A study in Britain has found that formal child care was not associated with any greater risk of kids being overweight than kids left home with mom.
For moms with a managerial or professional background or a college degree (but not for other moms), using informal child care was associated with increasing risk of being overweight.
My guess is it was less activity and more crap food, but the question is why for kids of management but not workers. More specifically, what is it about the child care or providers chosen by management and women with a college degree that makes their kids fat?
If I had guessed before seeing the results of the study, I'd have guessed it would be the other way around, on the theory that poorer people would be busier at home and would leave the kid alone more, but maybe richer people are just more indulgent and give them more crap food. Maybe the cost of crap food is less of an issue, or they're more likely to have a TV in every room. I love making up just so stories.
It doesn't rank up there with curing autism, but it would be worth finding out why this is, if only so moms can wave the study at their own moms and shout, "See what you did!"