Firstborn's higher risk of diseases

By SIMON COLLINS science reporter

Firstborn children are born smaller than average but then grow faster, making them more liable to develop diabetes and heart disease later in life, new findings show.

The Liggins Institute at Auckland University says firstborn children are lavished with attention by parents, making them gain weight faster than average in their first few years.

This combination means that they are "hardwired" at birth to find food hard to get, so their bodies become stressed, but in fact they receive far more food than they expected.

Researcher Dr Susan Morton, who returned to New Zealand at the end of last year after five years in Britain, has based her findings on a long-term study of all 15,000 children aged 7 to 12 at primary schools in the Scottish city of Aberdeen in 1962.

Their birth records were traced at the time, and Dr Morton contacted 99 per cent of them again with a postal questionnaire in 2001.

Her data give a unique picture of three generations - the children of 1962, their parents and now their own children.

Dr Morton found that a baby's weight at birth and in its first few years had long-lasting effects throughout life and into the next generation.

"Small size is associated with chronic cardiovascular [heart] disease, diabetes and the markers that go with those, such as high blood pressure and obesity," she said.

Firstborn children are more at risk because a mother's capacity to pass on food to her foetus increases with each successive pregnancy, making later babies usually larger than the first ones.

Although the Aberdeen children of 1962 are still aged only in their late forties and early fifties, the early signs are that those who were born first in their families are getting more heart disease and diabetes.

On average, the women who were firstborns are heavier than other women of their age and height.

They were also more likely to develop gestational diabetes when they themselves became pregnant. Although this is a short-term disease that hits about 3 per cent of pregnant women and usually disappears after the baby is born, it is a sign that chronic diabetes may develop later.

The historical data showed that children who were born small were still smaller than average when they started school about age 5.

But firstborns who were initially small grew to be heavier than average by the time they started school.

The weight order turned upside-down, with secondborn children the second-smallest at birth but the second-biggest at age 5, and so on down to the lastborn, who were the biggest at birth and the smallest when they started school.

"If you think about the firstborn, they tend to have time alone with the parents when they are the only child," said Dr Morton, herself a firstborn child and now the mother of three daughters aged 10 to 18.

"Emotionally, and maybe nutritionally, they are exposed to a more advantaged environment before the next sibling comes along. They have their parents' undivided attention.

"It's like a competition for resources. As you have more children, there is by necessity more competition for resources - nutritional or emotional or whatever."

But that quick weight gain makes the firstborn more vulnerable to the diseases of excess later.

Liggins Institute director Dr Peter Gluckman said human beings were still biologically designed to be hunter-gatherers like our remote ancestors.

"We are designed a bit like camels - to eat fat and store it when we could, because we had to have energy available for the days when we couldn't catch the woolly mammoth or sabre-toothed tiger," he said.

"That explains why the people who need to diet and exercise can't, because they are hardwired in utero [in the womb] to be like hunter-gatherers in a very constrained environment.

"Unfortunately since World War II, and perhaps before that in some societies, the amount of energy we consume and the amount of energy we have to expend to get that food have changed dramatically, and we are now living outside the environment for which we evolved."

He said children who were born big were better able to cope with the new environment, because they were well-nourished in the womb and therefore programmed to expect a relatively comfortable life with less need to store fat.

But firstborn children were now a growing proportion of all children, whereas "one in five of your grandparents were firstborn". Now in China "they are all firstborn".

"We believe this is going to cause a real problem in the next 20 years of increased diabetes and heart disease."

Dr Morton said the key lesson was that girls and young women needed to have a balanced diet and moderate exercise, and avoid smoking.