Cocoa Krispies boxes carry a banner saying, "Now helps support your child's IMMUNITY."
"By their logic, you can spray vitamins on a pile of leaves, and it will boost immunity," said Kelly Brownell, director of Yale University's Rudd Center for Food Policy and Obesity.
In a letter last week, the City of San Francisco asked Kellogg, the nation's largest cereal maker, to prove its claim, USA Today reported Monday.
"I am concerned the prominent use of the immunity claims to advertise a sugar-laden chocolate cereal like Cocoa Krispies may mislead and deceive parents of young children," city attorney Dennis Herrera said.
Kellogg spokeswoman Susanne Norwitz said the cereal maker added antioxidant vitamins A, C and E to its Cocoa and Rice Krispies products because studies show those vitamins play a key role in the immune system.