Although Portland voters resoundingly defeated a measure that would have fluoridated the city's drinking water last year, national guidelines are moving in the other direction.

This week, the American Dental Association ratcheted up its guidelines for toddlers, suggesting that parents begin brushing their children's teeth with the cavity-fighting substance as soon as the teeth are visible, rather than waiting until the children are old enough to spit out toothpaste. 

To fight the rising number of cavities in the very young, the dental group now advises getting a jump-start on prevention. However, they emphasize only the tiniest amount of fluoride toothpaste should be used to minimize the risk of mild discoloration, white spots or streaking of the teeth, a condition called fluorosis that is caused by ingesting fluoride toothpaste at a young age.

“We want to minimize the amount of fluoride consumption to reduce the risk of fluorosis while simultaneously adding a preventive tool for kids 2 and under that we haven’t recommended previously,” said Dr. Jonathan Shenkin, a spokesman for the A.D.A. and a pediatric dentist in Augusta, Me. Only a tiny amount of toothpaste should be smeared on the brush since some youngsters are likely to ingest some of the fluoride, he said.