“As an entertainment company, Nickelodeon’s primary mission is to make the highest quality entertainment content in the world for kids,” the company said in a written response to the senators. “That is our expertise. We believe strongly that we must leave the science of nutrition to the experts.”

Money, of course, is a significant factor. Food advertising on Nickelodeon has fallen 45 percent since 2008, according to the company, partly because of its own efforts to cut back on ads for certain sugary drinks and fatty foods, and partly because of self-regulation by food companies.

But food remains Nickelodeon’s third biggest advertising category (behind movies and toys), accounting for roughly 18 percent of annual sales. At a recent meeting on Capitol Hill, according to legislative aides, Viacom lobbyists reiterated this point and emphasized that their children’s networks — Nick, Nick Jr. and TeenNick — are more reliant on advertising than Disney’s flagship Disney Channel, which accepts limited sponsorships but does not accept 30-second spots.

Advertisers spend roughly $950 million annually on television tailored to children under 12, according to industry estimates. Across all the children’s cable channels tracked by Nielsen, the number of food ads was up 16 percent in the first quarter of the year versus the same quarter a year earlier, far outpacing the increase in all advertising, which was 6 percent.

Nickelodeon, which declined to comment for this article, is dug in for other reasons. Its executives ardently believe that critics, in particular the nonprofit Center for Science in the Public Interest, have ignored its many efforts to cut down on junk food ads and promote healthy lifestyles to its viewers.