way of assessing the role of the weight of one sibling effecting the other’s obesity risk. “This is really the first project to come out the bigger study, in which we are actually merging about 10 years of families’ food purchases with information about their health,” Pachucki said. The research comprised of 1,948 families – 807 families with two children and 1,141 with just a single child. Eating and exercising were looked at, in addition to height and weight data to determine who was obese. Around 12% of the only children could be classified as obese, with nearly a fifth not being physically active and over a third consuming fast food on two or more occasions each week. A similar pattern emerged for those households with two children, but only 8% of the elder children could be classified as obese, in comparison to 12% of their younger brother or sister. “We looked at one-child and two-child households, and in one-child households having an obese parent made a child more than twice as likely to be obese themselves,” Pachucki said. Unsurprisingly, if the parents of an only child had a large intake of junk food, this led to a higher chance that the child would be obese, with a lesser risk if those children were involved in vigorous physical activity. For two-child families, having an obese sibling led to a risk that was in excess of fives time of that than if the brother or sister wasn’t obese. It was also found that if both children in a two-child family were the same gender, the elder sibling being obese resulted in an even heavier likelihood of the younger child being obese. Girls with an older sister who was obese were 8 times more at risk of being obese themselves compared to girls with an older sister of a healthy weight, whilst boys with an older obese brother were a staggering 11 times more at risk. “Parents are often very explicit models of behaviour; they do the food purchasing, and they control a lot of aspects of their children’s lives,” Pachucki said. “I was expecting there to be a stronger correlation with parents’ obesity, but I was surprised that the siblings were stronger.”