Your gut transit time tells us how long it takes food to travel through your body. Our research shows that gut transit time is linked to gut microbiome health. As part of your ZOE test, you will eat two blue-coloured cookies for lunch that will allow you to measure your own gut transit time.

In the 14 days after eating your blue cookies, look out for any blue-greenish colour in your poo and on the toilet paper when you wipe. When you first see green-blue poo, log the time in your ZOE app. We'll use this information to continue advancing gut microbiome research.

That's fine, 30% of ZOE members have reported not seeing blue poo. Don’t worry, this experiment is purely for science so you will not need to re-do the blue cookies.

If you don't see any blue (or greenish-blue) poop, this could be because:

The dye used to bake blue cookies is a highly concentrated icing colouring that is safe to eat in small quantities. The pigment in the dye used to bake these blue cookies passes through your digestive tract unchanged, which is why they turn your poop blue. We have also made sure that dye quantities are within regulatory amounts.

Scientists working with ZOE published a paper in the peer-reviewed journal, Gut, all about blue poop. As part of our PREDICT studies, we measured the gut transit time — the time it takes for food to travel through the gut — of thousands of people using foods coloured with blue dye.

We discovered that:

You can read the full paper here.

If you don't see any blue poo, don't worry, this experiment is purely for science.

You can always redo the test at home in your own time by taking part in the Blue Poop Challenge and baking your very own blue cookies at home — it’s easy and fun. You can follow our recipe or choose your own.