Diet soda is a consistently fun target for psychology and neurobiology researchers. Past studies have linked drinking it to a plethora of badness, most ironically: weight gain (though I doubt those studies made a dent in sales. Coke Zero came out shortly after the weight gain findings were released and last I checked it was outselling Diet Coke). A recent study in the journal Psychological Science continues the tradition by investigating whether drinking diet soda makes people more impulsive.

Researchers used the always gratifying delayed gratification ploy to test the hypothesis. Participants responded to a series of questions asking, in different ways, whether they'd prefer to receive a moderate amount of money tomorrow or a larger amount at a later date.

The first few questions were asked before the participants drank either a regular soda (containing sugar) or diet soda (containing aspartame), and more questions were asked after they finished drinking. In addition, blood glucose levels were measured before and after the participants finished the sodas.

The results: participants who drank regular soda, and therefore had higher blood glucose levels, were significantly more likely to choose receiving more money at a later date. Those who drank diet sodas and had lower blood glucose levels were more likely to take the smaller amount of money upfront.

The study authors think the reason the diet soda drinkers didn't delay gratification is that higher blood glucose levels provide the mental juice for our brains to be more future-oriented. This could be because envisioning the future--in all of its fuzzy abstractness--drains more noggin energy than observing the concrete here-and-now.

So, when someone drinks a diet soda, which is designed to trick the brain into thinking it's getting a nice dose of sugar, the brain eagerly awaits an energy surge. When it never comes, panic alarms go off. The brain interprets the lack of blood glucose as a calorie shortage, and impulse is given free reign to get the body what it needs. Delaying gratification under those conditions isn't going to be easy.

The takeaway here is not to start drinking regular soda instead of diet soda--it's to stop drinking soda, or any other sugary or fake-sugary drinks. The impulse culprit in this study isn't really diet soda; it's erratic fluctuations in blood glucose levels caused by loading up on sugar or chemicals that mimic sugar. Theoretically, if you level off the glucose highs and lows, decision-making will benefit.

And your brain will stop smacking you around.

Copyright 2010 David DiSalvo

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