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The study authors noted their findings could have serious public health implications for countries like China, where a one-child policy means first-born children comprise a large portion of the overall population.
The study will be published in the March issue of the Journal of Clinical Endocrinology & Metabolism.
"Although birth order alone is not a predictor of metabolic or cardiovascular disease, being the first-born child in a family can contribute to a person's overall risk," study author Wayne Cutfield, of the University of Auckland, said in a news release from the Endocrine Society.
The study involved 85 healthy children ranging in age from 4 to 11. The researchers said they focused on younger children because puberty and adult lifestyles can independently affect insulin sensitivity.
Of all the children participating in the study, 32 were first-borns. After measuring the children's fasting lipid and hormonal profiles, height, weight and body composition, the researchers found all 32 first-born children had a 21 percent drop in insulin sensitivity and a 4 mm Hg increase in blood pressure levels compared with the other children.
The study also showed that first-borns were typically taller and thinner than their younger siblings. This was true even after considering the height and body mass index of the children's parents. Body mass index is a measurement that takes into account height and weight.
The researchers explained that changes occur in the uterus after a first pregnancy and better nutrient flow to the fetus during subsequent pregnancies could explain the metabolic differences in younger siblings.
"Our results indicate first-born children have these risk factors, but more research is needed to determine how that translates into adult cases of diabetes, hypertension and other conditions," Cutfield said.
-- Mary Elizabeth Dallas
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