Turning your poo blue can give you important information about your gut health, says a new study. Celebrities and scientists are all getting involved

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Would you take the ‘blue poop’ challenge? Thousands of people across the globe are turning their poo blue in a challenge issued by the eminent microbiome expert and self-experimenter Prof Tim Spector and health research app Zoe, to find out how healthy their gut is.

The idea involves baking muffins with added blue food dye, eating two for breakfast and monitoring how many hours it takes for your poo to turn blue. You then enter the details at on the Zoe website, which compares you to other poopers. It also tells you how your poop time relates to gut health and offers tips on how to improve it.

“The blue poop challenge is a simple way to find out what’s going on in your gut,” explains  Prof Spector on Instagram , where the #bluepoop hashtag has a lively following. “All you need are a couple of blue muffins and a spirit of curiosity.”

What's the science behind the blue poop challenge?

Sounds whacky, but it’s backed by science. A recent study of 863 blue poopers, published in leading journal BMJ Gut  concluded that knowing how long it takes food to pass through your system is more helpful than knowing what your poop looks like or how often you go. "Gut transit time, measured via the blue dye method, is a more informative marker of gut microbiome function than traditional measures of stool consistency and frequency," it states.

What does sluggish digestion mean for your microbes? “Slower transit time is associated with less favourable gut bacteria," explains dietitian and intestinal specialist Sophie Medlin to the BBC. "A faster transit time is associated with a better gut bacteria profile."

Who is getting involved?

The blue poop challenge has caught the imagination of hundreds of people who are posting their bakes – and resultant blue tongues – on Instagram. Celebrities are getting involved too. Actor couple Emma Thompson and Greg Wyse filmed themselves taking the challenge, eating their muffins while doing a hilarious film quiz, in a video that’s been viewed more than 28,000 times.

Emma Thompson and Greg Wyse take the bluepoop challenge Image: Instagram  @zoe

It’s also gone viral on Tiktok thanks to baker Julius Fielder, known as @bakinghermann, and his gourmet rye, roasted pecans and dark chocolate muffins (blue, of course), which have received 451,000 likes.


Test your own gut health with this science experiment. More info on my IG #BluePoopChallenge #guthealth #muffins #asmr #bluefood

♬ original sound - Julius Fiedler

Where can you find blue muffin recipes?

There’s a suggested recipe on the Zoe website, but essentially you can add blue dye to your favourite one. It can’t be any old dye though, you need the concentrated ‘professional’ gel type that is used to dye icing and is not water-soluble.

Does it have to be a muffin?

I'm guessing that the muffin idea was one that generated the most compliance. Who needs persuading to eat a muffin for breakfast, even if it is blue? Prof Spector recently added blue dye to his kefir smoothie too.

Is blue food colouring really safe to eat?

I was told never to touch any kind of bright blue food as it was filled with E numbers.   Zoe says  it’s fine in small quantities. The site adds that the test is not designed for those under 18, although Instagram is not short of toddlers with blue faces after helping themselves. You try keeping a kid away from a cake that channels Sonic the Hedgehog, especially if Nanny McPhee/Prof Trelawney is doing it.

What does a nutritionist think?

“E number food colouring isn’t great, however, used as a one-off isn’t going to cause much of an issue,“ says Daniel O’Shaughnessy. You don't have to use blue dye, he points out. You can measure your gut transit time with sweet corn or beetroot. “Beetroot turns everything purple-ish and, well, sweet corn just comes out the other end.”

Image:  ZOE  /  @tastyasfit 

What happened when we tried it?

For the sake of speed, I entered my gut transit time as I know it to be, most of the time. I typed in 27 hours plus some details about myself and my diet and immediatley received my ‘poop personality’. I was 'super pooper' coming in marginally better than the average test score of 29 hours. I'll still need to do the full blue poop test to be sure.

I was then matched with a 'gut twin' called Gabriella, who was the same age and size and who had an identical poop time and similar diet to mine. She’d taken part in a study to examine her gut microbes and diet and so her gut had real data behind it. The theory is that my microbe composition could be similar to hers. Her bio said that she had 11 good microbe types out of a possible 15 that were measured. If I was anything like her, I was doing well.

The site had further suggestions for how to take my gut health to the next level. “Eat 30 different plants a week,” it said. “It’s a fun challenge that your gut bugs will love.”

What else affects how long it takes for food to pass through your gut?

Gut microbes are only one, albeit imporatnt, factor in gut transit time says Daniel. “If it’s too fast, it could be a parasite, food sensitivity, IBD, stress, or overconsumption of things such as coffee. Too slow: not enough fibre, maybe SIBO [small intestine bacteria overgrowth] infection, hypothyroidism,”  he points out.

Ideally, we should be having two movements per day, he says, although personally, I'd struggle with that!  “This should be passing a stool with ease, it looking healthy and not having to clean much after.”

The bottom (!) line

To have a truly accurate idea of what’s going on in your gut, you need to have a stool test which is costly (I’ve just paid £350 per one) and needs to be assessed by a nutritional specialist, who can personalise your treatment. I went to Evekalinik.com.

The blue poop challenge is at least is a test anyone can do and helps poeple become more aware of the importance of gut health. As we know, having a thriving gut microbiome is linked to many health markers everything from immunity, to skin health and mood.

I can’t resist a challenge though and will be baking my blue muffins soon. Let us know if you’ve tried it!

Sign up for the blue poop challenge at  joinzoe.com/bluepoop

Follow Victoria on Instagram  @victoriawoodhall

MORE GLOSS: Surprising things abotu gut health a GP wants you to know