Birth order may impact more than just sibling rivalry. A study out of New Zealand has found that first-born children weigh about 14 pounds more by the time they reach middle age, with a higher body mass index (BMI) than their younger siblings. Older siblings also had a higher insulin resistance, making them more susceptible to type 2 diabetes later in life.
There is prior research that suggests birth order may influence the body in some ways from infancy to teenage years, but the potential of a mid-life impact has not been studied in depth.
An article at Live Science says, “During a woman’s first pregnancy, the blood vessels in the uterine lining undergo lasting structural changes, providing a more favorable environment for subsequent fetuses.” Other research has shown that first-borns may weigh less at birth but grow faster during infancy, and have a lower insulin sensitivity and higher daytime blood pressure.
The University of Auckland looked at 50 overweight but healthy men between the ages of 40 and 50. They compared the BMIs of the men who were first-born to the men who were born second and looked at their insulin sensitivity.
Insulin sensitivity in the first-born men was 33 percent lower than in the second-born men, which increases their risk of type 2 diabetes.
“This study has shown that within a cohort of overweight middle-aged men, first-borns had greater BMI and lower insulin sensitivity than second-borns. Importantly, the difference in insulin sensitivity was independent of fat mass,” the study states.
Being first-born doesn’t mean that one will be overweight, but it could mean that the risk is greater than that of younger siblings. The findings suggest that birth order may potentially have long-term health effects, but more research is needed to confirm this.
“Reduced insulin sensitivity is an independent predictor of type 2 diabetes mellitus, hypertension, coronary heart disease, stroke and cancer in non-obese middle-aged men,” the study states.