The study of 52,000 nurses was carried out over an eight-year period by researchers from the Harvard School of Public Health in the US and was published in this week's issue of the Journal of the American Medical Association (JAMA).
Type 2 diabetes affects about 17 million US individuals, and the prevalence of diabetes has increased rapidly during the last decades, the report's authors say, suggesting a link with soft drink consumption in the US, which has also increased dramatically during this time - by 61 per cent between 1977 and 1997 in adults and by more than 100 per cent in children and adolescents between 1977 and 1998.
"No study has examined the association between the consumption of soft drinks and other sugar-sweetened beverages and risk of type 2 diabetes," they wrote. "We therefore examined the relationships between sugar-sweetened beverage consumption and weight gain and diabetes risk in a large cohort of young and middle-aged women, controlling for potential confounding factors.
"Because the majority of sugar-sweetened beverages consumed in this cohort are soft drinks, we particularly emphasised soft drink consumption."
The study participants were asked how often they had consumed a commonly used unit or portion size of various types of food on average over the previous year, including both regular and diet soft drinks. At the same time, they were also assessed for signs of the onset of diabetes.
The authors report that woman who drank large quantities of soft drinks and showed signs of diabetes were also on average heavy smokers, physically inactive and had a poor diet - factors which could also lead to a higher diabetes risk - but that was still a "strong association" between soft drinks consumption and the disease even after adjusting the data to take account of these other factors.
But the authors also acknowledged that there was a lack of real evidence to support their claims. "Women who increased their sugar-sweetened soft drink consumption also increased energy intake from other foods, indicating that these beverages may even induce hunger and food intake," they said.
"However, experimental data on soft drink consumption and food intake have not provided support for this hypothesis. Our observation may, therefore, rather reflect dietary and lifestyle changes accompanying changes in soft drink consumption."
They also added that imprecise measurements, the observational nature of the study, and the reliance on self-reporting of body weight by the study participants could also have skewed the results.
The soft drinks industry, not surprisingly, has accused the researchers of unfairly singling it out, stressing that the scientists themselves had acknowledged that other factors could be responsible for the rising diabetes levels.
"It is scientifically indefensible to blame any one food or beverage for increasing the risk of type 2 diabetes, a disease which is commonly known to have multiple causes and risk factors," said the US National Soft Drinks Association in an angry statement published yesterday.
"This interpretation of the results reflects neither reality nor even this study's own results. In addition to soft drinks, this study found significant correlations between low cereal fibre intake, low protein intake, low magnesium intake, smoking, low physical activity, increased total caloric consumption and type 2 diabetes."
"A careful reading of the paper reveals that it was an unhealthy lifestyle, not consumption of a particular food or beverage, that increased the women's risk for type 2 diabetes."
Tellingly, the NSDA pointed out, the Harvard study showed little difference between increased sugared soft drink intake and diabetes and increased intake of diet soft drinks and the disease. "There is no reasonable scientific basis for any association between diet soft drink consumption and either weight gain or obesity," the association said. "The lack of difference between regular and diet soft drinks brings into question the validity of the analytical methods used in the current report."
The association also took umbrage at the suggestion that rising soft drink consumption among both children and adults was a bad thing. Much of the increase in sales in recent years came from diet soft drinks, water and sports drinks, it said, citing data from Beverage Digest: diet drink sales were up 6.3 per cent last year, water was up 21.5 per cent and sports drinks (which have less sugar and fewer calories than regular soft drinks) were up 17.9 per cent.
Consumer groups also criticised the findings - or at least the interpretation of them. The Center for Consumer Freedom, a non-profit coalition supported by restaurants, food companies and consumers, said that the authors had taken "a great leap to suggest a causal relationship between soda consumption and type 2 diabetes", pointing out that the data reported in the study showed no clear link between soft drink consumption and diabetes.
Instead, the group claimed, the researchers had suggested causality between soft drinks and diabetes because they had a particular axe to grind. Several of the authors of the Harvard study had close ties to the Center for Science in the Public Interest (CSPI), which it described as "an activist group leading the nation's anti-soda crusade".
"This report is the latest effort to use fizzy research to scare Americans over nothing," said Center for Consumer Freedom executive director Rick Berman. "The fact that the authors completely sidestep their own data that shows soda consumption has nothing to do with diabetes in the vast majority of women demonstrates that there is a biased agenda at work here by dietary Puritans."