A recent study concludes that firstborn girls tend to be more insulin resistant than their younger sisters, making them more likely to be overweight as adults and at greater risk for developing diabetes and high blood pressure. Conducted at the Liggins Institute of the University of Auckland, in New Zealand, and published in the Journal of Epidemiology & Community Health, the study also noted firstborn girls tend to be slightly taller than their younger, same-gender siblings.
To conduct the study, Dr. Wayne Cutfield—professor of pediatric endocrinology at the Aukland University—and his assistants used the Swedish Birth Register, gathering information on girls born between 1973 and 1988 who were pregnant between the years of 1991 and 2009. This gave the team complete data for more than 13,000 pairs of sisters.
Health Day cites the study’s finding that firstborns have a 29 percent higher risk of being overweight and a 40 percent higher risk of being obese than sisters born later. Cutfield says that the study was strictly observational and therefore did not provide a conclusive reason as to why this is the case. He noted, however, that he suspects less of the mother’s blood reaches her placenta during her first pregnancy, when blood vessels are relatively narrow, than in later pregnancies, when they are wider in circumference. This results in fewer nutrients reaching the fetus.
UnitedHealthCare’s quotes of Cutfield include the researcher explaining his theory as to why less blood flow to the mother’s placenta would predispose firstborns as overweight and at risk for diabetes and high blood pressure. Reduced blood flow, he said, results in a reduced supply of nutrients, which likely reprograms the fetus’s fat and glucose regulation. This would mean that later in life, the firstborn’s body would be more apt to store fat and become resistant to insulin.
Cutfield also suggested the obesity epidemic may share a link to the tendency of modern humans to have fewer children, leading to a larger percentage of firstborns. He added that he considered this a small contributor. The primary benefit of the study, said Cutfield, is that it forewarns firstborns of their likely health outcomes, giving them the opportunity to make lifestyle choices aimed at lowering their risks of high blood pressure, diabetes, and obesity.
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Cutfield said it is possible that being a firstborn also carries more life stresses than being a second-born child, which might have some bearing on the tendency to be overweight. He also suggested firstborns’ mothers may overfeed them once they are born, concerned that their child isn’t otherwise getting enough to eat. This would tend to be less the case with each subsequent baby as the mother grew to understand what is natural with infants.
Medical News Today reported that the new study appears to corroborate earlier studies that indicated firstborn boys are more likely to be at greater risk of being overweight or obese than their younger brothers. It also indicates that while firstborn sisters tend to weigh less at birth than their younger sisters, their body mass index is generally 2.4 percent higher.
Cutfield said he believes future research should focus on whether or not firstborn adult women are at higher risk of poor metabolic health overall. He also emphasized the need to continue to educate all people on healthy food choices and the importance of avoiding a sedentary lifestyle.Whizzco