Vinegar for Constipation? A New Japanese Trial Says Maybe

2026-08-31 · By the Diet for Constipation data editorial team · 1,034 words

A randomized controlled trial from Japan, accepted for publication in Scientific Reports in 2026, tested something unusually humble: a vinegar drink. Forty-four adults with mild constipation drank either a vinegar beverage or a placebo every morning for four weeks, and the vinegar group ended up with roughly 1.2 additional bowel movements per week. It is a small study with big headlines potential — so before you start drinking vinegar, let us walk through what it showed, what it did not, and how a data team reads a trial like this.

What the trial actually did

The study was randomized, double-blind and placebo-controlled — the strongest trial design for testing whether an intervention works beyond expectation. Researchers recruited 44 Japanese adults aged 20 to 69 who were otherwise healthy but constipation-prone: three to five bowel movements in the week before the study began. That detail matters. These were not people with severe or chronic constipation; they were mild cases at the edge of normal.

Participants drank 200 milliliters of liquid every morning for four weeks. The active drink contained 750 milligrams of acetic acid — the active component of vinegar. The placebo contained 30 milligrams of citric acid. Both were apple-flavored and sweetened identically, so neither the participants nor the researchers knew who was drinking what. Everyone kept their usual diet and lifestyle and recorded bowel movements in a diary throughout.

What the results showed

After four weeks, the vinegar group had significantly more bowel movements than the placebo group — about 1.2 more per week. The researchers also observed earlier signals: more frequent bowel movements and more defecation days during the first week, and a tendency toward increased stool volume in the first two weeks. Some participants reported a better sensation of complete emptying.

One interesting negative finding: gut microbiota diversity did not change. The researchers had wondered whether acetic acid might reshape the bacterial population, but the data did not support that. Their alternative hypothesis is mechanical rather than microbial — vinegar may influence the gastrocolic reflex (the signal that makes your colon move after a meal) or intestinal movement itself, though these were not directly measured either.

How much should this move your behavior?

Our honest read, in order of confidence. First, the trial is real and well designed for its size — randomized, blinded, placebo-controlled. Second, the effect is real within this trial but modest: 1.2 extra bowel movements per week, in a group that started at three to five. Third, the population was narrow — healthy Japanese adults with mild symptoms — and the study explicitly calls for larger and more diverse trials before drawing conclusions for the wider constipation population.

There are also limitations the researchers flagged themselves: a single vinegar dose was tested, bowel diaries are self-reported, and dietary intake was not closely monitored. Any of these can inflate or obscure effects in a 44-person study. We would add that 200 ml of flavored vinegar water is not the same as a splash of vinegar on your salad — the dose was deliberate and standardized, which is exactly how you should handle it if you experiment.

It is also worth asking who the study population was not: there were no older adults, no people with IBS, no chronic constipation cases on medication, and no children. The trial answers its own narrow question well — whether a standardized vinegar drink helps mild, diet-and-lifestyle-manageable constipation in healthy adults. The further your situation is from that description, the less the result transfers.

Where vinegar fits in the evidence stack

Compare this with the interventions we rate green. Prunes and kiwifruit carry randomized trials with head-to-head wins over established options. Psyllium has a deep trial record. Fiber as a principle has decades of clinical consensus behind it, plus mechanistic support from studies like the 2026 astronaut research we covered earlier this month — where gut bacteria starved of fiber shifted into a less healthy fermenting mode.

Vinegar sits at the bottom of that stack today: one small trial, one dose, one mild population, plausible-but-unproven mechanism. That does not make it useless — it makes it an interesting lead worth following, and a reasonable low-risk experiment for a curious adult with mild symptoms. It does not make it a proven fix, and it certainly does not displace fiber, water and movement as the foundation.

If you want to try it anyway

• Mirror the studied form: roughly 750 mg acetic acid diluted in 200 ml of water, in the morning. That is approximately one to one-and-a-half tablespoons of standard vinegar in a glass of water — not shots of straight vinegar.
• Protect your teeth: acetic acid is erosive to enamel. Drink it diluted, in one sitting, and rinse your mouth with plain water after.
• Give it the trial's timeframe: four weeks, tracked in a simple diary. If nothing changes by then, the honest conclusion is that it does not work for you.
• Check interactions: regular vinegar drinks are not automatically right for people with acid reflux, gastritis, or on certain medications. When in doubt, ask.

The takeaway

This is what early evidence looks like: a clean small trial, a modest positive effect, a plausible mechanism, and a clear call for replication. We will update our coverage if larger trials follow. Until then, vinegar goes in the "interesting, unproven, low-risk" column — behind prunes, kiwifruit, psyllium, oats, lentils and the entire green-rated list, all of which carry far stronger evidence and are food-first besides.

Sources

• Aoki Y, Yoshimoto J, Tokushige A, et al. (2026). Vinegar beverage consumption improves defecation status in constipation-prone healthy adults: a randomized, double-blind, placebo-controlled trial. Scientific Reports. DOI: 10.1038/s41598-026-67164-y
• Medical Dialogues, Medical Bulletin, Aug. 26, 2026 (trial summary).
• NIDDK: Eating, Diet & Nutrition for Constipation.

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→ Related: Best Foods for Constipation

This article is informational only, based on published research and USDA data. It is maintained by our data editorial team — not medical professionals. Nothing here is medical advice.