SEATTLE - As the country's population of overweight children swells, parents are flocking to baby exercise classes where tots as young as one day old can start getting fit.

While mainstream medical experts remain dubious, baby fitness advocates say getting babies and toddlers involved in exercise can set them up for a life of good health and improve motor skills and parent-child bonding.

"I have this little mantra and it goes like this: fit baby equals fit toddler equals fit child equals fit teen equals fit adult," said Helen Garabedian, author of "Itsy Bitsy Yoga: Poses to Help Your Baby Sleep Longer, Digest Better, and Grow Stronger." She also teaches an Itsy Bitsy Yoga class in Marlborough, Mass.

Yoga comes naturally to babies, who often learn the positions independently as they develop, said Garabedian, who works with babies as young as three weeks old. Babies will commonly move into "downward dog" just before they begin crawling, she said.

But do the kids really understand what they're doing?

Three-year-old Jasper Dean seems to. During the cool-down portion of his class in Seattle, Jasper sits in his mother's lap, eyes closed and legs crossed while serenely chanting, "Ohm." He then places his hands together, bows his head and murmurs, "Namaste."

"He has lots of energy, so it's a good place for him to run around and learn about his body," said his mother, Cindy Hazard, 40, of Seattle, who has been practicing yoga for five years.

At Christine Roberts' Nurturing Pathways class in Kirkland, babies as young as two months participate in various movement and stretching activities. The class uses music and props to keep the babies focused and helps improve their eye tracking and coordination, Roberts said.

"It's so good for them," she said. "We try to make it the whole meal deal for the brain and the body."

At a recent session, 7-month-old Jonah Justice muscles his way across a colorful workout mat, his tongue emitting a fine trail of drool as he pulls himself forward with his chubby arms.

"Look at how strong he is!" his 28-year-old mother, Tanya Justice, squeals in glee as Jonah finally reaches her.

Even though such classes are fun, medical experts say they do little to make babies physically fit. Very young children aren't capable of the sustained exercise needed to improve cardiovascular health, strength and flexibility, said Dr. Eric Small, chair of the American Academy of Pediatrics committee on sports medicine and fitness, and author of the book, "Kids and Sports."

"Fitness is an adult concept," Small said.

Nationwide, 30 percent of children and adolescents ages 6-19 are considered overweight or obese, according to government figures. Despite that, the academy doesn't recommend fitness classes for babies, arguing the fragility of an infants' bones can set them up for injury.

But Bonnie Prudden, who wrote the book "How to Keep Your Child Fit from Birth to Six," argues that babies' muscles can be strengthened through exercise. Prudden created a YMCA swim program for infants in the 1950s, and her research on childhood fitness helped create the President's Council on Youth Fitness during the Eisenhower administration.

"Every movement they're making in the water is exercise," said Vera Garibaldi, who teaches babies as young as one day old in a "waterbabies" aquatic class in Bellevue.

At a recent class, Garibaldi blows on 6-month-old Ethan Lux's face, triggering an automatic reflex that causes him to hold his breath. Then she dunks him under the water where he kicks his tiny legs and propels himself upward, breaking the surface wide-eyed and smiling.