Health advocacy groups hoping to persuade Viacom‘s (VIAB) Nickelodeon to change its food advertising policies appear to have hit a wall.

After receiving letters from four U.S. senators asking the kids-oriented cable channel to drop ads promoting foods consider unhealthy by anti-obesity activists, Nickelodeon has responded by saying that nutritional decisions should be made by experts and regulators, not media companies, the New York Times notes.

Over the past five years, Nickelodeon has limited ads for some junk foods. The channel’s ad revenue has declined 45% during the same period. Food ads comprise the channel’s third largest ad segment, after movies and toys.

Last year, rival Disney (DIS) banned ads for unhealthy foods on its children’s cable channels in response to pressure from advocates, but Nickelodeon says it depends more on ad dollars than Disney’s channels.

The channel may also be irritated that critics have failed to appreciate its previous limits on junk food ads as well as efforts to educate viewers about proper nutrition.

A representative of the Center for Science in the Public Interest, which prompted the senators to send their letter, said that Nickelodeon has moved in the right direction, but wasn’t proceeding fast enough.

Shares of Viacom inched up slightly in Wednesday midday trading.