THE BLOG
03/13/2014 08:59 pm ET Updated May 13, 2014

Will You Have Green Poop on St. Patrick's Day?

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"Will I poop green today?" That's probably not the most common question on your mind, but maybe for St. Patrick's Day, the potential hue of your fecal matter has your brain working overtime. I love to celebrate the holiday, especially sipping a pint of green beer. And what about those green frosted cupcakes someone in the office is sure to bring in? So much green food, so little time. But how will all of that green food and drink fare against your digestive system? Will it come out victorious, making your poop look like something left behind by a leprechaun? Or will your digestive system literally metabolize the crap out of it?

Let's focus on beer first, because most of us will probably have a green beer or two this St. Paddy's Day. When making green beer, a drop or two of food coloring is added to a light beer, like a Pilsner or an IPA. (If you're curious about exploring the science behind what separates light beers from dark beers, check out this article about the Maillard reaction, which is responsible for the variety of colors. But just a warning: This will give you enough organic chemistry to make your head spin, or maybe that's just the alcohol.) Even though a drop or two of food coloring is enough to turn a beer green, it's likely not enough to have noticeable results in the toilet. But what would happen if you were to keep eating green food coloring? Could it change the color of your poop, and why?

The green food coloring used in the U.S., like that of the McCormick brand, is composed of FD & C Yellow 5 and FD & C Blue 1. For simplicity, these colors have been given the names "Tartrazine" and "Brilliant Blue," respectively. Brilliant Blue is a byproduct of petroleum and is poorly absorbed by the GI tract. In fact, 95 percent of undigested blue coloring can be observed in feces. But Tartrazine, on the other hand, is digested by GI microorganisms and is not observable in feces, at least in rats, anyway.

After many questionable but completely humorous Google searches, I've found some information that may explain the green poop phenomenon. One might think the answer to this question is as simple as green food coloring dyeing our feces green. Not quite! Because Brilliant Blue remains chemically intact as it passes through the digestive system, it retains its dyeing abilities. Tartrazine loses its dyeing abilities after exposure to gut flora. OK, so green food coloring going in is definitely not green food coloring going out. This struck me as odd, because I would imagine excrement would then be blue. Clearly there is something else going on!

Among the Internet community, many people have experienced green poop after eating things like blue-raspberry licorice, Boo Berries, and Welch's Grape Soda, and Greg Laden even suggests that mixing Metamucil with blue Gatorade will result in a lovely green poop. Now, what do all these foods have in common? If you look at their ingredients, they all contain Brilliant Blue coloring. So here is an interesting piece to the puzzle: People are eating foods with blue food coloring but excreting green. What's the deal?

Sifting through all the fascinating discussions about stool colors on the Internet also led me to multiple statements that Brilliant Blue mixes with bilirubin, which is a yellow chemical component of bile in the digestive system that is responsible for turning urine yellow and poop brown. As bilirubin passes through the intestines, it first turns into an uncolored product called urobilinogen. At this point, urobilinogen can turn into the yellow urobilin and leave the body as urine, or into the brown stercobilin, which gives feces the characteristic brown color we all know and love. However, this process requires time, as bacteria in the intestines are responsible for breaking down bilirubin. If food moves too quickly through the intestines, it's possible that a lot of that yellow bilirubin could remain unchanged, which could then result in a yellow-colored poop. Now, if we were to mix that with Brilliant Blue, wouldn't it seem reasonable that we would be left with a lovely green hue? This is what I think might be going on, anyway.

Now, as humans, we are all different, and some of us may be more sensitive to food coloring than others. With that in mind, it's highly possible that you may experience something completely different after indulging in all the green treats this holiday. You may see no color change at all! If you're really inspired, maybe you could run your own little study like Dave J. did. And if you want to take it to the next step and turn your urine green this holiday, look into the combination of methylene blue and liquid vitamin B complex. I've heard the results are shocking! Most importantly, never stop wondering.

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