Doctors recommend introducing sippy cups at six months and weaning toddlers off bottles completely by the time they're 15 months old.
But 20 percent of two-year-olds and 10 percent of three-year-olds in the U.S. continue to use bottles, often drinking five bottles of whole milk every day, researchers said.
"Bottles can become a vessel for extra, or 'stealth' calories, because they are often used indiscriminately. For example, while in a stroller, or to put a child to bed," Karen Bonuck told Reuters Health in an email. She led the new study at Albert Einstein College of Medicine in Bronx, New York.
"Before you know it, a child can take in 150 calories of whole milk in a bottle on top of their regular diet," Bonuck said.
The researchers wanted to see if giving parents educational materials as part of a program called 'Proud to Be Bottle Free' and a sippy cup would reduce the number of bottles kids used and the calories they consumed.
They enrolled 300 pairs of parents and 12-month-olds at two Bronx Women, Infants, and Children (WIC) sites.
To be eligible for the study, children had to be consuming more than two bottles of milk or juice every day. The participants were randomly split into two groups: a bottle-weaning group that received the materials and sippy cup and a comparison group that did not.
The research team checked in with parents over the next year to find out how many bottles kids were consuming every day, as well as what else they ate and drank.
One hundred and four parent and child pairs completed the study.
After three months, bottle usage had dropped from 4.6 bottles per day to two bottles per day among kids in the bottle-weaning program. There was a smaller drop in the comparison group, from 4.4 bottles per day to 2.7, on average.
Sippy cup usage increased more in the bottle-weaning group.
By one year, toddlers in both groups were averaging about one bottle per day.
Kids in the bottle-weaning program consumed slightly fewer calories - 1,090 calories per day, on average, versus 1,186 among comparison children. But the difference was small enough that it could have been due to chance.
The program did not lower toddlers' chances of being overweight, according to results published in The Journal of Pediatrics.
"At first we were surprised that there was no effect upon overweight status," Bonuck said, "but looking at the data more closely, this seems partially attributable to the substitution of sippy cups for bottles in the intervention group."
She thought the program's benefits might also have been clearer had fewer families left the study early.
"Had we achieved our optimal sample size, and included messages about sippy cups, I would suspect this would have affected our overweight status outcomes," Bonuck said.
She said the advice to wean toddlers off bottles by 15 months should be extended to sippy cups.
Image: Girl with bottle, via Shutterstock