First-borns may be more likely than second-borns to have a greater body-mass index (BMI) and lower sensitivity to insulin, a study of 50 overweight middle-aged men suggests. The research, published in Scientific Reports this week, adds to our understanding of the potential long-term health effects of birth order, although larger studies that include sibling pairs are needed to fully evaluate this link.
Some evidence suggests that birth order may influence metabolism and body composition, from infancy to early adulthood, but the potential impacts in mid-life have remained unexplored. Wayne Cutfield and colleagues studied 50 overweight but otherwise healthy men between the ages of 40 and 50, who were recruited as part of two unrelated clinical trials. They report that first-borns tended to be 6.9 kg heavier than second-borns and had a greater BMI. Insulin sensitivity was also 33% lower in first-born men than in second-borns, despite adjustment for fat mass.
The research offers a broad assessment of the possible effects of birth order on metabolic health. The authors note, however, that the participant range was rather narrow (overweight males living in a large urban centre), and because sibling pairs were not studied, the study could have underestimated the magnitude of birth order effects on insulin sensitivity and other metabolic outcomes.