Chubby kindergarteners may have held tight to their bottles too long, according to a study out of Temple University in Philadelphia. In the first prospective (watching variables forward over time) study to examine the relationship between prolonged bottle use and the prevalence of obesity in early childhood, researchers found that kids who were obese at five years were often still drinking from a bottle at the age of two.

Those who were still drinking from a bottle at age two were 33 percent more likely to be obese.

Researchers studied 6,750 children born in 2001 and found that those who were still drinking from a bottle at age two were 33 percent more likely to be obese (BMI greater than or equal to the 95th percentile) at 5.5 years of age. Whether a child drank primarily from a bottle or was only put to bed with a bottle, they were considered prolonged bottle users.

Of the children that lead researcher Rachel A. Gooze and her colleagues studied, 20 percent were still taking a bottle at the age of two years. Among those long-term bottle users, about 20% were obese at the age of five years. About one in six children who had been weaned earlier were obese.

Other factors that were studied that could affect a child’s risk of obesity included the sex of the child, twin status, racial/ethnic group, maternal education, maternal weight, family income, whether the mother smoked, and whether the child had ever been breastfed. The research suggested that prolonged bottle use itself was linked to the increase in children’s risk of obesity, though Gooze said that the connection is not proof of cause and effect.

Prolonged bottle use may translate into the consumption of excess calories for toddlers. For example, an 8-ounce bottle of whole milk contains 150 calories; an 8-ounce bottle of apple juice contains about 120 calories. The average 2-year-old female requires approximately 1300 calories per day, so one nighttime bottle would provide 9 to 12 percent of her daily calorie needs. Two such bottles a day would provide up to a fourth of the child’s daily calorie needs.

In addition to problems with weight gain in toddlers who cling to their bottles, prolonged bottle feeding may also interfere with toddlers eating a varied and healthy diet. Parents should begin introducing solid foods to their infants around the age of six months, and as their intake of solid foods increases, the use of the bottle should decrease and the use of a cup introduced. According to the American Academy of Pediatrics (AAP), it is important to wean a child off the bottle by age 18 months. The AAP suggests that weaning begin at one year and that a cup replace a bottle in stages to make the transition smoother.

The study was published online in The Journal of Pediatrics on May 5, 2011.