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And some of those rushing to file may be fueling up with diet soda. Americans are drinking more diet soda and other artificially sweetened drinks than in years past. And while it may seem like a good idea to cut out extra calories, researchers have found swapping real sugar drinks for zero calorie options hasn't produced the expected results.

Now, as NPR's Allison Aubrey reports, a new study sheds light on why.

ALLISON AUBREY, BYLINE: Americans' love affair with diet soda has been decades in the making.

(SOUNDBITE OF DIET COKE AD)

UNIDENTIFIED SINGERS: (Singing) Just for the fun of it. Just for the one of it. Just for the taste of it, Diet Coke.

AUBREY: Diet Coke was introduced in the early 1980s and heavily marketed. As a nation, we started drinking lots of it, along with other diet drinks. And even in the last 15 years, consumption has continued to rise. People think by drinking it, they're helping control their weight.

RACHEL JOHNSON: Well, I think they make that assumption.

AUBREY: Rachel Johnson is a professor of nutrition at the University of Vermont.

JOHNSON: But what the mistake a lot of people make is that they compensate for the calories that they save in the diet soda with other foods and beverages, rather than eliminating those calories from their diet.

AUBREY: It happens so easily. You go to the vending machine for a midafternoon snack and after you push that diet soda button, you feel you've got a free pass for a bag of chips. This compensatory eating may be the reason that past studies have found that people who drink a lot of diet soda tend to gain weight over time. The weight gain also left researchers wondering if there's something about aspartame or other artificial sweeteners that primes people to want more sweet things, or in some other way makes them more susceptible to metabolic disorder – a condition characterized by excess abdominal fat, elevated blood sugar and high blood pressure. Some research has pointed in this direction.

To help shed light on the relation between diet soda and health, researcher Kiyah Duffey - at UNC Chapel Hill - decided to take a new approach. She says most of the prior studies had not really evaluated diet soda drinkers' overall patterns of eating.

KIYAH DUFFEY: We really did think that it was an oversight.

AUBREY: She wanted to know if a healthy eater who has a diet soda habit would end up better off than a person with a bad diet and a diet soda habit.

DUFFEY: You can imagine two different kinds of people – one who orders a Big Mac and a large fry and an apple pie and a Diet Coke; and someone else, who eats a grilled chicken salad and an apple and a Diet Coke.

AUBREY: To unravel the differences, Duffey and her colleagues studied data collected over 20 years from about 4,000 adults who had completed in-depth food frequency questionnaires, which detailed how often, and in what amounts, they'd eaten a hundred different kinds of foods.

DUFFEY: So it will ask something like, how often in the last 30 days did you drink low-fat milk? And then it would go on to ask about potatoes, fruit.

AUBREY: And, of course, it asked about soda and diet sodas. Duffey explains when she compared people who were eating what she calls a prudent diet - high in fruits and vegetable and low-fat diary - with people who were eating a standard Western diet - heavy on fried foods, fat and sugar - she found that there were very substantial differences. The prudent eaters were significantly less likely to develop the risk factors of metabolic disorder, even if they did drink diet soda.

DUFFEY: This kind of helps to make things a little bit more clear – that it's not necessarily the diet beverage that is the problem.

AUBREY: Duffey says the take-home message here is that people should pay attention to their overall diet.

DUFFEY: So what this suggests is that it's really the dietary pattern that's driving the association with health, and not the diet-beverage consumption.

AUBREY: Duffey says an important point to note is that the absolute healthiest people in her study were those who drank no diet soda, and had a prudent diet.

Allison Aubrey, NPR News.

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