The decline in tooth decay seen over much of the 20th century pretty well matches the increase in soft drink consumption

Professor William Bowen

Benzoates, which are found in soft drinks, ice cream and margarine, and sorbate, an ingredient in mayonnaise and dried fruits, were tested on rats.

The scientists at the University of Rochester Medical Center, New York state, divided the rats into four groups, giving them either fluoride, benzoate, both or neither.

The rats who received neither fluoride nor benzoate had an average of 16 cavities on their teeth after three weeks.

Those given fluoride had two cavities - holes in the outer layers of the teeth - and there were no cavities at all in rats given fluoride and a high dose of benzoate.

An earlier study by another team of researchers at Rochester University had found that preservatives mimicked fluoride in test tube experiments.

Bacteria and toxins

Benzoates are used to keep food free of bacteria and toxins. They usually make up just 0.1 or 0.2% of foods. They are also available from natural sources such as cranberries, prunes and cinammon.

Professor William Bowen, who presented the research to the International Association of Dental Research, said the use of fluoride in water supplies was the major reason for falling rates of tooth decay in the past few decades.

But he added: "During that same time frame, there's been a huge increase in the use of food preservatives. The decline in tooth decay seen over much of the 20th century pretty well matches the increase in soft drink consumption.

"In the United States, soda consumption is up to an average of two cans a day per person. That's a lot of benzoate."

However, he warned that relying on chance to prevent tooth decay was not advisable. "There may be a more structured way to take advantage of this, such as putting these preservatives into toothpaste."

In the meantime, people should avoid high-sugar foods, including most soft drinks, and brush their teeth and visit their dentist regularly, he said.

Dr Geoff Craig, head of the health and science policy group at the British Dental Association, said: "The main point to remember is that these additives are helping fluoride. On their own they won't do the job that fluoride does.

"It is also important to remember that many soft drinks are acidic and can erode teeth."

He advised people cut down the number of acidic drinks they have and use toothpaste which includes fluoride.