Just about every week, it seems, a new study warns of another potential health risk linked to soft drinks.
The most recent headlines have raised concerns that diet sodas boost stroke risk. Diet and regular sodas have both been linked to obesity, kidney damage, and certain cancers. Regular soft drinks have been linked to elevated blood pressure.
Several hundred soda studies have been published over the last two decades, but most of the ones done in humans (as opposed to mice or rats) relied on people’s memories of what they drank.
Observational studies like these can point to possible concerns, but they can't prove that sodas do, or don’t, pose a health risk.
If you drink sodas -- especially if you drink a lot of them -- what are you to make of all the headlines? Do you dismiss them, as the beverage industry does, as bad science and media hype? Or is it time to put the can down and take a hard look at what you're drinking?
Another Day, Another Soda Study
In the past six months alone, dozens of studies examining the health impact of drinking sugary beverages or diet soda have been published in medical journals. Some suggested a relationship; others did not.
Sometimes, the media coverage of these studies took the researchers by surprise.
That was the case for epidemiologist Hannah Gardener, PhD, of the University of Miami. In February, she presented early results from her ongoing research at a health conference, and was completely unprepared for the media attention it received.
The story appeared on all the major networks, in most major newspapers, and on the Internet, including WebMD.
The early findings showed a 48% increase in heart attack and stroke risk among daily diet soda drinkers, compared to people who did not drink diet sodas at all or did not drink them every day.
Most reports cautioned that the findings were preliminary and did not prove that diet sodas cause stroke.
But Gardener says many media reports overstated the findings. And even when the stories got it right, she says the headlines often got it wrong by leaving the impression that her research proved the diet soda-stroke connection.
“It was just an abstract presented at a meeting. It hasn’t even been published yet,” Gardener tells WebMD. “We are still working on the analysis. I don’t think the level of press attention it received would have been warranted even if it was a published paper.”
Gardener's team attempted to control for known heart attack and stroke risk factors, such as poor diet and lack of exercise, but she concedes that these factors could have influenced the findings.
Purdue University behavioral sciences professor Susan Swithers. PhD, had a similar experience in 2004, following the publication of her study in rats suggesting that no-calorie sweeteners like those in diet sodas increase appetite.
Swithers says she was shocked by the amount of news coverage her study received.
“Frankly, we were stunned,” she tells WebMD. “It really was a small study.”
The nonprofit Center for Science in the Public Interest (CSPI) sees sugary drinks as a major factor in the obesity epidemic and favors taxing them.
CSPI executive director Michael Jacobson, PhD, says sugary soft drinks deserve to be singled out in the battle against obesity because they are the biggest single source of empty calories in the American diet.
“According to the USDA, 16% of calories in the typical American’s diet come from refined sugars and half of those calories come from beverages with added sugar,” Jacobson says. “Sodas used to be an occasional treat, but now they are part of the culture.”
New York University professor of nutrition and food studies Marion Nestle, PhD, says there is plenty of evidence that sodas have contributed to America's growing girth, especially among children.
Nestle says pediatricians who treat overweight children tell her that many of their patients take in 1,000 to 2,000 calories a day from soft drinks alone.
“Some children drink sodas all day long,” she says. “They are getting all of the calories they need in a day from soft drinks, so it’s no wonder they are fat.”
“The first thing that anyone should do if they are trying to lose weight," Nestle says, "is eliminate or cut down on soft drinks."
Obesity Culprit or Scapegoat?
The American Beverage Association (ABA) argues that sodas are taking too much of the blame for obesity.
“A calorie is a calorie, and what the data clearly show is that Americans are eating too much and taking in too many calories, period,” says Maureen Storey, PhD, the ABA's senior vice president of science policy.
Not everyone agrees with that. Sugary soft drinks, in particular, have been shown in many studies to be associated with overweight and obesity, as in a review of 30 studies published in 2006 by researchers from the Harvard School of Public Health. Many of the studies included in that review showed that overweight children and adults drink more sugary beverages than normal-weight kids and adults, and several studies found that the more sugar-sweetened drinks people drank the greater their likelihood of becoming overweight.
At the time, the ABA criticized the review, claiming in a news release that the Harvard researchers “chose to ignore critical articles and studies that contradicted their hypothesis,” such as a 2005 study finding no link between soda and obesity in Canadian children.
Yale University researchers also examined the obesity issue, combing through 88 studies.
They found that people tend to eat more calories on days when they drink a lot of sugar-sweetened drinks, and that soda drinkers tend to be heavier than people who don't drink soft drinks.
The researchers hypothesized that the body does not easily recognize calories derived from beverages, so people end up eating more. But the Yale study wasn't designed to prove that.
As for diet sodas, nutrition researcher David L. Katz, MD, who directs the Yale Prevention Research Center, told WebMD in November 2010 that the research as a whole suggests sugar substitutes and other non-nutritive food substitutes have little impact on weight. “For every study that shows there could be a benefit or harm, there’s another that shows no ‘there’ there,” Katz says.
Patterns of Bias?
The ABA says the vast majority of studies supporting a soda-obesity link were done by researchers with strong anti-soda biases. Storey also says many of these biased or poorly done studies are covered by news media, while studies showing no link don't get the same attention.
“All too often, studies that don’t show a relationship between sugar-sweetened beverages and obesity or other health concerns are not reported, while the ones that show even a very weak relationship are,” she says.
Obesity researcher Kelly Brownell, PhD, who led the Yale study and supports taxing sugar-sweetened beverages, sees bias on the other side of the debate.
“Studies that do not support a relationship between consumption of sugared beverages and health outcomes tend to be conducted by authors supported by the beverage industry,” Brownell wrote in a 2009 New England Journal of Medicine article supporting a soda tax.
One such study, funded by the British sugar industry group The Sugar Bureau, examined sugar and soft drink consumption among 1,300 children in the U.K. The study found no evidence that soft drinks alone affected children’s weight.
Rachel K. Johnson, RD, PhD, MPH, is a professor of nutrition at the University of Vermont and an American Heart Association spokeswoman. She served on the American Heart Association's 2009 panel that recommended limiting added sugars, including those in drinks.
Johnson says she does not believe the science linking sodas to obesity and other health issues has been misrepresented or over-reported.
“I don’t think anyone would say that limiting sugar-sweetened drinks is the only solution,” she says. “But to me, it is an important step in the right direction.”