WASHINGTON — As K-12 students start the second year of more nutritious U.S. Department of Agriculture lunches, questions remain about how best to encourage consumption of healthy foods in the school cafeteria and what to do about less nutritious “competitive”— because they compete with U.S.D.A. meals — foods sold a la carte and as snacks eaten on school premises.

New standards for more nutritious school breakfasts are being implemented during the 2013-14 school year, the U.S.D.A. said in its September 2013 issue of Amber Waves. New, higher nutritional standards will begin to be required in 2014-15 for competitive foods offered in vending machines, snack bars and school stores as well.

Several important developments have occurred in the rollout of more nutritious school lunches and breakfasts at public elementary, middle and high schools in the United States. Students have increased their consumption of dark green and orange vegetables — though in most cases by very small margins — and the new healthier meals cost slightly more. The U.S.D.A. has upped its reimbursements by an additional 6c per lunch to schools that meet the more stringent nutritional guidelines that began January 2012 for lunches and school breakfasts. During the 2013-14 school year, the U.S.D.A. is paying schools $2.93 per lunch served free and $2.53 per lunch at a reduced price to low-income students and 28c for full-price lunches.

The U.S.D.A. said the main source of the higher food cost was offering a healthier mix of vegetables, which by itself raised costs by about 9.5c per meal. Countering that was the new, lower calorie standard, which reduced food costs by close of 9c per meal.

Looking at consumption patterns, Amber Waves highlighted the challenge of enticing children to actually eat the whole grains, low-fat milk, fruits and more variety in non-starchy vegetables   included in the healthier school offerings.

“To succeed in improving children’s diets, meals must not only be healthful but also appealing so that healthy foods are eaten, not wasted,” the U.S.D.A. said.

As for competitive foods, reducing purchases may have negative ramifications for schools and extracurricular activities that depend on revenue from a la carte and snack offerings. Plans are afoot to enhance the nutritional profile of these optional foods by the beginning of the 2014 school year. For instance, a grain product will need to  have whole grain as its first ingredient or, if not a grain, must have one of the major food groups such as fruits, vegetables, meat or seeds, as the first ingredient. Strict limits on sodium, saturated fats, sugar and calories are also planned and trans fats will be eliminated.

Competitive foods have been popular, especially as students grow older and in schools with lower numbers of students eating free and reduced-price lunches and school breakfasts, the U.S.D.A. said. In 2005, data showed that secondary students consumed twice as many competitive items at school as did elementary students: desserts, sweetened beverages, salty snacks, pizza and candy. Revenues from these competitive foods typically were significant in 2005, with just under $8,000 per elementary school but six times as much for middle schools and a whopping nine times more for high schools, with higher revenues correlating with more affluent districts.

As food in schools moves strongly in the direction of better nutrition, the U.S.D.A. still has a distance to go before it fully understands how much of the different options offered in school lunches and breakfasts and planned for snacks are actually consumed by students.

A fairly recent U.S.D.A. Economic Research Service (E.R.S.) study evaluated student intake of fruit, total vegetables, dark green vegetables, red/orange vegetables and legumes among 1,442 students in 242 schools. While the data were collected in 2005, before the adoption of new standards, some schools already were offering fruits and vegetables in amounts and varieties that would become required in 2012.

The outcome of the study showed an increase in consumption of a healthier mix of vegetables, although from a very low base. Schools that offered more orange vegetables showed an increase to 0.03 cups consumed at lunch on average, significantly higher than the 0.01 cup eaten by students at schools where the offerings had not been upgraded, but still a very low level of compliance, the U.S.D.A. said.

“The major reason the difference in average consumption is small is that many students did not eat the healthier options at all,” the U.S.D.A. said. Still, “more students in schools that offered vegetables in amounts consistent with new standards ate at least one kind of vegetable, 81%, compared with 70% in other schools,” the U.S.D.A. concluded.

In schools that had only healthy a la carte options, or no a la carte options at all, students ate more of the dark green vegetables on their plates, the U.S.D.A. said.

Most students at schools offering all types of lunches consumed at least some red/orange vegetables such as tomatoes and tomato products, a popular choice. Dark green and orange vegetables or legumes were much less popular, although students in schools with higher nutritional profiles did eat some of those foods, boosting intake substantially.

The E.R.S. reported that schools in 2005 that met revised daily nutrition standards had students who averaged 0.4 of a cup of total vegetables each day, more than the 0.3 cups per day of vegetables consumed at schools that did not meet daily nutrition standards.

The U.S.D.A. reported on efforts in the field of behavioral economics to “nudge” students to make healthy food choices from the higher-quality lunches made available to them.

“In a rushed, distraction-filled environment such as a school cafeteria line, choices that are more convenient and attention getting are more likely to be selected,” Amber Waves suggested.