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But perhaps it's a baby's weight at age 1 that matters more, experts say. Weight that is too high for the child's height — for example, being at the 75th percentile for weight but the 30th percentile for height — can spell trouble. Another study from Taveras' research group, published last year in Pediatrics, found that rapid increases in weight-for-length measurements during the first six months of life were associated with a greatly increased risk of obesity at age 3.

No one is sure why rapid weight gain in the first year is important. It could be that when a baby is fed more than it needs, the brain's development is affected so that it signals the need for excessive amounts of food, Stettler says. Likewise, too much food might program an infant's pancreas, and the body's response to insulin, in a manner that leads to obesity.

Whatever the cause, Taveras says, "excessive weight gain in those first six months of life is not baby fat that is going to go away. We're going to have to change perceptions about what's healthy and what's not healthy."

Whether a baby is breast-fed (and for how long) or bottle-fed, when it begins eating solid food and how much it sleeps have also been linked to obesity risk. A 2008 study in the Archives of Pediatrics & Adolescent Medicine found that babies in child-care centers or cared for by relatives tended to have lower rates of being breast-fed and had solid foods introduced earlier, both factors tied to weight gain.

But studies on breast-feeding are an example of a weakness in the argument that the early years influence future weight, Stettler says.

For example, one study randomly assigned the mothers of infants to a program that encouraged breast-feeding and compared them with women who did not receive the breast-feeding promotion program. More babies were indeed breast-fed in the first group. But when the children in both groups reached age 6, there were no differences in their weight. The paper was published in 2007 in the American Journal of Clinical Nutrition.

"We know that families that choose to breast-feed are very different from families that do not," Stettler said. They may have higher incomes or feed their children more healthful food. Thus, it's hard to say whether breast-feeding or other family characteristics affect a child's future risk of obesity.

That's why, Stettler says, "these associations are not ready for prime time or to be converted into public health recommendations."

Early childhood

Other experts say the soaring rates of child obesity warrant changes even without solid evidence.

In a study published recently in Pediatrics, Skinner's team found that obese children as young as age 3 had higher levels of C-reactive protein, a marker for inflammation that is linked to heart disease in adults. C-reactive protein levels can rise for a number of reasons, and about 15% to 20% of children have above-normal levels. But among obese 3-year-olds, 45% had elevated levels. It's not clear yet whether this inflammation remains high in obese children or causes any long-lasting harm.

"That indicates to me that when we see what appears to be a chubby kid on the outside, there may be something different on the inside, compared with healthy kids," she says. "It's a red flag that something is not quite right."

Levin didn't wait for someone to test her daughter's C-reactive protein.

The first-time mom recently consulted a doctor who specializes in weight and nutrition issues, and she learned that the family needed to make immediate changes. A toddler doesn't need a whole bagel with cream cheese and salmon for lunch, she learned. One-quarter of that bagel sandwich would suffice. Low-fat milk is more healthful than whole milk. Snacks need not include juice.

That doesn't mean she put her child on a diet; most doctors discourage such restrictions. "It's not about losing weight," says Levin. "It's about her growing into her weight. We're not focused on numbers and the scale and all of that. We just need to focus on healthy choices."

More parents of young children are asking for advice on weight and nutrition, says Dr. Elaine L. Rosen, director of the California Center for Healthy Living in Encino, where Levin took her daughter. A pediatrician, Rosen opened her center, which addresses weight or nutritional problems in babies through young adults, because of high rates of child obesity and because worried parents didn't know how to help their kids.

Too much concern on the part of parents can backfire, Rosen says, leading to eating disorders in children or contributing to children being overweight or underweight.

"There is a lot of misinformation out there," she says. "A lot of adults rely on diet-mentality notions, which is not appropriate for kids."

Families are taught what, when and where to feed their children, while leaving the questions of whether to eat and how much to eat to the child, Rosen says.

"Sometimes parents cross the line into the domain of control," she says. "The child does not learn to trust themselves to feed themselves properly. Some kids resort to under-eating and picky eating. Some overeat, learn to sneak food and defy their parents."

The emphasis on the early years just makes good sense, Taveras says. "Almost all of the risk factors we found we can change. We can counsel families about these issues."

Levin and her daughter began to see Rosen a few months ago. But already, Levin says, her daughter's preschool teacher has remarked that the child seems to have had a growth spurt — in height.

A late start

So far, the early years have been ignored when it comes to obesity practices and policies. First Lady Michelle Obama recently launched a campaign to address child obesity. But like many other programs, it focuses on school-age children. There is no organized public-health effort on obesity prevention from gestation to age 5. The Institute of Medicine, however, recently convened a task force to study that time period.

Because child obesity is linked to both maternal and child health, obstetricians and pediatricians must be enlisted to address obesity prevention in their patients, Taveras says.

But, she adds, "during pregnancy and the first two years of life, mothers and their infants are seen by physicians more often than any other time of life. It's kind of a golden opportunity. We have systems in place to reach mothers and children."