By Emma Wilkinson
Health reporter, BBC News
Grandma doesn't always know best when it comes to diet
Young children who are regularly looked after by their grandparents have an increased risk of being overweight, an extensive British study has suggested.
Analysis of 12,000 three-year olds suggested the risk was 34% higher if grandparents cared for them full time.
Children who went to nursery or had a childminder had no increased risk of weight problems, the International Journal of Obesity reported.
Nearly a quarter of preschool children in the UK are overweight or obese.
The researchers said very little research had been done on the influence of childcare on weight.
Yet childcare may have an effect on weight through diet and physical activity.
The study used data from the Millennium Cohort Study, which looked at the health of children aged between nine months and three years old, who had been born in the UK between 2000 and 2001.
The results showed that those looked after by grandparents part-time had a 15% higher risk of being overweight for their age compared with those solely looked after by their parents.
Those who were cared for by their grandparents full-time had a 34% increased risk of being overweight, the University College London team found.
Further analysis taking into account the child's socio-economic background, found the increased risk was only apparent in children from the most advantaged groups - whose mothers had a managerial or professional job, had a degree, or lived with their partner.
There was also an increased risk of being overweight associated with other informal care provided by relatives or friends but only if that was full-time.
The researchers said it was well-recognised that parents value care provided by grandparents and consider it to be the best alternative to full-time parent care.
They said the issue was about providing informal carers, such as grandparents, with better information and support around diet and exercise.
A recent announcement to provide grandparents with National Insurance credits for caring for grandchildren under the age of 13 years for at least 20 hours a week from 2011, "provides a potential opportunity for such health promotion", they advised.
Study leader Professor Catherine Law said this study, which was backed by other work done in the US, did not look at why grandparent care was associated with being overweight but that indulgence of children and lack of physical exercise were two possible explanations.
"One of the ways forward would be to talk to small groups of grandparents to see the challenges they face.
"Some of the things that might help would be educating the population in general about healthy lifestyles but also things like avoiding food as a reward and suggestions for building activities into daily life."
A Department of Health spokesman said: "We know that obesity is a very complex issue with a wide range of factors involved.
"The latest figures show that child obesity levels are the lowest reported since 2001. However, there's no doubt that levels of obesity in this country, as in the rest of the developed world, are far too high.
"That's why we're investing time, energy and money into preventing people from becoming obese in the first place."