With tens of thousands of schools in dozens of states now shuttered through the remainder of the school year because of the coronavirus pandemic, an estimated 55 million students will be home from school for double the length of their normal summer vacations, if not longer.

Now some experts are warning that one of the likely health consequences for many housebound children will be an increase in the unhealthy levels of weight gain typically seen during summer breaks.

“The stay-at-home orders and the social distancing are the right thing to do” to slow spread of the virus, said Andrew G. Rundle, the lead author of a new report in the journal Obesity on school closings and childhood weight gain. “But this six month period or longer is doubling out-of-school time, and it’s magnifying or exacerbating all of the risk factors that we think about for summer weight gain.”

While focusing on the immediate effects of the pandemic is a priority, Dr. Rundle and his co-authors point out that when it subsides one of its lingering effects could be a worsening of the obesity crisis among children. Childhood obesity rates have been on the rise in America for the past four decades, with more than a third of all youth under the age of 19 classified as overweight or obese. Studies show that overweight children are much more likely to become overweight adults, and that puts them at a higher risk of developing heart disease, cancer and Type 2 diabetes.

But in recent years researchers have begun to recognize that summer recess plays a major role in unhealthy weight gain. At least a half dozen studies have found that children gain weight at a faster rate during the summer months than during the school year. The effect is especially pronounced for children in minority groups or those who are already overweight.

One large study that tracked thousands of schoolchildren for five years starting in kindergarten found that school had a protective effect on their body compositions: On average, children saw their body mass index fall by 1.5 percentile points during the school year and then jump about 5 percentile points during their summer vacations.

“What is very apparent from the data is that kids experience unhealthy weight gain during the summer, that it’s more so for African-American and Hispanic kids, and that the weight gain that occurs during the summer does not get worked off during the school year,” said Dr. Rundle, an associate professor of epidemiology at the Columbia University Mailman School of Public Health. “It’s a stepwise pattern where the summer is the step up and the school year is the flat part of the step.”

Ultimately, some experts believe that the structured nature of the school day, with its scheduled exercise periods and limited chances to snack, is what helps protect children from excess weight gain. When children are at home for the summer, their days may be less regimented and less supervised, allowing them to indulge in more snacks and sedentary behaviors — similar to the factors that lead adults to eat more on weekends and pack on extra pounds during the holidays.

Data are limited, but there are early signs that the coronavirus shutdowns have prompted a rise in unhealthy behaviors that drive weight gain. Americans have stocked up on shelf-stable foods of all kinds, but sales of ultra-processed comfort foods like Oreo cookies, potato chips and macaroni and cheese have soared. Television and online video game usage have surged. Many playgrounds, especially in large urban areas, are now closed.

Though no two schools are the same, there are many reasons school environments can prevent excessive weight gain. Most schools offer students some level of exercise through physical education, team sports, and daily lunch and recess periods. Many children get about half their daily calories at school, which are required to provide meals that meet nutrition standards for components like sodium, whole grains, lean protein and fruits and vegetables. While junk foods can still be found in school cafeterias, many schools have worked to eliminate sugary beverages, candy and chips from their lunch lines and vending machines.

During the summer vacation, it’s the reverse. Studies find that children spend more time sitting in front of screens watching television and playing video games. They tend to consume more snacks and sugary beverages and eat fewer fruits and vegetables. That may especially be the case for children from low-income households that depend on schools to provide healthy meals. About 30 million children across the country receive free or subsidized school meals.

Dr. Rundle and his colleagues suggested in their new paper that schools, parents and policymakers could mitigate the long-term impact of the pandemic on children’s health by promoting exercise and healthy eating — where possible — during the lockdowns. Some schools, for example, have developed home lesson plans for exercise to go along with their lesson plans for math and English. Schools that are able to stream classes online might consider having their P.E. teachers stream exercise classes too, Dr. Rundle said.

Food is the trickier part of the equation. For some families right now, venturing outside to find healthy options at depleted grocery stores may not be possible. And with many parents stressed about their jobs, finances and other challenges, the prospect of fighting with their kids about food can be a hard sell — especially if they are worried about being able to put any food on their tables at all.

But with children facing limited options for physical activity, now more than ever is the time to try to limit unnecessary calories from sugar-sweetened beverages and excessive snacking, said Dr. Rundle. One good resource for stressed families is the American Heart Association, which provides quick, heart-healthy recipes on its website — like tuna stir fry, chicken salad, hummus and vegetarian three-bean chili — that can be made with canned foods and other inexpensive pantry items.

For families that rely on school meals to feed their children, many school districts across the country have been providing grab-and-go meals. Some are offering five meals at a time and allowing parents to pick up food without their children being present. In some districts, schools are packing up meals and having bus drivers deliver them to families along their normal routes.

Parents can find out where to get free meals through their school districts. They can also find meal service sites in their neighborhood using the federal government’s Summer Food Service Program website. For many children, these services could mean the difference between having a nutritious lunch and breakfast or not having any food at all, said Eliza Kinsey, a co-author of the new paper and a research scientist in the department of epidemiology at the Mailman School of Public Health.

That could set the stage for many consequences: Studies show that children who grow up in food-stressed households face a higher risk of obesity, behavioral issues and other chronic health problems.

“If kids are missing meals and not able to get replacements, then we’re looking at a very large increase in food insecurity,” she said. “There are a lot of serious long-term health implications for low-income families if those meals are missed and not being replaced.”