Anxious schools have waited more than a year to find out how sales of potato chips, candy bars, sodas and similar treats to students will be restricted. These rules on food sold outside traditional cafeteria meals are a key part of the first major overhaul on school food in more than three decades.
Officially, USDA said it expects the proposal by April, at which point a 60-day public comment period would kick in before final rules are issued - potentially for the next school year.
Vilsack said the delay was in part to give food and drink manufacturers, as well as schools, time to adjust to a revamp of cafeteria breakfasts and lunches in early 2012.
Those earlier sweeping changes, dictating more whole grains, fruits and vegetables on school menus, led to a few complaints and some hungry children. USDA later gave schools more flexibility on the new menus.
"The whole idea is that they eat more fruits and vegetables ... that's not going to happen overnight," said Gail Koutroubas, who oversees food services for the public school district in Andover, Massachusetts, near Boston.
The school nutrition overhaul seeks to make a dent in the nation's obesity epidemic at a time when government statistics show more than one-third of those younger than 18 are too heavy.
Health advocates want the snack changes to include smaller portions, reduced fat and less sugar. Acceptable drinks for most students would include low- or no-fat milks, 100 percent juices and water.
"We're not saying get rid of the vending machines. Just change what's in them," said Margo Wootan, head of nutrition policy at the Center for Science in the Public Interest, an advocacy group. "We, as parents, don't want our kids eating candy bars and Gatorade for lunch."
Recent USDA data shows most U.S. public schools sell snacks in some way, but access to vending machines varies.
Machines are in just 13 percent of elementary schools for young children, but are in 67 percent of middle schools, where students are around 11 to 14 years old, and 85 percent of high schools, USDA said in December.
USDA also found more than 80 percent of school districts have either restricted or banned sugary drinks. More than 75 percent also have some kind of limit or ban on snack foods.
Tops on the list of concerns are drinks, particularly high-calorie, sugar-sweetened beverages such as sodas and sports drinks that are popular among youth but cited by public health experts as empty calories with no nutritional benefit.
"The beverage industry from our perspective has been very cooperative. They understand and appreciate there has to be a different approach in terms of what's available in vending machines," Vilsack said.
Representatives from Coke and other drink makers met with White House and USDA staff last June to present industry-funded data on their voluntary effort in schools, according to one representative at the meeting.
The American Beverage Association said it wants USDA's snack rules to match the industry lobbying group's 2005 pledge to limit certain lower calorie sodas and sports drinks to mostly older students.
Cranberry growers also are pushing to allow sugar-sweetened cranberry juices, saying their fruits are too tart otherwise. Pizza companies are also seeking some exemptions.
Some health experts worry that the food and beverage industries' lobbying power will dilute the new nutrition law. They are also concerned that the delay signals a more cautious approach from the Obama administration.
Overall, food and beverage companies and groups spent more than $26 million in lobbying on a variety of issues over the last year, led by Coke and Pepsi, according to the Center for Responsive Politics.
"We're very concerned," said Maya Rockeymoore, head of a Robert Wood Johnson Foundation program targeting childhood obesity. "Certainly sugar-sweetened beverages ... need to be completely eliminated from the equation."
Many states and schools have plowed ahead with their own standards rather than wait for USDA. About half of all U.S. states have already passed laws addressing snack items, according to the National Conference of State Legislatures.
Money can also be an issue, since snack foods sales are often profit centers for schools, sports teams and other groups, especially given the current tight school budgets.
Advocates say the changes are necessary to help stave off a health crisis that impacts students' academic performance and even national security.
Vilsack says he has a personal stake in the issue, recalling how he was taunted as a "fatso" growing up on Pennsylvania.
"I've struggled with my weight all my life, and it's not an easy thing to deal with," Vilsack told Reuters. "And if you're dealing with it, you're dealing with a lot of consequences of feeling badly about yourself."
(Editing by Ros Krasny and Jackie Frank)