Firstborn Siblings More Likely to be Obese, Diabetic and Hypertensive, Large Scale Study Shows

A team of researchers from the Liggins Institute in New Zealand conducted the largest study of its kind in women and found that firstborn women have a higher likelihood of being overweight or obese than their second-born sisters. The study, titled, “First-borns have greater BMI and are more likely to be overweight or obese: a study of sibling pairs among 26 812 Swedish women” is available online in the Journal of Epidemiology & Community Health.

“We found that firstborns were nearly 30 per cent more likely to be overweight, and 40 per cent more likely to be obese than their second-born sisters,” says the senior investigator, Professor Wayne Cutfield of the Liggins Institute at the University of Auckland.

Based on gathered data from the Swedish Birth Register, which stores information on all births in Sweden dating back to the mother’s first antenatal visit, the study analyzed 13,406 pairs of adult Swedish sisters and affirms findings from his team’s previously completed studies on firstborn adult men and in children of both sexes.

“Collectively, these studies show that both men and women who are born first are at greater risk of being overweight or obese,” Professor Cutfield says.

The previous studies found that firstborns tended to be more insulin resistant, and first born children had higher blood pressure readings — risk factors for type 2 diabetes and hypertension respectively.

According to Professor Cutfield, while these findings can be alarming, they should not necessarily be viewed as predictors of whether or not they will become obese, diabetic or hypertensive in the long run. “The differences of about 20 to 25 per cent in obesity and insulin sensitivity between firstborns and those born later are not large enough to be a major determining factor. What this information about health risks does is to empower firstborns so they can make positive choices about diet and exercise,” he says.

While investigators have not yet fully understood why these differences occur between firstborns and their younger siblings, Professor Cutfield believes it may have something to do with differences in placental blood supply. “In a first pregnancy, the blood vessels to the placenta are narrower. This reduces the nutrient supply, thus reprogramming the regulation of fat and glucose, so that in later life the firstborn is at risk of storing more fat and having insulin that works less effectively,” he says.

Cutfield concluded by stating that this large-scale study’s findings may potentially contribute to how obesity is viewed and addressed as a global epidemic. “The steady decrease in family size over the last century has created a higher proportion of firstborns. That may be a contributing factor to the steady increase we are seeing in the adult body mass index or BMI around the globe.”