Dr. Steven Czinn is the chair of the department of pediatrics at the University of Maryland School of Medicine.

(Hans Pennink/Associated Press)

Five years ago, Congress passed legislation that transformed how the nation’s public schools feed students. The Healthy Hunger-Free Kids Act required these schools to serve more fruits, vegetables and whole grains, and less sugar, fat and salt.

The new standards are among the most important efforts to improve children’s health in the past two decades. Since 1980, obesity has more than doubled in children and quadrupled in adolescents. Nearly a third of children and adolescents are either obese or overweight. In some states, the combined rate is close to 40 percent; among African American and Latino children, these rates are particularly high. And millions more children consume more sugar and empty calories than is healthy.

As a pediatric gastroenterologist, I see firsthand every day the consequences of childhood obesity, so I was happy to see the law passed, and even more pleased that it’s working: A study published last year by researchers at Harvard, Columbia and George Washington University estimated that the updated meal standards could prevent more than 1.8 million cases of childhood obesity.

But Congress has not been as enthusiastic. Last month, the Senate Agriculture Committee approved a bill, sponsored by Sens. Pat Roberts (R-Kan.) and Debbie Stabenow (D-Mich.), that would allow schools to include fewer whole grains and extend by two years when schools must reduce levels of salt.

The content of school meals has long been controversial. The Reagan administration responded to funding cuts in the Federal School Lunch Program by suggesting that pickle relish could be counted as a vegetable. In 2011, Congress blocked the U.S. Department of Agriculture from updating its guidelines, a move that, among other things, allowed pizza with two tablespoons of tomato paste to qualify. This time, opponents argue that trying to improve our children’s diet and health is “nanny state” meddling. Once the law was passed, these groups did their best to weaken and disable it.

Conservative members of Congress, as well as the School Nutrition Association, a trade group for school meal providers that receives half of its funding from food companies, say the new rules have reduced meal participation and caused massive amounts of uneaten food, known in the industry as “plate waste.” Rep. John Kline (R-Minn.) is chairman of the House Education and Workforce Committee, which oversees the law, offered a typical objection: “These regulations have created an environment where students are not getting the nourishment they need, and food and taxpayer dollars wind up in the trashcan.”

As evidence, those who oppose the law point to the many media stories over the past three years suggesting that the new approach hasn’t been working. The stories included examples of students throwing out the new meals because they supposedly weren’t appealing enough, and cafeteria directors complaining about uneaten food. This seemed to support Kline’s view that pie-in-the-sky do-gooders were trying too hard to legislate human behavior and limit choice: You can lead kids to carrots, this argument goes, but you can’t make them eat.

These claims are based on anecdote, old data and a spurious connection between the new standards and cafeteria usage, which has dropped slightly in recent years. To the extent that it occurred, students’ initial reaction to the new meals isn’t surprising. After all, who among us wouldn’t have trouble adjusting to a diet of salads and whole grains after years of eating pizza and tater tots? And when tens of thousands of large-scale dining establishments significantly change their menus in a short time, there are bound to be hiccups, especially when the 30 million diners are between the ages of 5 and 18.

Since the new standards took effect in 2012, school districts have worked hard to adjust, devising more effective ways to serve food that is healthful as well as appealing. Many have developed salad bars with more diverse and interesting choices, introduced a range of spices besides salt, and increased the use of frozen vegetables rather than canned, to improve taste and lower sodium content.

There is strong evidence that this effort is succeeding. A 2014 survey of school leaders found that most were receiving high marks from students about the healthier meals. In a study published last year, researchers at Yale, Berkeley and the University of Connecticut found that at schools serving the new fare, students ate more fruit and vegetables and threw away less.

The subjects, a group of nearly 700 students in a low-income, urban school district, “responded positively to the new lunches,” the researchers concluded. “Overall, the revised meal standards and policies appear to have significantly lowered plate waste in school cafeterias.”

In January, a much larger national study provided more support. For this project, the largest and most complete look at the new standards yet, scientists at the University of Washington School of Public Health looked at more than 1.7 million meals eaten by 7,200 students in a diverse urban school district. The researchers compared students’ eating habits before and after the new rules took effect. After the change, kids chose food that was lower in calories and more nutritious. Just as important, there was almost no difference in how many students participated in the school lunch program: Before the standards kicked in, 47 percent of students ate school lunches; after, the rate was 46 percent. That comes down to a loss of about seven students, hardly a stampede toward the cafeteria exits.

The bill approved by the Senate Agriculture Committee could have been worse. The proposal leaves standards for fruit and vegetables intact, but it was far from ideal. House Republicans are expected to introduce their own bill, which probably will include further harmful and counterproductive nutritional rollbacks. In other words, the fight is far from over.

The science clearly shows that the standards are working. Do some students still push back against healthier school food? Of course. Any parent knows that it can be hard to get kids to eat vegetables (even some adults have been known to shirk their dietary responsibilities). But overblown claims of wasted food don’t constitute proper evidence. The science shows that the standards are having a real effect, and are improving the quality of school meals — meals that are being eaten, not wasted.