People who drink more than half a litre of fizzy drink a day are also more likely to have asthma or serious lung conditions - especially if they are smokers as well, a major study has found.
University of Adelaide researchers found that 10% of people in South Australia drank 500ml or more soft drink every day, and such heavy consumption was associated with an increased chance of airway problems.
Lead author Zumin Shi said the study of 16,900 people appeared to show that the more soft drink someone consumed, the higher chance they had of having asthma or chronic obstructive pulmonary disease (COPD).
Dietitians in New Zealand said New Zealanders were the ninth-highest consumers of soft drink in the developed world, consuming about 84.2 litres each a year.
Australians were placed sixth, with 100.1 litres.
While the study made a connection between an unhealthy diet and lung problems, the authors said the cause of this was not clear.
One of several theories was that sugar consumption made airways more vulnerable to allergic inflammation.
New Zealand Asthma Foundation medical director Bob Hancox said it did not yet prove there was a causal connection between an unhealthy diet and asthma.
"What they've shown is that it's an association that needs to be looked at further. But it certainly hasn't demonstrated that soft drinks are necessarily the cause of airways disease," he said.
New Zealand Juice and Beverage Association Kerry Tyack said there was some uncertainty about the study because it did not differentiate between sugary drinks and sugar-free soft drinks.
But he said the industry welcomed any robust research on its products, especially if it caused people to adjust to a healthier diet.
"There is no producer of non-alcoholic, recreational drinks that advocates high consumption of their drinks.
"These drinks are designed to be enjoyed in the appropriate environment, on appropriate occasions and in moderation." Dietitian Rob Quigley said New Zealand's food and nutrition guidelines "categorically stated" that water was the best drink, followed by trim milk.
He suggested the guidelines should possibly be extended to advise against soft-drink consumption.
"It's pretty stark. You just don't need them, frankly. They're completely superfluous. They provide no benefit to a person beyond energy." The study, published last week in the journal Respirology, also found that people who both smoked and consumed 500ml of soft drink a day were 6.6 times more likely to have COPD than a non-smoker who drank no soft drinks.
The authors adjusted the study to take into account the fact that smoking was a risk factor for airways disease. They also considered another confounding factor - that people who drank more sugary drinks might also be obese, a risk factor for asthma and COPD.
Once the study was adjusted for people who were overweight, the positive association remained between fizzy drinks and lung problems.
Fizzy facts
• New Zealanders are the ninth-highest consumers of soft drink in the world, drinking 82.4 litres a year per person.
• Australians are the sixth-highest (100.1 litres) and United States the highest (216 litres).
• New Zealand children get 6% of their energy from soft drinks.
• An Australian study of 16,900 people shows that people who drink 500ml of soft drink a day were more likely to have asthma or serious lung conditions.
• 11.4% of the participants reported daily soft drink consumption of more than 500ml.
• 13.3% of people with asthma and 15.6% of those with COPD reported consuming this amount of soft drink.