There may be a scientific explanation for why some people order a diet soda with a burger and fries. A new nutritional study suggests many people who drink diet soda tend to eat more junk food to compensate for the lower calories.

The study, conducted by University of Illinois health researchers, examined the dietary habits of more than 22,000 U.S. adults and found that diet-beverage consumers often compensate for the absence of calories in their drinks by gorging on food loaded with sugar, sodium, fat, and cholesterol.

University of Illinois kinesiology and community health professor Ruopeng An examined 10 years of data from the National Health and Nutrition Examination Survey, conducted by the National Center for Health Statistics, which asked participants to recall everything they ate or drank over the course of two nonconsecutive days.

"It may be that people who consume diet beverages feel justified in eating more, so they reach for a muffin or a bag of chips," said lead researcher Ruopeng An. "Or perhaps, in order to feel satisfied, they feel compelled to eat more of these high-calorie foods."

A third possibility is that people opt to drink diet beverages because they feel guilty about indulging in unhealthy food.

"It may be one — or a mix of — these mechanisms," An said. "We don't know which way the compensation effect goes."

For the study, An compared participants' daily calorie intakes, including their consumption of junk foods and five types of beverages — diet or sugar-free drinks; sugar-sweetened beverages, such as sodas and fruit drinks; coffee; tea; and alcohol.

Using a U.S. Dept. of Agriculture food database, An compiled a 661-item list of junk foods — known scientifically as “discretionary foods” because they are not among the major food groups or required by the human body but may add variety to a person's diet. Such nutrient-poor foods include products such as cookies, ice cream, chocolate, fries, and pastries.

An’s findings, published in the Journal of the Academy of Nutrition and Dietetics, indicated adults who drank diet beverages consumed more discretionary foods.

"If people simply substitute diet beverages for sugar-sweetened beverages, it may not have the intended effect because they may just eat those calories rather than drink them," An said. "We'd recommend that people carefully document their caloric intake from both beverages and discretionary foods because both of these add calories — and possibly weight — to the body."