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Children whose parents are divorced are more likely to get fat than those whose parents stay together, say researchers.

The weight gain is particularly marked in children whose parents divorce before they are six, the study found.

Researchers from the London School of Economics and Political Science analysed data on 7,574 children born between 2000 and 2002.

The authors say their findings back calls for better health support for families going through a break-up.

The paper suggests a range of reasons why children might put on weight after a divorce, both economic and non-economic.

They include:

less money in separated households for fresh fruit and vegetables

parents having to work more hours, leaving less time to prepare nutritious food

less money for extra-curricular activities, including sport

parents with less time and energy to establish healthy eating habits in their children

emotional problems leading to parents who overfeed and children who eat too much sugary and fatty food

The information on the children was collected by the UK Millennium Cohort Study, which followed the lives of a representative UK-wide sample of children born at the start of the new millennium.

The children were surveyed at the ages of nine months, three years, five, seven, 11 and 14, although this particular study excluded the data collected at 14, as the researchers wanted to focus on the period before adolescence.

'Long-lasting consequences'

Of the children studied, 1,573 - or about one in five of the total - had seen their parents separate by the time they were 11.

The study also looked at the children's heights and weights, ages and genders to calculate their body mass index (BMI) - a widely used measure of whether individuals are a healthy weight, overweight or obese.

The results showed that children of separated parents gained more weight during the 24 months after their parents separated, than children whose parents stayed together over the same period.

And children of separated parents were more likely to become overweight or obese within 36 months of the separation, the study found.

The paper says the results underscore the idea that parental separation is "a process with potentially long-lasting consequences".

The authors suggest that, as the study stopped when the children were 11, the data might underestimate the full extent of the children's weight gain over time "because the magnitude of this association becomes stronger as the time since separation increases".

The authors argue that efforts to prevent children at risk from gaining weight should start soon after separation.

"Intervening early could help to prevent, or at least attenuate, the process that leads some children to develop unhealthy obesity," they write.

The study focused on the consequences of the first separation of children's biological parents, so children whose parents were later reconciled were not included in the analysis.

The authors also controlled for socio-economic disadvantage.

The article is published in the journal Demography.