A new study shows children with several rotten teeth are nearly twice as likely to get rheumatic fever.
The research suggested sugar, which causes tooth decay, can also be linked to the childhood disease that can lead to severe complications in later life and sometimes early death.
Auckland Regional Public Health Service epidemiologist Dr Simon Thornley said he was inspired to explore the link after reading a throwaway line in a 1930s dentist's magazine.
He said Weston Price's controversial Nutrition and Physical Deterioration was one of the first texts to highlight the failings of a modern western diet with flour and sugar.
Dr Thornley scoured the dental and health records of 20,000 five- and six-year-old Pacific and Māori children in south Auckland over seven years.
He said his findings, which were in a preliminary stage and still have to be published, were a breakthrough.
"We looked at the state of their teeth and looked at whether they got rheumatic fever during follow-up, and it showed that kids who had five or more decayed teeth when they had that first visit, they were about twice as likely to develop rheumatic fever compared to kids who had less than five decayed teeth."
Sugar 'providing fuel for the bacteria'
Rheumatic fever mostly affects poor Pacific and Māori children.
It starts with strep throat and can lead to scarring and damage within the heart.
Children who have suffered rheumatic fever often need heart surgery later in life and some will die prematurely.
Dr Thornley, who campaigns against sugary drinks, said the bugs that cause tooth decay and strep throat are closely related and feed on sugar.
"Having a lot of sugar in the diet causes both bad teeth to develop but also it increases the chances that if that child comes into contact with a bacteria that it will take hold and cause disease in the mouth and in the body."
He said sugar was the culprit.
"To me, the sugar is providing fuel for the bacteria, which we know are important in both those diseases."
New options for prevention
Dental therapist Rachel Bridgeman of Simply Dental, a mobile dental programme for adolescents, said she was extracting and filling more teeth than ever.
"The sugary drinks, what these kids eat, is insane. It's just setting up for not just dental issues but a huge range of other issues that result from a bad diet."
Ms Bridgeman said the study should be used to put pressure on sugary drinks makers.
"Actually putting a little bit of responsibility back on the people who are actually selling this and the way we market the stuff because we're teaching people that its normal."
Dr Bryan Betty, a GP in Cannons Creek in Porirua, which has one of the highest rates of rheumatic fever in the country, said research linking the two diseases would be exciting.
"It's been well-known for a long time that tooth decay is associated with morbidity both in adults and children. It's certainly one of the hidden health issues that New Zealand is facing, and I think if there is research that is starting to look at the link between rheumatic fever and tooth decay it's actually very, very important."
The government has spent more than $65 million tackling rheumatic fever.
Dr Betty said a lot of focus was on treating sore throats but this study opened new ways of thinking about how to prevent it.
"The whole thing ties up with diet and sugary drinks and the sugar story, I think it has a role to play in terms of dental problems and it's an aspect of health care that I think needs some serious, serious focus."
Dr Thornley said his study was still to be published and needed further research but his next step was to make the link between rotten teeth and other childhood diseases including glue ear, asthma and cancer.