Smoking, a poor diet and a lack of physical activity were all been identified as risk factors for diseases such as cancer, which children may carry into adulthood.
Overall, grandparents were found to have an adverse effect, despite meaning well.
The behaviour of grandparents may inadvertently be having a negative impact on the health of their grandchildren, according to a new study..
Grandparents are overfeeding children and letting them get away with not exercising when they are in charge, an overview of 56 studies in 18 countries has concluded.
Grandparents pose a potential health risk to children and may even put them at risk of cancer, scientists say.
However the studies did not take into account the positive emotional benefit of children spending time with their grandparents.
Previous research has studied the way parents can affect their children's susceptibility to cancer and other diseases, but less attention has been paid to the role of part-time carers.
Dr Chambers said: "From the studies we looked at, it appears that parents often find it hard to discuss the issues of passive smoking and over-treating grandchildren".
The phenomenon is coming under increasing scrutiny as social trends - such as the growing proportion of women in the workforce, rising childcare costs, and increasing numbers of single parents - result in more children being placed in the care of their grandparents. 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.
"They bring out the biscuits at the slightest hint of a tantrum and, as the researchers report, they are also often too protective in loco parentis".
Tam Fry, chairman of the National Obesity Forum, said: "Finding a doting grandparent who is confident enough to follow rules laid down by mum and to the letter is frequently a rarity".