Fluoridation of water, long credited with the large decline in tooth decay in much of the world during recent decades, might actually have played only a minor role, an Australian public health researcher has concluded after reviewing many previous studies.

However, an official of the American Dental Association challenged the interpretation, asserting that many studies show fluoridated water to be the chief factor behind a reduction in the tooth decay rate that, in the United States, fell between 30 and 35 percent during the 1970s alone.

As evidence minimizing the role of fluoridated water, the Australian researcher cited studies showing that the incidence of tooth decay had already begun to decline in many cities in Western Europe, the United States and Australia before the start of fluoridation in those places.

In some cases, the prefluoride decline had already attained most of the improvement that would later be credited to fluoridation.

Moreover, he found that tooth decay has been declining in cities with unfluoridated water about as fast as it has in those with fluoridation.

The rate of tooth decay is even continuing to fall among those who have been drinking fluoridated water all their lives.

For example, today's 10-year-old, after a lifetime of fluoridation, has less tooth decay than the 10-year-old of a decade or so ago who also had a lifetime of fluoridation.

Taken together, the observations suggest that something other than the addition of fluoride to drinking water has been causing the reduction of tooth decay, Mark Diesendorf of Australian National University's Human Services Program, asserts in the current issue of Nature, a British scientific journal.

Diesendorf said that he did not know the main cause or causes of declining rates of dental caries, or decay, but that they may include reduction of sugar in the diet, or a reduction of sugar in the forms and the high frequencies that have the greatest effect on teeth. Another factor, he said, may be the widespread use of antibiotics that have the side effect of suppressing mouth bacteria.

One factor might have been the increasing application of fluoride directly to teeth in the form of toothpaste and in the much higher concentrations used in treatment by dentists.

All these factors were operating in populations with fluoridated water as well as those without, Diesendorf said, and therefore could have accounted for much of the presumed benefit of water fluoridation.

Diesendorf cited the example of Sydney, where advocates of fluoridation once boasted that the percentage of children with "naturally sound" teeth had increased from 3.8 percent in 1961, before fluoridation, to 28 percent in 1972, after fluoridation. Overlooked was the fact that Sydney's water was not fluoridated until 1968 and that the percentage had grown to 20.2 perecent by the year before fluoridation started.

Diesendorf is not claiming that fluoridated water is useless, only that the evidence suggests that other factors, including other sources of fluoride, play larger roles.

"We've heard this line of reasoning before," said Lisa Watson, director of fluoridation and preventive dentistry for the American Dental Association. "There's a long list of studies that show water fluoridation works."

Watson cited a 1983 study comparing two towns in Britain, one with fluoride in the water and one without. After examining hundreds of children of given ages who had lived all their lives in each town, the study found a significantly lower rate of tooth decay in the town with fluoridation.

For example, the average 12-year-old in the town without water fluoridation had 4.46 "decayed, missing or filled teeth," while children of the same age with fluoridated water averaged 2.59 such teeth, a 43 percent reduction.

"We know that water fluoridation works," Watson said. "We know it costs about nothing. But we certainly don't deny that there are other factors, such as fluoridated tooth paste. That's a big one and the use is now almost universal, well over 95 percent of the population."

Watson also discounted the notion that dietary changes have cut tooth decay. "We don't see any basis for that," she said. "If anything, the consumption of total sugar per person has gone up and there's more snacking now than ever." Watson said it was now well established that it is worse on the teeth to consume a given amount of sugar in separate small doses than to consume it all at once.