Firstborns may be at higher risk for heart disease and diabetes than their later-born counterparts, new findings suggest.
In a small study from New Zealand, researchers looked at 50 overweight men ages 40 to 50, and compared the body mass indexes (BMIs) of the men who were firstborn in their family, with those who were born second. The researchers also examined the men's level of sensitivity to insulin, the hormone that regulates blood sugar levels.
They found that although the two groups of men had similar average heights, the firstborns were 15 pounds heavier on average. And among firstborns, the average BMI was 29, compared with 27.5 for second-born men. People with higher BMIs are known to be at increased risk of heart disease.
Insulin sensitivity in firstborns was 33 percent lower than in second-born men, according to the study. In people with low sensitivity to insulin, body cells are less able to respond to insulin, so the pancreas has to make more to compensate. The condition brings an increased risk of developing Type 2 diabetes.
The results held after the researchers controlled for other factors that could affect BMI and insulin sensitivity, such as the men's body fat, age and physical activity, according to the study published today (Feb.6) in the journal Scientific Reports.
The findings suggest that birth order could potentially have long-term health effects, according to the study researchers at the University of Auckland. However, larger studies — that, ideally, compare siblings to each other — are needed to confirm the findings, they said.
Several earlier studies suggested that birth order may influence metabolism and body composition in children. Research has shown for example, that firstborns weigh less at birth but grow faster during infancy, and have reduced insulin sensitivity and higher daytime blood pressure, as well as higher cholesterol during young adulthood.
It's unclear how birth order might influence metabolism, but it is possible that differences in placenta blood flow play a part, the researchers said. 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.
The researchers cautioned that their study only looked at overweight men, living in urban areas, and the findings may not apply to other groups of people.