The lawmakers asked the GAO to report back on the cost of the new rules and whether they have prompted students to drop out of school meal programs, among other concerns.
Several Republican House members slammed the rules as evidence of a "nanny-state" approach to reducing U.S. obesity. Rep. Steve King (R-Iowa), a member of the House Agriculture Committee, introduced a bill to repeal them in September.
But the Obama administration argues that the rules are necessary to avert a public health crisis.
"We are going full steam ahead to making sure that the school day is healthier," Agriculture Undersecretary for Food Kevin Concannon told reporters in August. "These are the first major changes to school meals in 15 years. They are much-needed and long overdue."
First lady Michelle Obama was another major advocate for the policy, arguing that kids should get "the same kind of food we would serve at our own kitchen tables" at school.