Firstborn children more likely to be obese

THE obesity crisis may be partly driven by women having fewer children, as research has suggested firstborns are far more likely to be overweight.

A study of nearly 30,000 sisters found that those born first were 29 per cent more likely to be overweight as adults, and 40 per cent more likely to be obese. They were also slightly taller.

It is thought that firstborns tend to be larger because of “resource dilution hypothesis”, a theory which suggests that single children benefit because they did not have to share family resources such as food, affection and care. Other studies suggest that first- borns often suffer malnutrition in the womb which can trigger epigenetic changes in metabolism, such as the increased storing of fat, which continue into later life.

Whatever the reason, researchers from the University of Auckland in New Zealand believe that shrinking family size could be contributing to the general rise in adult body mass.

The average family size in Britain is now 1.9 children, compared with 2.4 in the Eighties, meaning there are far more only children in the population.

“There has been a steady decline in birth rates worldwide, especially in Europe,” said Prof Wayne Cutfield. “We showed that firstborn women have greater BMI and are more likely to be overweight or obese than their second born sisters. The steady reduction in family size may be a contributing factor to the observed increase in adult BMI worldwide.”

In Britain, 25 per cent of adults are obese — 12 million people — compared with fewer than 3 per cent in the Seventies. The proportion is predicted to grow to more than half by 2050.

Previous studies have shown that firstborns are at greater risk of health problems such as diabetes and high blood pressure in later life than their siblings.

The research, published in the Journal of Epidemiology & Community

looked at data from the Swedish Birth Register. Scientists looked at 303,301 girls who were born between 1973 and 1988, then narrowed the study to 13,406 pairs of sisters.

At birth, firstborns were very slightly lighter than their second born sisters, but as adults their BMI was marginally higher (2.4 per cent) than that of their younger sisters. BMI is calculated by dividing a person’s body mass in kilograms by the square of their height.

Maureen Talbot, senior cardiac nurse at the British Heart Foundation, said: “Since sibling rivalry can be a great motivator, brothers or sisters could use it as encouragement to eat as healthily as possible and stay active.”