Why does Burger King’s black burger turn poo green in the U.S. and the U.K., but not Japan?

If you’ve gone on a few overseas trips, you may be familiar with the phenomenon of travel poo, wherein your stool takes on a different hue for a few days as you adjust to local ingredients. It’s far less common for the opposite to occur, but that’s what seems to be happening with Burger King’s black burgers, which have become a repeating success story in Japan.

Burger King is currently offering its darkly colored sandwich in the U.S. and the U.K., but many are reporting that while the company turned the burger’s bun black, the burger is turning their poo green. But what’s behind this transformation, and why didn’t it happen in Japan?

While in Japan Burger King seems to offer the Kuro Burger (kuro being the Japanese word for “black”) whenever it feels like it, it’s timed the sale of its American and British counterpart to coincide with Halloween, dubbing it the Halloween Whopper.

With the year’s spookiest holiday becoming increasingly popular, and the Western palette increasingly adventurous, plenty of customers in the U.S. and U.K. have already tried the Halloween Whopper at their local Burger King. People seem generally pleased with the taste, but what most diners are talking about is the color: first of the bun, and then their poo.

Other online comments from the English-speaking world have included:

“It made my poo the color of Piccolo from Dragon Ball.”

“It was a toy-like, or anime-like, green color.”

“Mine was blue-green. Where has my brown poo gone?”

“It took three days for mine to come back.”

“I bet Burger King wasn’t expecting this to happen.”

We were a little confused by all this chatter, and not just the puzzling line about green poo in anime (is that some new dojiinshi trend?). See, Burger King has released multiple versions of the Kuro Burger in Japan, and not once have we heard people here say it changed the color of their semi-solid waste.

But maybe it’s just that people in Japan are more discreet about what goes on in their bathrooms, and are thus uncomfortable telling the entire Internet about their poo. So to get to the bottom of things, we checked with two of our RocketNews24 reporters who have both eaten the Japanese Kuro Burger and have no qualms about grossing people out, Mr. Sato and P.K. Did they have green poo after eating the Kuro Burger?

▼ Mr. Sato: “No, mine wasn’t green.”

▼ P.K.: “I didn’t really think about it.”

We’re taking that to mean that P.K. didn’t experience any unusual coloring, and not simply that his lifestyle is such that he’s become blasé to making particularly colorful deposits in the First Bank of Porcelain. So what’s the reason for the regional lockout on green poo?

Probably the ingredients used. In Japan, the various versions of Kuro Burgers get their shadowy shade from squid ink or bamboo charcoal, neither one of which is especially shocking to Japanese palates. In America and the United Kingdom, however, squid itself isn’t so widely popular, let alone its ink, and the number of people who find the prospect of eating charcoal (bamboo or not) alluring constitutes an even smaller demographic.

As such, neither of those ingredients is used in the Halloween Whopper. Website Metro theorizes that the Halloween Whopper’s black bun is the result of dark green food coloring, which apparently is effective in changing the color of what comes out of your digestive tract along with what goes into it.

The explanation makes sense, especially when we recall that eating Burger King Japan’s red Aka Burgers, colored with tomato powder, didn’t change the color of our stool either (although its super-spicy “Angry Sauce” did provoke some extra bathroom runs). In response to the Halloween Whopper’s surprising effect, Burger King has said that food coloring makes up less than one percent of the sandwich’s ingredients, and while many diners’ poo has changed color, there haven’t been any complaints about the black burger causing any pain or indigestion. In other word, it seems like the Halloween Whopper is safe to eat, even if its Halloween spirit is so strong it helps your poo get into costume.

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