Gut transit time measured with a novel blue dye method is a more informative marker of gut microbiome function compared with traditional measures of stool consistency and frequency, according to a study in Gut.

The Personalized Responses to Dietary Composition Trial (PREDICT 1), a single-blinded intervention study conducted from June 2018 to May 2019, sought to quantify individual metabolic responses to standardized meals. The study included data from twins and unrelated adults from the United Kingdom (n = 1002; 723 female; average age, 45.58 years) and the United States (n = 100, 68 female; average age, 41.33 years). Data were available for 866 participants (n = 778 from United Kingdom; n = 88 from United States).

The participants consumed 2 muffins with blue food coloring paste within a 10-minute period along with chocolate milk. With use of the blue dye method, transit time was measured from the time of muffin consumption to the first visualization of blue within a stool.

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The median gut transit time for the whole study population was 28.7 hours. The study authors found that gut microbiome taxonomic composition accurately discriminated between gut transit time classes (0.82 area under the receiver operating characteristic curve). In addition, longer gut transit time was associated with Akkermansia muciniphila, Bacteroides spp, and Alistipes spp (false discovery rate-adjusted P values <.01).

Scores on the Bristol Stool Form scale and bowel movement frequency were weaker, according to machine learning analyses, compared with the association between the gut microbiome and gut transit time using the blue dye method. Gut transit time had an independent positive association with visceral fat (beta = 0.83) and postprandial responses (beta = 0.69), with longer gut transit time predictive of greater visceral fat and higher postprandial responses.

Among several study limitations, the researchers noted that the blue dye method has not been validated in comparison with other gut transit methodologies, such as the radio-opaque marker technique or scintigraphy. Additionally, stool consistency was assessed based on recall within the past 3 months, which may have led to some inaccurate data.

“The blue dye method is a novel, inexpensive, and scalable method of gut transit assessment providing valuable gut health and metabolic insights,” the study authors commented. “Its wide use in both research and clinical settings could facilitate the advancement of our understanding of gut function and its determinants, as well as the complex interactions between gut physiology and health outcomes.”

Disclosures: This work was supported by Zoe Global Ltd and other organizations, and some of the authors reported affiliations with Zoe Global Ltd. Please see the original reference for a full list of disclosures.


Asnicar F, Leeming ER, Dimidi E, et al. Blue poo: impact of gut transit time on the gut microbiome using a novel marker. Gut. Published online March 15, 2021. doi: 10.1136/gutjnl-2020-323877