Nearly half of all U.S. preschool-age children don't get outdoors at least once a day for parent-supervised playtime, researchers reported Monday, causing concern among experts who say early exercise habits could protect children from obesity later in life.

Many children might not be getting enough outdoor exercise because of barriers faced by single parents and families with two working parents, said Dr. Pooja Tandon, a pediatrician with the Center for Child Health, Behavior and Development at Seattle Children's Research Institute, who led the research.

It might also come down to a "cultural shift as to how families spend their time," Tandon said, citing the way parents tightly schedule their kids' play dates and classes while seemingly forgetting the value of free play and outdoor time. "There may be missed opportunities for kicking those kids outside the door when it's appropriate and safe."

Tandon also said that the lack of daily outdoor exercise among preschool children (defined here as those in the year before kindergarten) also might stem from parents assuming "that young children are spending their day running around, that they're active," she said, suggesting that some day-care centers and babysitters are not getting children outside often enough, or for long enough, to meet the 60 minutes of daily exercise recommended by the National Association for Sport and Physical Activity.

The reasons child-care providers are not be meeting these recommendations might be diverse, including  "some real, some perceived," Tandon said. Yesterday's rain should not prevent an outdoor outing today, she said. And, she added, "Some child-care providers say children didn't bring a jacket or they wore flip-flops. Depending on staffing, maybe the class doesn't go outside. "

The good news, Tandon said, is that "these young children are naturally programmed to be active if given the opportunities."

Tandon's study, which appeared online Monday, on the website of the Archives of Pediatric & Adolescent Medicine, was based on parental surveys from a national study of nearly 9,000 U.S. children, a sample size representing about 4 million children. The children, all born in 2001, were followed for several years and their parents, usually mothers, were interviewed several times, including the year before their children entered kindergarten.

Along with finding that 49 percent of children were not getting outdoors with a parent at least once every day, she and her colleagues from the research institute and the University of Washington found that those youngsters whose parents took them outdoors to play tended to be boys, children with lots of playmates and those whose parents were exercisers.

Children more often fell short of recommended exercise if their mothers were Asian, African-American or Hispanic, although the study didn't delve into the reasons. "Being physically active is good for your brain, for your learning," Tandon said in an interview.

Previous research, she said, has found that hyperactive kids with wandering attention do better after they have had nature breaks, which seem to make it easier to return to class, sit down and refocus.

John Cave Osborne, a Knoxville, Tenn.,-based freelance writer, real estate investor and father of five, said he and his wife are avid exercisers who enjoy the outdoors. As a result, he said, the children "know we're always on the go; they just get thrown into the mix with us."

For a couple of hours each day, the kids ride bikes or scooters, play on swings or swim under the watchful eye of his wife, Caroline, a stay-at-home mother. On weekends, they put their 4-year-old triplets in a stroller, their 10-year-old daughter gets on her bicycle and all take off on 2- to 3-mile walk through their neighborhood. On family vacations, everyone is riding a bike, or riding along. "It's not like we're Olympians; we're just active."

They'll take the triplets with them this summer for the first time on their big summer camping trip, he said.

Osborne, who blogs for Babble.com, a Disney-owned online parenting magazine (Disney is also the parent company of ABC News), said he and his wife have no choice but to keep all their kids moving. "If those guys don't expend some energy, we're dead," he said.

Parents might not realize the importance of getting preschoolers to exercise when they tend to be thin, but doing so instills good habits that can protect them against childhood obesity later on, Tandon said.

Dr. Reginald Washington, a Denver cardiologist and chief medical officer for Rocky Mountain Hospital for Children, said he suspects most pediatricians aren't bringing up exercise during preschoolers' appointments. "They're talking about immunizations, about safety, about development," he said. "They almost never ask how often and how long is your child physically active?"

Exercise, Washington said,  "is just as important as learning how to read."

The preschool years represent a time in youngsters'  development when they learn skills like throwing a ball, picking things up, running, and jumping.   "If you deprive them of that," Washington said. "They're less likely to be active when they get older."

Although the National Association for Sports and Physical Education recommends at least 60 minutes a day for children in this age group, Washington said parents should put the focus on just getting them outdoors "even if it's 30 minutes outside, playing, exploring your environment, learning these physical skills, blowing off steam."

Tips for Getting the Kids Outside

Tandon offers some tips from her own experiences:

• If your child is in day care, preschool or cared for by others, ask about outdoor playtime;
• Talk to your friends to increase their awareness of the importance of outdoor play time;
• Because boys tend to play outdoors more than girls, encourage and support girls in outdoor play;
• Don't let darkness or rain deter your family from heading outside for exercise. Consider taking a "flashlight walk" or rainy day hike together;
• Involve your children in walking the dog.

The married mother of sons ages 2 and 6 said she and her husband strive to get their boys outdoors for gardening in the yard,  picnic dinners on warm nights and play dates with other children. Weather doesn't deter them, Tandon said.

"If our plan was to go to the park, we'll still try to go. We're the only crazy people out there sometimes," she added. "If we can rope in a few friends, then it becomes more fun."