(CNN) -- A 4-year-old lumbered into a Boston pediatric clinic. He walked with a limp.
"He was carrying so much weight, he displaced his hips," recalled Dr. Elsie Taveras, co-director of the Obesity Prevention Program at Harvard Medical School.
The boy, an extreme example of childhood obesity, carried more than 100 pounds and had a body mass index that was over the 99th percentile for his age group.
He is part of a disturbing trend among the youngest Americans.
Poor diet, huge portion sizes, lack of physical activity, inadequate sleep and uninformed parents are contributing to larger numbers of overweight or obese young children.
Last week, the Institute of Medicine released its first report focusing on obesity prevention policies for children under the age of 5.
Almost 10 percent of infants and toddlers carry excess weight for their length. Their growth is measured using charts that compare them with peers.
One in five kids (ages 2 to 5) is overweight or obese before entering kindergarten.
"This is not school lunch causing this," said Dr. Sandeep Gupta, director of Pediatric Overweight Education and Research Program at Indiana University Health. "They're not in school yet."
Families and adults who take care of children are well-meaning but unintentionally making poor food choices, Gupta said.
The good news is that this is an ideal age to make lifestyle changes. Parents and adults have more control over what infants and toddlers eat, compared with teenagers.
"It's phenomenal," Gupta said. "Even though the children are heavier, they improve better compared to older kids. The parent is the police at the point, setting the agenda."
One problem he observed is that young kids are constantly drinking empty calories such as energy drinks, fruit punch and sugary juices.
"They have been on the bottle too long, and they're transitioning to sippy cups," said Gupta, who practices at Riley Hospital for Children. "They're sipping constantly, all day long, on empty calories."
"There's a disconnect in people's minds. The mothers don't know. They're used to thinking juice is good; juice is fruit. What they don't tell them is all the sugar in the package."
Obese toddlers have shown metabolic abnormalities in their insulin, liver enzymes and cholesterol -- usually problems detected in older adults.
Taveras, who served on the Institute of Medicine's committee on childhood obesity, said she has seen arthritis in 8-year-olds and early signs of diabetes.
The young patients have high glucose insulin resistance and skin discoloration called acanthosis nigricans that forms around their necks and arms. This skin problem is linked with diabetes and insulin resistance.
"Even though we're not seeing frank type-2 diabetes in 5- or 6-year-olds yet, we're seeing early indication that the child is heading to that direction," Taveras said.
Although not all cases of obesity in toddlers are so dire, early health problems can follow throughout childhood and into adulthood.
Not all problems are solely lifestyle-related.
Tasha Secuskie noticed that her daughter, Tylynn, who had been breast-fed, gained six pounds between her sixth and eighth weeks.
Her doctors were puzzled. The child started gaining weight dramatically at 18 months.
Secuskie reduced Tylynn's portion sizes and cut out junk food, but that didn't slow her rapid weight gain.
"We have the problem when she's asking you for food and she thinks she should get an adult-sized plate," Secuskie said.
Tylynn's heaviest weight was 81 pounds.
The family cut out fried food and transformed its diet to mainly fruits, vegetables and lean meats. They take Tylynn and her younger sister to dancing and swimming lessons and hikes. While her younger sister is skinny, Tylynn remains overweight.
"It's hard for a kid because of that," Secuskie said. "You're supposed to increase your exercise, and that makes her hungry."
They learned that Tylynn, 4, has hypometabolism, which makes her body burn fuel slowly and therefore gain weight quickly.
"It breaks my heart, because people stare at you, like 'What are you feeding that kid?' " her mom said.
Beyond the physical toll, obesity can have a long-lasting emotional effect on children, Taveras said.
"The other important adverse affect we're seeing is mental health and trouble with self-esteem, depression," she said. "The mental health implications of being overweight that young -- we're realizing what a big impact it's having on children's quality of life."
"The good news was that (young kids) tended to respond to treatment and lifestyle modifications, because most of the kids are obese because of the lifestyle."
Here are recommendations from pediatricians and the Institute of Medicine about raising a healthy baby:
A baby is more likely to be obese if its parents are obese.
During pregnancy, the mother's health, such as her pregnancy weight gain and gestational diabetes, are risk factors of the baby's obesity.
"One thing women hear when they're pregnant is that they're eating for two," Taveras said. "That's wrong."
Eating for two doesn't mean eating twice as much. Here are general guidelines for pregnancy weight gain.
Studies show that formula-fed babies who were given solid foods before age 4 months have higher risk of becoming obese.
Formula-fed infants could be consuming more food and therefore more calories than breast-fed children.
The Institute of Medicine recommends breastfeeding during infancy. This echoes the American Academy of Pediatrics' advice that moms should exclusively breast-feed for the first six months. The breast-feeding should continue for a year.
Feeding a baby solid foods too early in life may increase his risk of becoming obese before reaching preschool, according to a Pediatrics study.
Various websites offer helpful breastfeeding tips, such as the American Academy of Family Physicians, pregnancy.org and La Leche League International.
Desperate breast-feeding moms reveal secrets
Children's growth should be monitored by health care professionals because parents tend to underestimate their child's weight.
"There is a popular belief, culturally embedded in our minds, that a chubby baby is a healthy baby, and children will grow out of it," Taveras said. "Those two perceptions are inaccurate. Parents don't recognize they gained too much during pregnancy and that their infants are overweight."
In a 2006 study that surveyed parents of obese children, only half recognized that their child was overweight.
Toddlers and preschool children should be encouraged to be physically active throughout the day. They should have outdoor playtime, access to playground or open grass and an adequate indoor play environment at child-care centers.
"Food and physical activity and TV shouldn't be used for encouragement or punishment," Taveras said. "It's obvious there are other ways to promote behaviors or discourage behaviors."
Adults should set the example of healthy nutrition and encourage children to eat fruits, vegetables, lean protein, whole grains and low- or non-fat dairy.
Kids should be allowed to sleep, because the heaviest children slept less than their peers who were of normal weight.
TV affects sleep of preschoolers
Experts also advise limiting kids' screen time using TVs, computers and cell phones and limiting their exposure to junk food ads.