School is still in session, albeit online, but many American children are no longer sitting in a traditional classroom with scheduled gym periods or the chance to run around at recess. They are distance learning at home, where it’s tempting to snack all day in front of a screen.

Prompted by fears of coronavirus transmission, numerous schools shut down in March, stretching the usual two-to-three-month interval at home into five months or longer. Childhood obesity experts are worrying that children — who often gain weight during the summer when they’re home — will add even more pounds, escalating an already serious public health problem.

“Weight gained each summer accumulates year after year since children don’t usually lose it when they return to school,” says Andrew Rundle, who heads the childhood obesity research project within the Columbia (University) Center for Children’s Environmental Health.

“We’ve basically doubled their time out of school, and all the risk factors for weight gain we see in the summer will be magnified by the lockdown.”

Research has found that children, especially racial and ethnic minorities, are at greater risk of weight gain when they are out of school. Not only are they out of school now, social distancing is also keeping many indoors.

“This pandemic will have multiple impacts on childhood health and development, one of them that it places kids at higher risk of experiencing obesity,” Rundle says. “This is likely to have lasting impacts throughout their lives.”

A 2016 study, for example, found that obesity rose from 8.9 percent to 11.5 percent during the two summers between kindergarten and the end of second grade, while overweight increased from 23.3 percent to 28.7 percent, with no rise during the actual school years.

Joseph Workman, assistant professor of sociology at the University of Missouri at Kansas City and one of the study authors, projects that nearly six months of school closure could result in a 4.86 percentage point increase in childhood obesity.

“This would be a case of one public health crisis exacerbating another public health crisis,” Workman says.

Obesity afflicts 13.7 million American children and adolescents ages 2 through 19, or 18.5 percent, according to the Centers for Disease Control and Prevention. It is 13.9 percent among 2- to 5-year-olds, 18.4 percent among 6- to 11-year-olds and 20.6 percent among those ages 12 to 19, the CDC says.

Moreover, minority children suffer from higher rates of obesity than white children, with Hispanics and African Americans at 25.8 percent and 22 percent respectively, compared with whites, at 14.1 percent, according to the CDC. Research suggests these disparities result from factors such as genetics, physiology, culture, socioeconomic status or a combination.

“Childhood obesity is what I like to call the insidious pandemic,” says Joan C. Han, associate professor of pediatric endocrinology at the University of Tennessee, who also directs the pediatric obesity program at Le Bonheur Children’s Hospital in Memphis. “Unlike coronavirus, which rapidly became a worldwide threat, childhood obesity has spread more quietly, gradually tripling in prevalence over the course of decades, but which one might argue is even more deadly, and should be taken very seriously.”

The rate of childhood obesity has more than doubled in recent decades among preschool children, as well as those ages 12 to 19, and more than tripled among those ages 6 to 11, according to research.

“Once upon a time, city kids might have spent the summer playing stickball, or rural kids might have spent a summer keeping the chicken coop clean,” says Paul von Hippel, associate professor of public policy, sociology, statistics and data science at the University of Texas in Austin, and another co-author of the 2016 study on summer weight gain among children.

Today, however, “the biggest change in behavior that we see when summer vacation starts is that kids watch a lot more television, and that’s especially true for kids from lower income families. Television is a major predictor of obesity,” von Hippel says, adding it’s still unclear whether TV’s influence encourages children to exercise less, snack more, exposes them to junk food advertising — or something else.

Shelter-in-place orders, with parents working from home, likely contribute to obesity by decreasing physical activity, “especially for children living in urban areas with limited access to outdoor space safe for social distancing,” says Eliza Whiteman Kinsey, a postdoctoral research scientist at Columbia University’s Mailman School of Public Health. “Parents working from home . . . is also likely to increase screen time [for children.]”

Moreover, shoppers are buying more packaged foods, which often are unhealthy, but last longer than fresh ones, experts say.

“I was in a supermarket recently where the fresh fruit and vegetables were well stocked, but the shelves were empty when I got to the cookies and snacks,” Rundle says. “People were loading up on ice cream and cookies and microwave dinners, and bringing these high calorie foods into their homes. All the food companies that make processed foods are reporting major sales bumps. Sitting in math class may not burn many calories, but the kids aren’t snacking.”

Also, many economically disadvantaged children rely on public schools for meals during the week. With schools closed, and families suffering financial hardship from job layoffs, “the loss of schools as a safety net for providing nutrition is very concerning,” Han says. “The cheapest foods are among the unhealthiest. This contributes to the disproportionately higher risk of obesity among children in poverty.”

Studies have found that childhood obesity portends dangerous health consequences in adulthood.

“Unhealthy weight gain, even as early as age 5, creates a trajectory of weight gain throughout one’s whole life,” Rundle says, having studied obese children through adulthood to age 50. As adults, they have high body mass indexes, body fat, as well as hypertension, diabetes, heart disease, cancer and stroke, he says. “That’s what we’ve seen in the long term. It’s stunning.”

Experts urge parents to provide kids with food that includes whole grains, vegetables, fruits, lean proteins and dairy products. Substituting low sugar/low salt frozen and canned items is a good alternative if fresh produce is difficult to find, or if parents worry about food waste or spoilage. Avoid buying sugary drinks and packaged snacks, such as chips or candy, tough to resist once they are in the house. Reduce screen time and encourage more physical activity — even indoors — if possible.

“This situation is worse in part because families are under more stress,” von Hippel says. “We went into it without much warning, without a plan. And we didn’t know how long it would last. It might be kind of fun to eat toaster pastries . . . for a week or two, but as the weeks stretch into months, you need a more sustainable plan.”