The group last week said that as of January 2009 its licensed characters on food packaging will be limited to 'better for you' products, in accordance with government dietary guidelines.
The move was announced in a letter sent to Representative Markey, chairman of the Subcommittee on Telecommunications and the Internet.
"Over the past three years we have consistently communicated to our food marketing partners the importance of evolving their messages and products to kids, and they have been incredibly responsive, whether it's changing their formulations of their products, depicting more active and healthy lifestyles in their advertising, or offering healthier alternatives using our characters," wrote Nickelodeon president Cyma Zarghami.
"We are confident that with the continued focus of all stakeholders in this issue that we will continue to make progress going forward and help reduce the number of obese children and children at risk in this country."
The entertainment group, which is part of the Viacom kingdom, said it has committed $30m in resources and 10 percent of its air to health and wellness messaging to children.
It is also currently in its fifth year of its initiative, Let's Just Play, an on-air, online effort to mobilize kids to adopt healthy lifestyles. By last year, over 100,000 kids had pledged to 'Go Healthy' as a result of the campaign.
In 2005, Let's Just Play entered into a partnership with the Clinton Foundation and the American Heart Association's Alliance for a Healthier Generation to combat the spread of childhood obesity. The three organizations teamed up to launch a media and public awareness campaign designed to encourage kids to engage in healthy and active lifestyles.
But despite implementing a number of health-promoting initiatives, media conglomerate Viacom was last year threatened with a lawsuit by parents and consumer groups for marketing junk food to children.
Together with cereal company Kellogg, the entertainment firm was accused of "directly harming kids' health" by pressure groups Center for Science in the Public Interest (CSPI) and the Campaign for Commercial-Free Childhood.
At the time, the CSPI had claimed that the "overwhelming majority of food products they market to children are high in sugar, saturated fat, or salt, or almost devoid of nutrients."
Indeed, Nickelodeon's characters have so far primarily been associated with products such as cereal, ice cream and candy. However, the group in 2005 launched a licensing initiative to encourage a healthier diet for kids, and said it expects to continue promoting healthy products. Since then, other 'healthy' moves include a licensing agreement with fruit firm Summeripe Worldwide, to bring Nickelodeon's most popular characters, SpongeBob SquarePants and Dora The Explorer, to packs of peaches, plums and nectarines.
Previous agreements by the network have seen its characters used on fruit and vegetable products including fresh baby carrots, clementines, apples, pears, cherries, spinach and organic edamame.
According to Nickelodeon, its two characters - SpongeBob and Dora The Explorer - currently bring in around $1bn each per year in sales of branded goods.
Nickelodeon's letter to Representative Markey follows his statement earlier this year that the Federal Communications Commission (FCC) is considering a rulemaking that would place limits on the types of advertisements seen by children if the industry does not respond to growing childhood obesity concerns with adequate voluntary measures.
Speaking in June at a hearing of the House Energy & Commerce Subcommittee on Telecommunications and the Internet, Markey said the "potentially harmful content" of advertisements for unhealthy foods is "deeply concerning".
"There is a terrible inconsistency in policies that require broadcasters to air three hours of educationally nutritious programming for kids, and then to have this programming and other children's shows surrounded by a barrage of junk food ads," he said.
"As the House Sponsor of the Children's Television Act, I believe that parents and children deserve better. And that act already grants the FCC authority to address many of these issues if the industry does not respond to this problem on its own swiftly and concretely."
Markey said that in the absence of a proper response from industry, he is prepared to "press the FCC to put on the books rules that will protect the children of our country from these unhealthy messages".
In its letter to Markey, Nickelodeon said that although it will be implementing dietary criteria for the licensing of its characters, these may still be used on "a limited number of occasional treats designed for special occasion/celebration purposes, including birthdays and holidays".