Dental researchers, using a new method of ranking foods according to the amount of cavities they cause, have uncovered some new relationships between eating and tooth decay that run counter to the common wisdom.

One finding appears to be a "sugar threshold" in some foods. For example, a breakfast cereal that contains about 1 percent sugar causes very few cavities. But a cereal that is 8 percent sugar causes a large number -- as many, in fact, as one that is 60 percent sugar.

Another finding is that the amount of sugar eaten is less important than how often it's eaten. If you're going to eat a lot of sweets during the day, it's best to devour them in a short time, the researchers say.

The mouth can recover in a matter of hours from a single dose of sugar, gradually neutralizing the decay-causing acid generated by bacteria in the mouth as they share the sugar feast. But if small amounts of sugar are taken in over a long period, the mouth cannot neutralize the acid, and decay continues.

A third fact learned from the studies, done at the National Institutes of Health by Dr. William Bowen and a team of dental researchers, is that the addition of calcium phosphate to sweets cuts their cavity-causing potential by more than half.

"You may be able to modify many sweets or desserts by adding this phosphate, and thus cut down on the damage they will do," Bowen said.

Bowen has so far tested sugar and 12 different snack foods to begin a far more extensive ranking that the group hopes to accomplish over the next year or two. The first goal is to make a list of snack foods rated by cavity-causing potential so consumers can make decisions about what to eat among the foods that are one of the biggest sources of decay.

To rank the foods, the researchers gave rats a normal diet through a stomach tube, and fed them additional controlled amounts of sugar or snack foods the normal way. Thus, only the foods being studied touched the teeth.They began by giving rats 17 small meals of pure sugar spread evenly over the day. They went on to give other rats different foods, as well as smaller numbers of sugar-only meals.

Here are the foods ranked so far, with their relative cavity-causing power expressed as a ratio compared to pure white sugar (rated 1.0):

Confectioner's sugar, 1.0; chocolate cookie with soft chocolate creme filling, 1.4; vitamin-fortified breakfast cereal with 14 percent sugar content, 1.06; breakfast cereal with 8 percent sugar, 96; sugar-coated breakfast cereal with 60 percent sugar, 94; sugar-coated chocolate candy, .91; potato chips, .84; caramels, .73; chocolate bar, .72; unsweetened cereal with about 1 percent sugar content, .45; white powdered starch, .45; confectioner's sugar with 7 1/2 percent calcium phosphate added, .42, and no food by mouth at all, 0.

"The fact of overriding importance," Bowen said, "is the frequency of eating sweets. When we gave rats . . . sugar meals only 10 or 20 minutes apart (so they ate them all in less than five hours,), they got half as many cavities as when we gave them the same [amount of sugar] spread over 13 to 17 hours."

He explained that cavities are caused by bacteria which, multiplying on the teeth when sugar is present, manufacture the yellowish plaque on the teeth, then use it as home as they begin to secrete their decay-causing acid.

"The plaque is like a bacterial aspic to hold colonies on the tooth surface, and it quickly becomes an acid-filled sponge held against the tooth as the decay begins," Bowen said. Decay proceeds rapidly as long as sugar supplies for the bacteria last.

However, saliva over a period of hours can seep into the plaque and neutralize the acid. New supplies of sugar make new production of acid, but if there is a long period between additions of sugar, much acid can be neutralized and decay can be reduced perhaps by half.