Mixing alcohol with diet drinks can increase intoxication, U.S. researchers are warning consumers.

Drinking on an empty stomach is well known to increase alcohol concentrations. How diet drinks containing artificial sweeteners raise risk of intoxication hasn't been explored to the same degree, despite the greater potential for impaired driving.

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Participants' peak alcohol concentrations were above the 0.08 legal limit. (Eric Risberg/Associated Press)

When researchers had 16 men and women come to a lab three times for different doses of vodka, sweetened and diet pop or a placebo in a random order, they found that consuming alcohol with a diet mixer resulted in 18 per cent higher breath alcohol concentration compared with having the same amount with a sweetened mixer.

"Many people probably chose to mix their alcohol with diet mixers, because they're concerned about the number of calories they're consuming," study author Cecile Marczinski of Northern Kentucky University in Heighland Heights, Ky., said in an interview with CBC News. 

"But really having a higher blood alcohol concentration is much more harmful to your body," in terms of potential damage of the brain, liver and risk for alcoholism.

For the study published in this week's online issue of the journal Alcoholism Clinical and Experimental Research, Marczinski and colleague Amy Stamates measured the volunteers' breath alcohol concentrations and had them do computerized tests measuring response time and error rates like those of a driver who has decide to hit the gas or brake, they also found higher rates of impairment with the diet mixers.

"What I think is going on is that the stomach recognizes a sugary drink a bit like food. There's something to digest," Marczinski said. In contrast, the researchers suspect the diet mixers get through the stomach faster so blood alcohol spikes faster — like drinking without having something to eat.

"Sugar slows things down."

The researchers said while the peak alcohol concentrations were above the 0.08 legal limit for driving, willingness to drive ratings didn't differ between the alcohol and placebo groups. Participants appeared unaware of the differences in breath alcohol.

"The elevation in breath alcohol concentration associated with diet mixers warrants greater consideration and consumers should be made aware of this phenomenon," the study's authors concluded.

"Those that drink and drive should be made aware that they may go above the limit and that could have severe implication for them," Bernard Le Foll, who heads an alcohol research and treatment clinic at the Centre for Addiction and Mental Health in Toronto, said in an email.

The researchers cautioned that the findings should be repeated while measuring blood alcohol levels directly.

Earlier, smaller studies also found higher alcohol concentrations with diet drinks in men only.

Dennis Thombs, a professor and chair of the department  of behavioral and community health at UNT Health Science Center  was the author of the field study. In a release, Thombs cautioned that young women are more likely to drink diet colas at bars and nightclubs than young men and women should be aware of the greater risks.

Becky Robertson is a 22-year-old student in Toronto who favours mixing with regular soda. She compared the newly measured effect to ordering a double.

"Money wise, it's not getting more alcohol, but it's an added effect," she said. "People might opt for it."

The research was funded by the U.S. National Institute on Alcohol Abuse and Alcoholism and the U.S. National Institutes of Health.