The health risks (Alzheimer's?!) may not be worth the calorie savings.
Evidence about just how terrible diet soda is for your health is mounting. The latest? People who consumed at least one diet drink a day were nearly three times as likely to have a stroke or develop Alzheimer's disease compared to people who had them less than once a week, per new research in the journal Stroke. And you thought saving the calories and sugar would be worth it.
Before you spit your soda across the room, you should know how the study was conducted. Participants in the Framingham Heart Study filled out food-frequency questionnaires, which asked them to recall what they ate throughout the year. For instance, they might say they drank soda, on average, twice per day, once a month, or never. (As you can imagine, this can be an inaccurate way of gathering data. It's easy to forget what you eat. Quick: What did you have for lunch yesterday?)
Researchers had participants fill out these questionnaires three times over seven years. After ten years, they looked at how many cases of stroke and dementia occurred in two study groups: One group of 2,888 people over 45 were monitored for stroke, and a second group of 1,484 people over 60 were monitored for dementia; most of the participants were white (a notable fact given that people of color have higher stroke risk in the first place). They found that about 3 percent of the participants had a stroke (97 people), while 5 percent were diagnosed with dementia (81 cases of dementia, 63 of which were Alzheimer's disease). Diet soda-drinkers were 2.96 times more likely to have a stroke and 2.89 times more likely to develop Alzheimer's.
Ultimately, this was an observational study and it can only show an association between diet soda and these conditions, not that it caused them. The researchers did control for overall diet quality—that is, based on how well participants adhered to the US dietary guidelines—to rule out other nutrition habits at play. (They also controlled for age, sex, caloric intake, physical activity, and smoking.)
So though they can't say that diet drinks cause stroke, it still raises some questions, and "suggests diet [soda] may not be a healthy alternative," says study co-author Matthew P. Pase, a senior fellow in the department of neurology at Boston University School of Medicine. "People should be cautious about consuming diet sodas," he says. Ultimately, observational studies are helpful because they tell researchers, Hey, there could be something going on here—we need more research. And that's especially important considering about one in five Americans drink diet soda daily, according to 2009-2010 data from the Centers for Disease Control and Prevention.
There could be other factors, like body mass index or waist circumference, that are actually causing the increased risk. It may also be that people who have health problems like high blood pressure and diabetes switched from regular soda to diet since they think it's healthier—in which case it would be these conditions, not the soda, driving the increase in risk, notes Melina Jampolis, a board-certified physician nutrition specialist and author of The Doctor on Demand Diet, who was not involved in the study. The study did not control for BMI, diabetes, or high blood pressure.
Then again, it could be that diet soda is really, really bad. "We do know that artificial sweeteners have a particularly detrimental effect on the bacteria in your gut, which some research has shown increases risk of pre-diabetes and diabetes," she says. And having diabetes raises your risk for stroke and dementia.
While the study looked at stroke in adults over age 45 and dementia in those over 60, it's possible that these drinks could affect the brains of young people as well. Pase published a different study last month in Alzheimer's & Dementia, which found that drinking both diet and regular soda was associated with smaller brain volumes, a marker of early cognitive aging.
Ultimately, three times the risk for these diseases sounds scary—and it's not something to take lightly—but remember that only a small percentage of participants developed them. Translation: The results are significant and worrisome, but you're not destined to get dementia or have a stroke, Pase says.
One of the key things that will be picked out of the study was that the researchers didn't find an association between drinking regular soda and developing these conditions. Pase noted in a release that people in the study didn't drink as much regular soda as they did diet, which could explain why the didn't see a link. And it's not a pass to go back to drinking the full-sugar stuff.
"Regular soda has been associated in other studies with an increased risk of obesity, diabetes, high blood pressure, stroke, and dementia," Jampolis says. "My advice will not change. I have been recommending that people cut back on both diet and regular soda for both weight control and improving health."
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