Think the yogurt you buy in the grocery store is making a difference to your balance of good or bad bacteria? Think again.

Jeffrey Gordon, a microbiologist at Washington University in St. Louis, and his team gave a commercially-available probiotic yogurt containing five strains of bacteria to healthy adult volunteers and administered the same five strains to mice that harbored a subset of genetically-characterized human gut microbes.

The yogurt bacteria did not significantly alter population structure in any of the entrenched gut microbes, in humans or mice—a result that is not surprising, according to Mills. “To assume that you could eat a yogurt and numerically challenge what’s in your gut is kind of like dumping a gallon of Kool-Aid in your swimming pool and expecting it to change color,” he said.

But that doesn’t mean the yogurt didn’t change anything. Even though tests showed no difference in the population of bacteria, there was a change in the way certain carbohydrates were digested. So while you still have the same bacteria in your gut as before you ate the yogurt, now you’re getting even more out of those carbs you eat as well.

So eating special yogurt is no substitute for a good probiotic supplement.