In a small study from New Zealand, researchers looked at 50 overweight men ages 40 to 50, and compared the body mass indexes (BMIs) of the men who were firstborn in their family, with those who were born second. The researchers also examined the men's level of sensitivity to insulin, the hormone that regulates blood sugar levels, Fox News reports.

They found that although the two groups of men had similar average heights, the firstborns were 15 pounds heavier on average. And among firstborns, the average BMI was 29, compared with 27.5 for second-born men. People with higher BMIs are known to be at increased risk of heart disease.

Insulin sensitivity in firstborns was 33 percent lower than in second-born men, according to the study. In people with low sensitivity to insulin, body cells are less able to respond to insulin, so the pancreas has to make more to compensate. The condition brings an increased risk of developing Type 2 diabetes.

It's unclear how birth order might influence metabolism, but it is possible that differences in placenta blood flow play a part, the researchers said. During a woman's first pregnancy, the blood vessels in the uterine lining undergo lasting structural changes, providing a more favorable environment for subsequent fetuses.

The researchers cautioned that their study only looked at overweight men, living in urban areas, and the findings may not apply to other groups of people.