A soda a day? That's not so bad—a 150-calorie blip, burned off with a brisk half-hour walk. But it's not only your waistline that's at stake. A study released today in the journal Diabetes Care found that people with a daily habit of just one or two sugar-sweetened beverages—anything from sodas and energy drinks to sweetened teas and vitamin water—were more than 25 percent likelier to develop type 2 diabetes than were similar individuals who had no more than one sugary drink per month. Since the overall rate of diabetes is roughly 1 in 10, an increase of 25 percent raises the risk to about 1 in 8. One-a-day guzzlers in the study also had a 20 percent higher rate of metabolic syndrome, a collection of indicators such as high triglyceride levels suggesting that diabetes is not far off.
"Previous studies have shown that sugar-sweetened beverages are strongly associated with weight gain," says lead author Vasanti Malik, a research fellow in the Harvard School of Public Health Department of Nutrition, who says the decision to examine the relationship between sugar-sweetened beverages and risk of diabetes was "the logical next step."
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The researchers conducted a study of studies—a meta-analysis—to reach their conclusions. They identified eight studies with enough data to let them check for a link between sugary drinks and type 2 diabetes and three similar studies of metabolic syndrome. The largest diabetes study, which followed more than 91,000 American women ages 24 to 44 for eight years, made the strongest case for a relationship, and it wasn't just because higher consumption of sweetened drinks added excess calories that turned into pounds. While weight gain is a known diabetes risk factor, the diabetes-beverage link persisted even after adjusting for that. "Other factors independently put you at risk for developing diabetes," says Malik.
The main one is spikes in blood glucose and insulin because sweetened drinks are often consumed quickly and in large quantities and their sugar content is rapidly absorbed. Frequent spiking can lead to insulin resistance, inflammation, and hypertension—often precursors to diabetes. High-fructose corn syrup, the sugar in many sweetened drinks, is emerging as possibly riskier than other sugars because it seems to produce more belly fat. Fat that accumulates around the middle is closely tied to high blood pressure and other cardiovascular problems.
Americans love sweetened drinks. Consumption climbed to an average of 142 calories a day, or nearly one 12-ounce can of soda, in 2006, from 65 in the late 1970s. And many people down far more than that, notes Frank Hu, a senior author of the study and a professor of nutrition and epidemiology at Harvard, which puts them at a much greater risk of diabetes. A report from the Centers for Disease Control and Prevention released earlier this week projects that by 2050, 1 in 3 Americans will develop the disease. "Soft drink consumption has significant public health implications in terms of the diabetes epidemic," says Hu.
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Earlier this year the American Heart Association issued a recommendation advising consumers to set a limit on sweetened drinks of 450 calories a week, or three 12-ounce sodas, in a 2,000-calorie diet. Calorie-counting is a convenient way to keep track, but it can be misleading. "Consumers are overly focused on calories," says Constance Brown-Riggs, a spokesperson for the American Dietetic Association, who would like people also to understand that a 12-ounce can of soda contains the equivalent of 15 teaspoons of sugar. "They think it's not that bad, without taking into consideration the other components that are putting them at risk."
Type 2 diabetes and metabolic syndrome aren't the only risks of a one-a-day habit. In a 2004 study published in the Journal of the American Medical Association of 88,000 women followed for 24 years, those who guzzled two or more sugary drinks a day had a risk of coronary artery disease 35 percent higher than non-guzzlers, even after adjusting for other unhealthy lifestyle factors. "You receive no benefits out of drinking these beverages," says Malik, who lists additional hazards from dental cavities to gout. "It's a wake-up call for the American public."