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Researchers from New Zealand report that these children have reduced insulin sensitivity and higher daytime blood pressure than kids with older siblings.
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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SOURCE: The Endocrine Society, news release, Feb. 12, 2013