Being the oldest child could mean more than added pressure from parents. A new study suggests first-born children may be more likely to face Type 2 diabetes and heart problems.

Researchers looked at 85 healthy children between ages 4 and 11, recording information like height, weight and blood profiles from physical exams, in addition to giving them tests to check their blood sugar. Thirty-two of the children studied happened to be first-borns.

The study found, on average, a 21 percent drop in insulin sensitivity among the first-borns, compared to the other kids. First-born children also had an average blood pressure 4 mmHg higher than that of non-first borns.

“This finding may have important public health implications, in light of a worldwide trend toward smaller families,” the researchers concluded.

Insulin in the body is made by the pancreas and is used to help regulate the amount of sugar in the blood stream. If someone becomes less sensitive to insulin, that means the body is using insulin less effectively than normal, causing blood sugar and fat levels to rise as a result, according to the National Institutes of Health.

Insulin resistance can lead to metabolic syndrome, a collection of risk factors that increase a person’s risk for heart disease, stroke and Type 2 diabetes. High blood pressure, or hypertension, can lead to kidney disease, heart attack, heart failure and stroke.

The study, which appeared Feb. 12 in the Journal of Clinical Endocrinology & Metabolism, was observational and did not measure cause-and-effect.

“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 Dr. Wayne Cutfield, a pediatric endocrinologist at the University of Auckland in New Zealand, said in a press release.

Levels of blood lipids, such as cholesterol and fatty acids, were not affected by birth order. Also, the oldest children were found to be about 3 centimeters taller and slimmer (based on body mass index, or BMI) than later-born children, even after taking into account their parents’ sizes.

The researchers weren’t certain what explained their findings, but theorized that physical changes that occur in a mother’s uterus from a first pregnancy to a second could lend clues. For subsequent pregnancies, nutrients tend to flow better to the fetus, which may explain these metabolic differences. They said more research is needed to find out how these findings translate into adults.

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Not all experts were convinced of a link.

“As far as I know there’s not a strong association between insulin sensitivity in an ... 8-year-old and an adult at 40 or 50 or 60,” Tamara Wexler, an endocrinologist based in Boston, told NPR. “They’re not relating this to pediatric obesity and pediatric diabetes.”

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Ryan Jaslow
CBS Interactive Inc.