The path toward obesity starts at a young age — even before babies transition to a solid diet, according to a new study.

Almost one-third of 9-month-olds are obese or overweight, as are 34 percent of 2-year-olds, according to the research, which looked at a nationally representative sample of children born in 2001. The study is one of the first to measure weight in the same group of very young children over time, said lead researcher Brian Moss, a sociologist at Wayne State University in Detroit. The results showed that starting out heavy puts kids on a trajectory to stay that way.

"If you were overweight at nine months old, it really kind of sets the stage for you to remain overweight at two years," Moss told LiveScience.

Tracking obesity

According to the Centers for Disease Control and Prevention (CDC), childhood obesity has tripled over the last three decades. In 2008, 19.6 percent of kids between the ages of 6 and 11 were obese.

But less is known about obesity rates in very young children. In fact, researchers hesitate to label children that young as "obese."

Recent studies have raised the alarm about particularly large babies, however. One 2009 paper published in the journal Pediatrics found that babies who gain weight rapidly in the first six months of life are at increased risk of being obese by age 3. Another study, published in April 2010 in the Journal of Pediatrics, found that heavy 6-month-olds are more likely to be obese as 2-year-olds. 

What can be done?

"Studies have shown that exclusive breastfeeding — breastfeeding alone, not breastfeeding combined with bottle-feeding —prevents obesity," said Dr. David McCormick, senior author of the April study at the University of Texas Medical Branch at Galveston. "Getting enough fiber — eating apples instead of drinking apple juice, for example — also helps keep babies on track to a healthy weight. By contrast, improper early introduction of cereal by adding it to an infant's bottle promotes obesity."

Overweight infants

Moss and his co-author, William Yeaton of the University of Michigan, used data from a survey called the Early Childhood Longitudinal Study-Birth Cohort, which contains data on 8,900 babies at nine months and 7,500 of those same babies at 2 years. (Some families moved out of the country or didn't respond to the second round of surveys.) The researchers classified the babies' weights based on CDC growth charts, which compare a baby's growth to a standardized growth curve. Kids in the 95th percentile of weight were categorized as "obese," while kids in the 85th to 95th percentile were counted as "at-risk," similar to the adult category of "overweight," Moss said.

Even in the first year of life, many babies fall into these two categories, Moss and his colleagues report in the January/February issue of the American Journal of Health Promotion. In the 9-month age group, 15.2 percent of babies were at-risk and 16.7 percent were obese. Among 2-year-olds, just under 14 percent were at-risk and almost 21 percent were obese.

"So you're seeing, combined, more kids being at-risk and obese [in the 2-year-old age group]," Moss said. "Of that combined total, more kids are obese than at-risk at two years."

The find hints at an unfortunate pattern: Kids who start out heavier end up heavier. Of kids who were normal weight at 9 months old, 75 percent were still normal weight at 2 years. But kids who were at-risk at nine months had only a 50 percent chance of being normal weight at age 2. More than 28 percent of at-risk kids ended up obese by their second birthday.

Obese 9-month-olds showed an even starker pattern. Only 37.6 percent were normal weight at age 2. Just over 18 percent did improve to the at-risk category, but 43.9 percent remained obese.

It's not likely that doctors are going to be recommending infants go on a diet  anytime soon, Moss said. However, the results could help target health education and other interventions to the populations that need them, he said. The study found that Hispanic babies were at the highest risk of becoming overweight or obese. Babies in families of low socioeconomic status were also more likely to be heavy.

"It would be pretty important if we could, instead of reacting to kids who are already obese, maybe try to get them on a healthy track at the very early ages of their lives," Moss said.

You can follow LiveScience Senior Writer Stephanie Pappas on Twitter @sipappas.