Switching to diet soda doesn’t help people lose weight the way they hope. Two recent studies shed light on what is going wrong.  

In February 2008, a study by Denver-based scientists on brain reaction to real and artificial sugars was published in the journal Neuroimage. Employing functional magnetic resonance imaging (MRI), these researchers watched the brain activity of 12 women as they tasted water sweetened with either sugar or an artificial sweetener.  

Both substances activated a response in the women’s brains, but in different areas. Only real sugar engaged the dopaminergic areas in the midbrain that signal pleasantness. In addition, sugar triggered a stronger brain response than the artificial sweetener. It seems that the brain distinguishes between real and fake sweeteners, even though the study participants were unable to tell the difference.  

In a study published in the May 2012 issue of Physiology and Behavior, researchers from San Diego took this a step further. Twenty-four volunteers were selected; half drank diet soda at least once a day. The other 12 rarely, if ever, drank diet soda.  

Using MRIs, the researchers analyzed how the subjects’ brains responded to small amounts of water sweetened with either sugar or saccharin. Again, while participants couldn’t distinguish between the sweeteners, their brains did—or at least the brains of those unaccustomed to artificial sweeteners did. 

In the group that didn’t drink diet soda, sugar and saccharin lit up distinctly different parts of the brain. In those who drank diet soda regularly, no clear difference was detected. In addition, the more often they drank diet soda, the less the brain region called the caudate head was activated. This region is associated with food motivation and reward sensations, and decreased activation is linked to a higher risk for obesity.  

The conclusion: If we fool the brain too many times with “fake” sweeteners, it forgets how to sense energy intake or feel satiety from eating sweets. The more diet soda consumed, the worse this gets. 

This may finally explain why switching to diet foods seems to backfire. The theory is that initially the brain knows it’s being cheated and increases caloric intake. Then at some point it loses the ability to track calories and no longer believes that anything is sweet, and thus it never feels satisfied. 

If you want to lose weight, research shows it works best to eat real food, but less of it, and exercise more.