Same potato starch, same dose, run a second time: the bifidobacteria rose again and stools settled. The gut bug the trial was built to find didn't show

Resistant starch is starch that gets past the small intestine undigested and is fermented by bacteria in the large bowel. Raw potato starch is rich in it, and one Canadian company, MSP Starch Products, sells a processed version called Solnul as a prebiotic, meaning a food for gut bacteria that is supposed to bring a health benefit. A 2023 trial of the product in Nutrients reported that 3.5 g a day for four weeks raised two groups of bacteria, Bifidobacterium and Akkermansia, and made stools more regular. A new paper in Nutrients, published on 27 October, describes an attempt to get the same results again.

Repeating a trial exactly is rarer than it sounds. The authors note that two trials of 12 g a day of chicory inulin, a well-known prebiotic fibre, failed to reproduce each other's results on stool form or bifidobacteria. So this trial copied the first: the same 3.5 g dose (about 2.1 g of actual resistant starch), the same four weeks, the same kind of healthy adults from the same part of Ontario, and the same placebo, a fully digestible corn starch matched for calories. It ran at a contract research site in Guelph between May and October 2025, with 74 adults aged 18 to 69 randomised and 70 included in the main analysis.

The main test this time was Akkermansia, a mucus-eating bacterium that gets a lot of attention in gut research. It failed: the change in Akkermansia on potato starch was no different from placebo (p = 0.49). Akkermansia did rise from the starting level within the potato starch group and not in the placebo group, but that is a weaker comparison. The bifidobacteria result, a secondary goal, did come back: Bifidobacterium rose more on the potato starch than on placebo. That held for the 70 people who stuck to the protocol, but not when the three people who skipped too many doses were counted as well (p = 0.10).

Stools changed in a quieter way. Participants scored their last seven bowel movements on the Bristol Stool Form Scale, a seven-point picture chart from hard pellets to watery. The first trial had found fewer very hard and very loose stools; this one did not. What both trials showed, once the first trial's data were re-analysed the same way, was a larger share of 'normal' Type 3 or 4 stools on the potato starch: 65% against 53% on placebo here, and 71% against 57% in 2023. People on the starch also had almost one day fewer without a bowel movement over the final two weeks. Symptom scores for bloating, pain and reflux did not change, and no side effect was put down to the potato starch; the only two possibly linked to a product were bloating on the placebo.

Two things are worth keeping in mind. The study was run by the company: the lead author works for MSP Starch Products, and one co-author has been paid as a consultant by other fibre makers (but not by MSP). And the effects are small and measured in healthy people, so they say little about constipation or IBS, which the authors suggest as the next step. The authors put the weaker microbe results partly down to a different lab method for extracting and sequencing stool DNA, and say the trial may have been too small to pick up the Akkermansia change they expected.