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This post has been updated.

I admit it: I drink a Diet Pepsi just about every day. I love the stuff — with a meal, after a long run or when I'm just really thirsty. It's not the caffeine. For that, I reach for coffee. I can't really explain this, because I don't like the taste of Diet Coke or most other diet beverages.

Despite evidence that some of the things in my favorite diet soda are not particularly good for me, I've always justified the habit with the idea that at least I'm not consuming the huge amounts of sugar in a regular Pepsi. There are 69 grams of sugar and 250 calories in a 20-ounce Pepsi, according to the Pepsico Web site.

Now comes a study that threatens to shatter my carefully-crafted self-delusion. Researchers examined data taken periodically for nearly 10 years from 749 Mexican-Americans and European-Americans ages 65 and older in the San Antonio Longitudinal Study of Aging (known by the fine acronym SALSA).

[Parents are still being duped into giving their kids high sugar drinks]

They determined that daily and occasional diet soda drinkers gained nearly three times as much belly fat as non-drinkers, after they ruled out other factors such as age, exercise and smoking. Occaional diet soda drinkers added an average of 2.11 centimeters (.83 inches) 1.83 inches to their waist circumferences, while the non-drinkers added .8 inches .77 centimeters (.3 inches) over the 9.4 years of the study. Daily consumers like me gained a striking 3.16 inches 3.04 centimeters (1.19 inches).

Men, European Americans, people with a body mass index greater than 30 and people who did not have diabetes fared the worst.

You don't want belly fat (visceral fat in technical terms), especially as you reach your later years, when it is associated with greater incidence of mortality, cardiovascular disease, Type 2 diabetes and insulin resistance. High waist circumference is also one component of metabolic syndrome, a cluster of risk factors that also includes high triglycerides, blood pressure and blood glucose.

"This is a more vulnerable population," Sharon Fowler, an adjunct assistant professor in  the Department of Medicine at the University of Texas Health Science Center in San Antonio and lead author of the study said in an interview. According to one study, about a fifth of the U.S. population consumed some form of diet drink on any given day in 2009-2010, and 11 percent of those people drank 16 ounces or more.

A couple of caveats here that are worth mentioning: There is considerable debate over the impact of diet soda and artificial sweeteners, with various studies showing conflicting results. (Another Fowler study in 2008 showed significant increases in body mass index among diet soda drinkers.) This study, because of the way it was designed, could not prove cause and effect; it showed an association between drinking diet soda and increases in waist circumference. Most strangely, the data revealed no relationship between consumption of regular, sugary soda and waist circumference growth, which, Fowler acknowledged, would have been expected.

The research appeared online in the Journal of the American Geriatrics Society.

In a statement, the American Beverage Association, said that "previous research, including human clinical trials, supports that diet beverages are an effective tool as part of an overall weight management plan. Numerous studies have repeatedly demonstrated the benefits of diet beverages--as well as low calorie sweeteners, which are in thousands of foods and beverages--in helping to reduce calorie intake."

[World Health Organization to basically everybody: Stop eating so much sugar]

The Calorie Control Council, which represents producers of no- and low-calorie foods and beverages, also urged that the study "be treated with caution" due to some limitations. The organization noted that older people tend to lose muscle mass and gain waist circumference as a result of aging and contended that some important information on Mexican-American lifestyles, diet records and family histories were not available to the researchers.

Nevertheless, Fowler said, there are a number of possible explanations for the findings. A psychological one may be that regular diet soda drinkers conclude (as I have) that they are saving calories by not consuming sugary drinks and let themselves go overboard on other foods.

"There can be underestimation of the impact of other foods," she said. "People can give themselves extra permission [to eat]. They also can just do bad [calorie] math."

Based on other research, she said, the sweeteners and/or the acid in diet soda may have an impact on gut bacteria, the ability to handle sugar from other food and drink or the part of the brain that signals us to stop eating.

With so much uncertain, Fowler said, a safe path is to drink water, milk, 100 percent fruit juice, tea and coffee — perhaps adding a tiny bit of sugar or fruit juice for added sweetness in some.

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