A Single Vitamin Might Affect How Often You Poop, Huge Study Finds

We’ve written before at HuffPost UK about the best (and worst) fruits and teas to help you poop. But bowel movement frequency is a different issue. 

The NHS said that going for a “number two” anywhere from three times a day to three times a week can be considered normal if it’s how your gut usually behaves. However, 2025 research suggests that the “Goldilocks zone” is once or twice a day; and this is linked to fewer diarrhoea- or constipation-related “toxins”.

In a paper published in Gut, researchers found that a B vitamin may be linked to your foods’ “transit time”, too.

Vitamin B1, or thiamine, may help to determine how often you poop 

This study looked at DNA and health information from 268 ,606 participants; naturally, it recorded how often they pooped, too.

The scientists identified 21 genomes that seemed to affect stool frequency (SF). Some of these – like those which were linked to bile-acid regulation and the muscles that contract in our guts – were expected. 

But 10 hadn’t been reported before, “including a previously unrecognised role for vitamin B1”, the paper reads. 

Genes SLC35F3 and XPR1 were among this list of 21 relevant genes. Both of them help your body to transport and make use of vitamin B1, or thiamine, which turns our food into energy.

To see whether that result played out in real life, the study authors then looked at health information from 98,449 participants in the UK. They wanted to see if people with higher thiamine intake seemed to pass stools more often – and they did.

Even here, though, DNA mattered. Natural variants in the SLC35F3 and XPR1 genes seemed to give vitamin B1 more or less of an impact on participants’ bowel habits.

This could give a “roadmap” for better gut care 

The study’s first author, Dr Cristian Diaz-Muñoz from the Gastrointestinal Genetics Lab (GGL), said: “We used genetics to build a roadmap of biological pathways that set the gut’s pace.

“What stood out was how strongly the data pointed to vitamin B1 metabolism, alongside established mechanisms like bile acids and nerve signalling.”

And Prof Mauro D’Amato, also of the GGL, added: “Gut motility problems sit at the heart of IBS [irritable bowel syndrome], constipation and other common gut-motility disorders, but the underlying biology is very hard to pin down.

“These genetic results highlight specific pathways, especially vitamin B1, as testable leads for the next stage of research, including lab experiments and carefully designed clinical studies.”