Want to evade tooth decay? Then avoid too much sugar, say researchers whose study identified sugar as the sole cause of dental cavities in both adults and children.

Treatment of tooth decay, which affects 60 to 90 percent of school-age children and a similar number of adults, accounts for between 5 and 10 percent total health spending in industrialized countries, the study points out.

Researchers at University College London and the London School of Hygiene & Tropical Medicine, in a study reported in the BMC Public Health journal, say free sugars should be no more than 3 percent of total energy intake in a person's diet in order to avoid tooth decay.

The World Health Organization Nutrition Guidance Advisory Group defines free sugars as including "monosaccharides and disaccharides added to foods by the manufacturer, cook or consumer, and sugars naturally present in honey, syrups, fruit juices and fruit concentrates."

Analyzing public records of dental health and diet from around the world, the researchers found tooth decay rose dramatically at any sugar intake level of above zero percent of energy.

While the World Health Organization recommends 10 percent of free sugar energy intake as a maximum, with 5 percent as a preferred "target," the new study suggests the lower figure should be considered a maximum, with a preferred target below 3 percent.

"Tooth decay is a serious problem worldwide and reducing sugars intake makes a huge difference," says UCL dental public health Professor Aubrey Sheiham.

He cites the example of Japan, where the population was without access to any sugar during or immediately after World War II and data shows a huge reduction in tooth decay during that period.

When the country eventually began imports of sugar again, the incidence of tooth decay rose dramatically, he says.

Fruit juices and sodas are a chief source of sugar in the diet of Americans, contributing to tooth decay as well as other health problems such as obesity and diabetes.

For this reason, the researchers recommend vending machines serving the sugar-laden and unhealthy beverages be removed from places financially funded or controlled by government entities.

"We are not talking draconian policies to 'ban' such sugar-rich products, which are available elsewhere, but no publicly supported establishment should be contributing to the expensive problems of dental caries, obesity and diabetes," they say.

Tooth decay is world's the most common non-communicable disease, they note.