For the first time in 15 years, the USDA has issued new guidelines governing the content of meals served through the National School Lunch Program and other federally funded school meals programs, which together serve some 32 million children a day.

U.S. First Lady Michelle Obama gets her lunch at Parklawn Elementary School in Alexandria, Va., Jan. 25, 2012. (Kevin Lamarque/Reuters)

The guidelines, crafted to comply with the 2010 Dietary Guidelines for Americans, place new restrictions on the number of calories (according to age group), milligrams of sodium and percentages of calories from saturated fat children’s meals may contain. They require half (and eventually all) of the grains served to be whole-grain-rich. And they suggest a number of other measures designed to make school meals more healthful than they currently are. Some of the requirements, such as those for whole grains and sodium reduction, are to be phased in over time.

The reforms, presented by First Lady Michelle Obama and Agriculture Secretary Tom Vilsack Wednesday, are a major plank of the Healthy, Hunger-Free Kids Act of 2010, which aims to improve children’s nutrition and to curb the widespread problem of childhood obesity. About a third of U.S. children are overweight or obese, according to the Centers for Disease Control and Prevention.

Implementing the new guidelines will cost $3.2 billion over five years, according to the USDA; that cost will be offset in part by a 6-cents-per-meal increase in federal reimbursements and other measures.

Despite controversies over listing tomato paste and potatoes among the acceptable vegetables to be served through school meals programs, both foods remain included among the vegetable options, though their use must comply with other guidelines such as the sodium restrictions.

Here’s the USDA's comparison of sample school-lunches following the old guidelines and a week’s worth of meals that comply with the new rules.