They are the neurotic working mother's double-edged sword: she can't cope without their support, but she hates that she can't micromanage what they are feeding her children. Now grandparents have emerged as the nutritional bogeymen in a major new study that reveals mums are right to fret.

The report by the University of Helsinki, which was published in the journal Evolutionary Psychology, warns that children looked after by their grandparents may be more likely to be overweight. All those sugary treats and ice creams are increasing the rate of childhood obesity, which carries a greater risk of premature death, as well as an increased chance of diabetes and heart disease.

One in every three children aged three who were looked after by grandparents was obese, the study found. The results flip tradition on its head: in the past, and in traditional societies today, grandparental care has always been crucial to improving child survival rates. But not in today's affluent world.

The research was based on 9,000 families in the UK. "Our results show that the same grandmother behaviour that may have had beneficial effects for grandchildren in our evolutionary past may have opposite effects in modern societies," say the researchers.

"Grandmother investment aimed at improving grandchildren's nutrition in subsistence societies may have different outcomes in contemporary affluent societies."

Psychologists said the findings suggested grandparents were exacerbating the trend towards less healthy eating patterns. Aric Sigman, a health education lecturer and psychologist, also pointed to the fact that grandparents today are older than ever, which means "their nutritional understanding is from a different era so their simple understanding of what makes children fat is not as developed as that of later generations".

Dr Sigman added: "People who are older have less energy and are less mobile, so they simply can't be as active with their grandchildren as previous grandparents."

The researchers compared the weight of children at the age of three who were cared for mainly by their grandmothers with those cared for by their parents. Overall, 23.6 per cent of the children were overweight. Among children who received primary childcare from their maternal or paternal grandmothers, 26.2 per cent were overweight, compared with 22.9 per cent who were looked after by parents.

That, according to the report, equates to a 31 per cent increased risk for children looked after by grandparents compared with those looked after by their parents.

The double blow of prohibitively expensive childcare and families where both parents work, means grandparents are increasingly being asked to step in.

- INDEPENDENT