"I would not consider Frosted Flakes the food of an Olympian," the New York Daily News quoted nutritionist Rebecca Solomon of Mount Sinai Medical Center as saying.
"I would rather see him promoting Fiber One. I would rather see him promoting oatmeal. I would even rather see him promoting Cheerios," she added.
Phelps, the winner of a record eight gold medals at the Beijing Olympics, has signed a deal to endorse Frosted Flakes and Corn Flakes boxes rather than the traditional athlete's choice of Wheaties.
His decision has left many perplexed as frosted Flakes has three times the amount of sugar as Wheaties, and one-third the fibre.
Experts are worried that Phelps' iconic image could send the wrong message across America, where childhood obesity is an alarming issue.
"For a guy like Michael Phelps who isn't worried about obesity because he's burning thousands of calories as an athlete...eating Corn Flakes and Frosted Flakes every so often is not an issue," Solomon said.
Cereal boxes emblazoned with Phelps' pictures are to hit supermarket shelves in mid-September.