(CNN) As politicians debate spending and cuts in President Donald Trump's proposed budget, there have been questions about the effects of nutrition programs for kids.

From before birth and through the school years, there are decades-old food programs designed to make sure children won't go hungry. Experts agree that the nutrition provided to millions of children through school meal programs is invaluable for their health.

What are the school-based meal programs?

The National School Lunch Program, a federally assisted meal program, has been around since In 1946, when it began operating in public schools as well as some private schools and child-care institutions. The program provides nutritionally balanced low-cost or free lunches to children each school day.

In 1966, the School Breakfast Program, which was designed for "nutritionally needy" children, started as a pilot project, and it received permanent authorization in 1975. Decades later, Congress authorized the reimbursement of snacks served to children enrolled in after-school programs.

Under these programs, any child at a participating school may purchase a meal. Local schools set the prices for meals and offer a sliding scale to students depending on family income.

Children from families with incomes at or below 130% of the poverty level are eligible for free meals. ( Poverty is calculated annually by the United States Census Bureau based on family size .) Families with incomes between 130% and 185% of the poverty level are eligible for reduced-price meals, which means a student can be charged no more than 40 cents. Children from families with incomes over 185% of poverty pay full price.

School breakfast and lunch programs, which are administered by the US Department of Agriculture, remain untouched in Trump's proposed budget. However, the 21st Century Community Learning Centers program , a US Department of Education program that supports before- and after-school and summer programs, may be on the chopping block. Though independent of the USDA, some of these programs may provide snacks to children.

What nutrition programs exist for younger children?

The Special Supplemental Nutrition Program for Women, Infants and Children , often referred to as WIC, gives federal money to states, which then distribute these funds to programs helping low-income pregnant and postpartum women so they can buy additional food while receiving nutrition education and health care referrals. The point is to benefit infants and children up to age 5 who are at nutritional risk.

Since its launch in 1972, the program has had a "really quite extraordinary" impact, and its return on investment for every dollar invested is "vast," according to Dr. David Paige, a professor at Johns Hopkins Bloomberg School of Public Health.

According to Paige, the program is responsible for a reduction in low birth weights and preterm births, as well as a range of other positive short-term effects. It will lead to positive intermediate and long-term health effects for the children as well, he said.

"Over the last five to 10 years, the issue of long-term considerations have emerged quite convincingly in the scientific literature," Paige said. "By that, I mean intermediate in terms of cognitive development and intellectual performance but even more strikingly in terms of chronic disease in terms of later in life."

This $6 billion USDA program program has been trimmed in recent years and may face additional cuts soon. Former President Barack Obama signed off on a substantial cut -- a $273 million decrease -- between fiscal years 2015 and 2016, and Trump's budget would withdraw about $150 million from the budget over fiscal 2016.

Such cuts would probably result in negative health and academic consequences, suggests research published Tuesday in the journal Child Development

Children who experienced food insecurity during infancy and toddlerhood had lower cognitive and social-emotional skills in kindergarten compared with peers who had enough nutritious food during their earliest stages of development, said Anna Johnson, lead author of the new study and an assistant professor of psychology at Georgetown University. " Food insecurity " is a term used by the USDA to describe households in which residents lack adequate access to food.

When food insecurity occurred in the preschool years, it also had a negative impact on school performance, said Johnson and her co-author, Anna Markowitz, a postdoctoral research associate at the University of Virginia. Johnson and Markowitz investigated 3,700 low-income households for their study.

However, the effects of food insecurity during preschool appeared to less consistently impact kindergarten performance than did hunger at an earlier age.

Yet how often food insecurity happened mattered. Three episodes of food insecurity versus one or two was linked to poorer outcomes in kindergarten across all areas of development, Johnson and Markowitz said.

How can nutrition affect children's performance in school?

"There's a pretty large proportion of kids who just don't have any access to food at home," said Sibylle Kranz, an associate professor of education at the University of Virginia and a certified child nutrition epidemiologist.

And kids' nutritional needs are different from those of adults, Kranz said -- "very different."

Children have much smaller stomachs than adults and restricted access to food during the day. Not being able to eat the same quantity as an adult and not being able to "just whip out something to eat in the middle of their lessons as needed" means that children require "nutrient-dense meals and snacks," she said.

"Children are growing, so they are in an anabolic state: Muscle is being built; bone is being built; the brain is developing," Kranz said.

Generally, adults are in a steady state of maintaining tissue; exceptions might be a person trying to build muscle mass or heal from an injury.

"But in kids, if you take a 5-year-old, by the time that child is 12, that child has doubled its size," Kranz said. This translates to a need for lots of good food every day.

"There is pretty solid evidence that children who are hungry are not able to focus, so they have a low attention span, behavioral issues, discipline issues in the school," she said. "So having children who are well-fed and not hungry makes a difference in their individual performance and also how much they are contributing or disrupting the classroom situation."

Dr. Tanya Altmann, a pediatrician and an American Academy of Pediatrics spokeswoman, agrees.

"I think there is good research that shows that nutrition is critical for a child's brain and for concentration and learning at school. So whether breakfast is provided at home or at school, as a pediatrician, I do see a difference in kids that get good nutrition in the morning, such as protein, fresh fruit and enough calories, and how they function during the day at school," she said.

"While adults may be able to focus and concentrate better with poorer nutrition, with kids, they cannot necessarily control that, and they might be more distracted and less able to sit and learn if their basic needs such as sleep and nutrition aren't getting met."

What food is served in school meal programs?

Breakfast and lunch meals must meet federal requirements based on the latest Dietary Guidelines for Americans. Yet each school or local district decides which foods to serve and how to prepare them.

"I think that school nutrition has improved dramatically over the past five years. The quality is so much better than what it was five and 10 years ago," Altmann said. "For instance, there's more fresh fruits, more whole grains are used, less extra, unneeded sugar and calories."

Altmann, whose practice is based in Southern California, says this is happening across the country, with school districts doing a "fairly good job" working within budgets and choosing meals that are appropriate for kids at different grades and age levels.

"I do visit a variety of schools in my area, and I find that the meals look more appetizing to me," she said. Ten years ago, school food "used to look more like airplane food," she said, but today "healthier choices are available -- healthier choices that taste good and kids like to eat."

Some lobbyists have suggested that the nutritional guidelines need revision . The School Nutrition Association, which says it's a national nonprofit organization representing over 57,000 members in the school food service industry, called for "practical flexibility under federal nutrition standards to prepare healthy, appealing meals." It recommended that the USDA allow saltier foods than would have otherwise been allowed and cutting current whole-grain requirements in half.

Is there a downside to school meal programs?

"There is and always has been a debate on whether or not school breakfast and school lunch are in fact contributing to the childhood obesity problem," Kranz said. In other words, some kids may be eating both at home and at school, while other kids are eating only high-calorie selections they receive, avoiding the vegetables and fruits on their plates.

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"Experts are having different opinions on this," she said.

Still, she said she does not side with those who argue against school meals on the basis of childhood obesity. There's not one program that would meet the needs of every child in this country, Kranz said, noting that "our population is not a homogenous map."

"You have kids who are very much dependent on getting nutrition in school, and you have others who would be fine without it," she said. "So it's not an easy answer."

Paige, the Johns Hopkins professor, believes that obesity is too complex a health issue to be blamed on just one factor.

Obesity arises in part from maternal eating patterns during embryonic and fetal development, he noted, and later, other factors help decide whether a child becomes obese, including what and how much food a child eats and the amount of exercise a child gets.

"I wouldn't make the case that providing school lunch is a determinant of obesity," Paige said. Overall, he said, good nutrition from an early age is key.