LEAVING children with the grandparents could turn them into overweight smokers as adults with an increased risk of cancer, a new Glasgow University study shows.

They are over-indulged with cakes, biscuits and sweets and allowed to play video games or watch TV instead of running about outside, warn scientists.

Grandparents also tend to smoke in front of them, exposing them to second hand fumes and making them more likely to pick up the habit as they get older.

The harm is unintentional but is causing tension in families and public health campaigns are required to address the issue, said the researchers.

Changes in social conditions - such as more women in the workforce, childcare costs and an increase in lone parenting - have led to an increased focus on the role of grandparents' in children's lives.

Lead author Dr Stephanie Chambers, of the University of Glasgow, said: "From the studies we looked at, it appears parents often find it difficult to discuss the issues of passive smoking and over-treating grandchildren.

"Given many parents now rely on grandparents for care, the mixed messages about health children might be getting is perhaps an important discussion that needs to be had."

Her team analysed 56 studies with data from 18 countries concerning the care provided by grandparents and found overall they were inadvertently having an adverse impact on youngsters' health.

This was especially in the areas of weight and diet through 'treating' and overfeeding and lack of physical activity.

Grandparents tend to be less energetic, so are more likely to stay in the home with the children than take them outside.

The review also identified problems regarding tobacco smoke. Grandparents were not complying with parents' wishes regarding secondhand smoke, and their importance as role models not to indulge in unhealthy behaviours.

Dr Chambers said: "While the results of this review are clear that behaviour such as exposure to smoking and regularly treating children increases cancer risks as children grow into adulthood, it is also clear from the evidence that these risks are unintentional.

"Currently grandparents are not the focus of public health messaging targeted at parents and in light of the evidence from this study, perhaps this is something that needs to change given the prominent role grandparents play in the lives of children."

She said many lifestyle patterns are established in childhood. Dr Chambers added: "In the tobacco studies reviewed, grandparents smoked around grandchildren."

Until now research has largely focused on the potential role of parents in contributing towards risk factors for diseases such as cancer.

But there has been limited investigation of the role of grandparents, and other part time caregivers.

The study published in PLOS One was financed by Cancer Research UK and aimed at identifying any potential influence grandparents' habits may have on their grandchildren's health.

Dr Chambers and colleagues said they found grandparents do appear to have a negative impact on children's long-term cancer risks.

Smoking, diet and a lack of physical activity, along with excess weight, have been identified as risk factors for non-communicable diseases, particularly cancer, as well as heart disease and diabetes.

There is evidence to suggest exposure to risk factors in childhood increases an individual's likelihood of cancer morbidity or mortality in adulthood.

Factors associated with children's long term cancer risk are first experienced within the family setting, said the researchers.

Dr Chambers said most studies included in the review looked at these issues from a parents point of view.

She also found the types of grandparent behaviour described by the parents also caused tension in families.

However, the studies did not take into account the positive emotional benefit of children spending time with their grandparents, she said.

Professor Linda Bauld, Cancer Research UK's prevention expert, said: "Children's health can be affected by range of factors, and this study reinforces the importance of the broader family picture.

"With both smoking and obesity being the two biggest preventable causes of cancer in the UK, it's important for the whole family to work together.

"Children should never be exposed to second hand smoke. But it's also important for children to maintain a healthy weight into adulthood, and in today's busy world it's often the wider family who have a role to play in keeping youngsters healthy.

"If healthy habits begin early in life, it's much easier to continue them as an adult."

The study was also funded by the Medical Research Council and The Scottish Government Chief Scientist Office.

In 2010 a study of 12,000 three-year-olds by University College London found those regularly cared for by their grandparents were a third more likely to be overweight.

More than four million British children are now overweight or obese.