"Research shows that food marketing is an important factor contributing to children's poor diets and obesity," the letter stated.
"The majority of foods marketed to children remain of poor nutritional quality. The [federal Institute of Medicine] concluded that marketing puts children's health at risk."
Childhood obesity has more than tripled in the last three decades, portending a major health crisis as overweight children age.
Actions to limit junk-food consumption — like New York City Mayor Michael Bloomberg's (I) large-soda ban — have prompted questions about the best role for government in curbing the growth of U.S. waistlines.
Monday's letter came from the American Cancer Society, the American Heart Association, the Center for Science in the Public Interest, and more than 50 other groups.
The authors pointed to a move by the Walt Disney Company to implement nutrition standards for food and drinks marketed on its media platforms.
"Nickelodeon lags behind the efforts of other children's entertainment companies," the letter stated.
Disney announced its commitment alongside first lady Michelle ObamaMichelle LeVaughn Robinson ObamaYouTube confirms it picked kids featured in Harris video Photos of the Week: Congressional Baseball Game, ashen trees and a beach horse The Hill's Morning Report - Presented by Alibaba - Gears begin to shift in Congress on stalled Biden agenda MORE in June.
The first lady has been a prominent nutrition advocate, in part through her "Let's Move!" campaign, which seeks to combat childhood obesity.
Nickelodeon responded to the groups in a statement.
"No entertainment brand has worked as comprehensively and with more organizations dedicated to fighting childhood obesity over the past decade than Nickelodeon," the channel said.
"Our commitment has included dedicating 10 percent of our airtime to health and wellness messaging; our partnerships with Let’s Move, The Alliance for a Healthier Generation, Boys and Girls Clubs of America and professional sports leagues among many others ... as well as integrated licensing and marketing partnerships with companies like Birds Eye that encourage kids to eat vegetables."
Nickelodeon added that the "vast majority" of its advertisers have signed a pledge to encourage kids to eat healthily.
—This post has been updated.