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Obesity is a disease that is caused by many factors such as a lack of exercise, poor nutrition and genetics. In a new but small study, researchers suggested that firstborn males might have a greater risk of obesity.

For this study, the researchers from the University of Auckland in New Zealand compared the health of older and younger brothers. The men were between the ages of 35 and 55 and had a body mass index (BMI), a measurement of weight in relation to height, between 25 and 30, which classified them as overweight. The researchers exempted all male participants who had diabetes, high cholesterol, hypertension, or were taking any kinds of medications that could affect those health conditions. The researchers also excluded smokers and other tobacco users. The total sample set ended up involving 26 firstborns and 24 second-born male siblings.

The researchers reported that the firstborns had an average height of five feet 10 inches whereas the younger brothers were slightly shorter with an average height of five feet nine inches. The researchers found a much larger difference in weight between the two groups. The older brothers weighed just over 200 pounds whereas the younger brothers weighed an average of 185 pounds. The difference in weights was represented in their BMIs as well. The firstborns had an average BMI of 29.1 and the second-borns had an average BMI of 27.5. The researchers reported a slight but not significantly significant difference in body fat between the groups.

The researchers reported that one major difference between the two groups was insulin sensitivity. The firstborns had a 33 percent lower insulin sensitivity, which meant that their bodies had a harder time responding correctly to insulin. Poor insulin sensitivity can lead to many health problems such as diabetes, heart disease and obesity.

The researchers reasoned that something different could have occurred when the firstborns were in the womb. This difference could then affect the firstborn's metabolism. The team stated that one possibility was placental blood flow. During a woman's first pregnancy, arteries in the uterus undergo permanent changes, which alter the womb for all children born after the first child.

The researchers stated that their study could not be generalized to women or to men who are not middle-aged overweight Europeans. The findings were published in Scientific Reports.