Sugary drinks are linked to a heightened risk of developing Type 2 diabetes, but a large US study that confirmed this shed little light on whether caffeine - suggested in past studies to have a link to sugar processing - helps or hinders.
Among more than 100,000 men and women followed for 22 years, those who drank sugar-sweetened drinks were as much as 23% more likely to develop diabetes than those who didn't, but the risk was about the same whether the drinks contained caffeine or not, according to the study that appeared in The American Journal of Clinical Nutrition.
"We found that caffeine doesn't make a difference at all," said lead author Frank Hu of Harvard University. "Coffee can be beneficial and the caffeine doesn't appear to have a positive or negative effect on diabetes risk."
Numerous past studies have linked regular consumption of soft drinks, both sugar- and artificially-sweetened, to an increased risk of diabetes. Research over the past decade has also suggested that caffeine temporarily prevents the body from processing sugar efficiently - a problem that those who live with diabetes deal with all the time.
That at least suggests that caffeine in conjunction with sweetened drinks might raise diabetes risk even further. However, other research has found a protective effect from coffee and tea, suggesting caffeine does the opposite.
Hu and his coauthors wanted to know if people who regularly drink sugary and caffeinated beverages might only be exaggerating their risk of developing a disease that affects nearly 26 million adults and children, or about 8% of the US population, according to the American Diabetes Association.
They examined the health habits of 75,000 women and 39,000 men involved in long-term health studies that began in the mid-1980s.
Compared to people who didn't consume sugary drinks, the likelihood of developing diabetes over the years for those who did was higher by 13% for caffeinated sugary drinks and 11% for decaffeinated among women, and by 16% or 23% among men, respectively.
Caffeine-free artificially sweetened drinks were also linked to a 6% increase in risk among women.
However, coffee drinkers showed slightly lower risks compared to non-drinkers. The chances of developing diabetes were 8% lower among women, whether they drank decaf or regular coffee, and for men, 4% lower with regular coffee and 7% lower with decaf.
Hu and his team have used this same dataset, which contains the health habits of mostly white health professional, to suggest that regular coffee drinking in general is tied to a lower risk of diabetes.
But past studies, like the current one, have also found that the risk falls even lower if adults drink decaffeinated coffee.
"Our understanding of the body's tolerance to caffeine is not complete," said James Lane of Duke University, who has done short-term studies that linked caffeine to a disruption of the body's ability to process glucose.
The latest study suggests that people who currently drink sugary beverages could substitute coffee or tea, though tea was associated with fewer benefits, instead.
But other researchers said that more work is necessary to untangle caffeinated coffee's complicated relationship with diabetes risk, and that it is still far too early to advise people to drink coffee if they don't do so already.