Tooth decay is on the rise among kids in the US, according to the largest government study of the nation's dental health in more than 25 years.

Sugary foods and drinks and nonfluoridated bottled water may be contributing to the tooth decay of young children, it suggests.

An estimated 28 percent of U.S. children ages 2 to 5 had a cavity in at least one baby tooth between 1999 and 2004. This compared to a 24 percent rate from 1988 to 1994.

But there was some good news: Older children have fewer cavities and adults have less periodontal disease than in the past, and more of the elderly are retaining their teeth.

"Overall, we can say that most Americans are noticing an improvement in their oral health," said the study's lead author, Dr. Bruce Dye a dentist and epidemiologist with the National Center for Health Statistics.

He cited several likely factors, including parents serving young children more prepackaged foods with high sugar content, more sugary juices and sodas and more bottled water, much of which is not treated with fluoride.

Many experts credit fluoridation of public water supplies with driving down tooth decay starting in the middle of the 20th century.

"One of the interesting paradoxes of this is that we've actually had expansion in community water fluoridation in the United States over this time period," Dye said in a telephone interview.