Think you’re making a healthier choice when you reach for diet soda instead of a sugary soft drink? Think again.
Diet soft drinks may have minimal calories, but they can still have a major impact on your waistline, according to two studies presented at a meeting of the American Diabetes Association in San Diego.
Researchers at the Texas Health Science Center at San Antonio tracked 474 people, all 65 to 74 years old, for nearly a decade, measuring the subjects’ height, weight, waist circumference, and diet soft drink intake every 3.6 years. The waists of those who drank diet soft drinks grew 70 percent more than those who avoided the artificially sweetened stuff; people who drank two or more servings a day had waist-circumference increases that were five times larger than non-diet-soda consumers.
The findings are in line with those of a 2005 study, also conducted by researchers at the Texas Health Science Center, in which the chance of becoming overweight or obese increased with every diet soda consumed.
“On average, for each diet soft drink our participants drank per day, they were 65 percent more likely to become overweight during the next seven to eight years, and 41 percent more likely to become obese,” said Sharon Fowler, who was a faculty associate in the division of clinical epidemiology in the Health Science Center’s department of medicine at the time.
But how does something with no calories cause weight gain? Turns out that even if our taste buds can’t tell the difference between real and fake sugar, our brains can. Another study, also presented at the American Diabetes Association meeting on Sunday, found that after three months of eating food laced with aspartame (which is also found in many diet soft drinks), mice had higher blood sugar levels than rodents who ate regular food. According to Fowler, who worked on all 
three studies and is now a researcher at UT Health Science Center at San Diego, the aspartame could trigger the appetite but do nothing to satisfy it. That could interfere with your body’s ability to tell when you’re full—and could lead you to eat more in general.
It happens in humans, too. A 2008 study found that women who drank water sweetened with sugar and water sweetened with Splenda couldn’t taste a difference, but functional MRI scans showed that their brains’ reward center responded to real sugar “more completely” than it did to the artificial sweetener.
“Your senses tell you there’s something sweet that you’re tasting, but your brain tells you, ‘actually, it’s not as much of a reward as I expected,’” Dr. Martin P. Paulus, a professor of psychiatry at the University of California San Diego and one of the authors of the study, told the Huffington Post. So you chase that no-calorie soda with something more caloric, like a salty snack. The sweet taste could also trigger your body to produce insulin, which blocks your ability to burn fat.
Aside from the health problems that go along with a widening waistline, diet soft drinks have also been linked to an increase in diabetes, heart attack, and stroke. One study of more than 2,500 people found that those “who drank diet soda daily had a 61 percent increased risk of cardiovascular events compared to those who drank no soda, even when accounting for smoking, physical activity, alcohol consumption and calories consumed per day,” ABC News reported in February. And a 2008 University of Minnesota study of nearly 10,000 adults ages 45 to 64 found that drinking a single can of diet soda a day led to a 34 percent higher risk of developing metabolic syndrome, a collection of health problems that includes high blood sugar, high cholesterol, and high levels of belly fat.
“Drinking a reasonable amount of diet soda a day, such as a can or two, isn’t likely to hurt you,” writes Katherine Zeratsky, a nutritionist at the Mayo Clinic. “The artificial sweeteners and other chemicals currently used in diet soda are safe for most people, and there’s no credible evidence that these ingredients cause cancer.”
“Some types of diet soda are even fortified with vitamins and minerals,” she added. “But diet soda isn’t a health drink or a silver bullet for weight loss.”
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