Washington ? A food pyramid just for the under-2 set? Contrary to popular belief, children don’t usually outgrow their baby fat — and a new report urges steps to help prevent babies, toddlers and preschoolers from getting too pudgy too soon.

That’s a growing problem: Already, one in five preschoolers — 2- to 5-year-olds — is overweight or obese.

Topping the list of proposed changes: better guidelines to help parents and caregivers know just how much toddlers should eat as they move from baby food to bigger-kid fare. And making sure preschoolers get at least 15 minutes of physical activity for every hour they spend in child care.

Thursday’s recommendations, from the Institute of Medicine, aren’t about putting the very young on diets. But those early pounds can lead to lasting bad effects on their health as children grow, says the report.

“It’s a huge opportunity to instill good habits at a time when you don’t have to change old ones,” said Leann Birch, director of Pennsylvania State University’s Center for Childhood Obesity Research, who chaired the IOM panel.

Consider: Babies drink milk until they’re full and then turn away. But children as young as 2 or 3 are sensitive to portion size, important in not inadvertently training them to overeat.

“If you give them larger portions, they eat more,” Birch explained.

Pediatricians generally give pretty explicit directions on how to feed babies. And the nation’s dietary guidelines include a special section for preschoolers, including information that a portion size generally is about 1 tablespoon of each food type per year of age.

But overall, those national guidelines are aimed at ages 2 and older — though surveys show even very young children eat too few of the fruits and vegetables they need. So the institute called on the government to create consumer-friendly dietary guidelines for birth to age 2.

That would capture the “dramatic dietary transition that occurs, from consuming one single food to, by the time they’re 2, ordering up things from McDonald’s and, we hope, having also learned to eat a lot of healthy foods,” Birch said.