Even if it doesn't make you fat, overloading on sugary drinks can still cause metabolic problems like diabetes, researchers say.
People who drank one or two sugar-sweetened beverages a day, like soda or vitamin water, had a 26 percent increased risk of developing type 2 diabetes, compared with those who barely drank any, according to Harvard's Vasanti Malik and colleagues.
"Part of sugar-sweetened beverages' contribution to diabetes is via obesity, but there are other pathways as well," Malik told MedPage Today. They reported their findings in the journal Diabetes Care.
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Malik and colleagues said the findings provide further evidence that patients should replace sugary drinks with healthier alternatives like water in order to reduce their risk of both obesity and chronic diseases.
Dr. Larry Cantley of Wake Forest University in Winston-Salem, N.C., who wasn't involved in the study, told MedPage Today that the findings are "very important."
"A lot of decisions [to avoid junk food] have been guided by weight," he said. "We now have outcomes data to say that you can actually have a higher risk of diabetes by making these kinds of choices."
High intake of sugar-sweetened beverages has long been associated with weight gain and an increased risk of obesity. That risk has only grown as the global consumption of these drinks increases.
While it's been thought that the drinks can lead to ill health effects directly through weight gain, science has been revealing that they may have other, more direct disease-generating mechanisms.
For instance, they've been shown to raise blood sugar and insulin levels rapidly and dramatically, potentially contributing to glucose intolerance, insulin resistance, beta-cell dysfunction, and inflammation.
Recent studies have also shown that fructose, the main sweetener in these drinks, may be linked to high blood pressure as well as increased production of blood fats such as triglycerides and bad LDL cholesterol.
Since the direct relationship between these beverages and metabolic disease is not as well understood, the researchers conducted a meta-analysis of 11 studies: three that focused on the metabolic syndrome, and eight looking at type 2 diabetes.
The studies, totaling 310,819 patients, largely used food frequency questionnaires to evaluate dietary intake.
The researchers found that patients with the highest intake of sugary drinks -- one or two servings per day -- had a 26 percent increased risk of developing diabetes, compared with those who drank hardly any of the beverages.
There was a similar risk pattern for the metabolic syndrome, as those drinking the most sodas or juices having a 20 percent increased risk of developing the condition.
Since obesity may mediate this relationship, the researchers attempted to control for its effects. When they excluded the three studies that controlled for body mass index (BMI) and total energy intake, there was a "slight increase in the magnitude of the association between diabetes and sugar-sweetened beverages," Malik said.
"One of the studies included in our analysis found that when adjusting for BMI, the association decreased by about half, suggesting that the association between [sugary drinks] and diabetes is partly mediated by body weight, but there is an independent association as well," she added.
The researchers also cautioned that higher levels of sugar-sweetened beverage intake could be a marker of an overall unhealthy diet. Those who drink too many sodas may also eat too much saturated or trans fat and not enough fiber, they said.
They concluded that the data "provide empirical evidence that intake of sugar-sweetened beverages should be limited to reduce obesity-related risk of chronic metabolic diseases."
Cantley added that the results could be stratified better, to look at patients who are "ultra-high" consumers of sugary drinks -- downing six to eight cans of Coke a day, for example.
"How much higher would the risk be if we took the really high consumers and focused on them?" he said. "The population [in this study] is diluted out by people drinking one or two beverages a day, which could underestimate how bad it might be."