Birth order linked to risk of adult disease

A small study by New Zealand investigators shows that first-borns may be at higher risk of metabolic and cardiovascular disease (CVD) as adults than their younger siblings. The results, published in The Journal of Clinical Endocrinology & Metabolism, may have important public health implications.

Included in the analysis were 85 healthy, prepubertal children (32 first-born; 53 later-born) aged 4 to 11 years, born 38 to 40 weeks’ gestation and at weight appropriate for gestational age. The investigators focused on children because puberty and adult lifestyle can affect insulin sensitivity.

Clinical assessments of the participants included height, weight, fasting lipid and hormonal profiles, and dual-energy x-ray absorptiometry-derived body composition. The children also underwent 24-hour ambulatory blood pressure monitoring and frequently sampled intravenous glucose tests with Berman’s minimal model.

Compared with the later-born children, the first-borns were approximately 3 cm taller and slimmer (height SD scores 0.889 vs 0.39; P=.009; body mass index SD scores -0.05 vs 0.39; P=.048). IGF-1 concentrations also were 27% higher in the first-borns, in keeping with their stature (227 vs 173 ng/mL; P=.002). Birth order did not impact blood lipids but first-borns had 27% lower insulin sensitivity compared with later-borns (8.4 vs 10.6 x 10^-4/min[mU/L]; P=.019). Both daytime systolic and diastolic blood pressures were higher in first-borns (+5 mm Hg; P=.032 and +4 mm Hg; P=.029, respectively).

Despite first-borns’ taller, slimmer bodies, the combination of reduced insulin sensitivity and increased daytime blood pressure suggests they may be at greater risk of metabolic disorders and CVD in adult life. The findings, say the investigators, may have significant public health implications because of the worldwide trend toward smaller families. They theorize that the metabolic differences in the groups may be caused by physical changes in a woman’s uterus during her first pregnancy, which may lead to increased nutrient flow to fetuses in subsequent pregnancies.