How much grandparents should spoil their grandchildren has often been a source of friction for parents and now scientists have concluded what many families have long suspected: too many unhealthy treats from care-sharing relatives can be bad for a child's health.
Poor diet, excess weight, smoking and lack of physical activity are all known to increase the risk of cancer, said the researchers.
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 overall were found to have an adverse effect on the health of the children despite meaning well.
Candies were also often used to reward, express love or strengthen the bond between grandparent and child.
The authors emphasised that none of the reviewed studies took account of the positive emotional benefit of children spending time with their grandparents.
Dr Stephanie Chambers of the University of Glasgow 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".
Changes in social conditions, such as more women in the workforce, childcare costs, and a rise in lone parenting, have led to a rise in grandparents looking after their grandchildren.
"Finding a doting grandparent who is confident enough to follow rules laid down by mum and dad to the letter is frequently a rarity", he said.
Tam Fry, chairman of the National Obesity Forum, said it was the case that some grandparents "bring out the biscuits at the slightest hint of a tantrum".