Skipping Bowel Prep Cut Post-Surgery Brain Fog Risk in Small 80-Patient Trial

If you've ever had "brain fog" after a major surgery, trouble concentrating, fuzzy memory, that isn't quite right for weeks or months afterward, you weren't imagining it. Postoperative cognitive dysfunction (POCD, a measurable decline in memory and thinking speed after surgery and anesthesia) is a recognized complication, and researchers have long suspected it has something to do with the body's inflammatory response. A new pilot study now points a finger at an unexpected contributor: the bowel-cleansing routine given before colorectal surgery.

Key takeaways:

• In a small trial of 80 men, patients who skipped standard mechanical bowel prep before colorectal cancer surgery had a 7.5% rate of postoperative cognitive dysfunction, versus 55% in patients who received it.
• A gut-barrier marker linked to bacterial leakage into the bloodstream was elevated in the bowel-prep group, suggesting a possible gut-brain mechanism.
• This is a small, single-center, male-only pilot study (40 patients per group). The authors themselves say their design cannot prove bowel prep causes the cognitive decline, only that the two are associated.
• Nothing here means patients should decline bowel prep on their own; that is a surgical decision with its own safety rationale and needs a much larger trial to settle.

Mechanical Bowel Preparation Tied to Postoperative Cognitive Dysfunction

Patients who underwent colorectal cancer surgery without standard mechanical bowel preparation (MBP, the laxative-based bowel-cleansing routine typically given the day before intestinal surgery) had dramatically lower rates of postoperative cognitive dysfunction than patients who received it. In this pilot randomized controlled trial out of Erciyes University in Turkey, researchers randomly assigned 80 men undergoing elective colorectal cancer surgery to either no mechanical bowel prep (40 patients) or standard mechanical bowel prep (40 patients), then tracked their cognitive function before surgery and afterward. POCD, defined as more than a 1 standard deviation drop from a patient's own baseline score on the Montreal Cognitive Assessment (MoCA, a standard bedside test of memory and thinking), showed up in just 3 of 40 patients (7.5%) who skipped bowel prep, compared with 22 of 40 patients (55%) who received it (P<.001). The study is published in Therapeutics and Clinical Risk Management (DOI: 10.2147/TCRM.S613539) and was registered in advance as NCT05942404.

Why This Matters for Anyone Facing Colorectal or Major Abdominal Surgery

Postoperative cognitive dysfunction is common enough after major surgery, particularly in older patients, that it's become its own area of research, distinct from the temporary confusion of anesthesia (delirium) that resolves within days. POCD can linger for months and, in some patients, contributes to longer-term cognitive decline. For anyone scheduling colorectal surgery, or supporting a parent through one, a factor that plausibly triples or more the odds of postoperative brain fog is worth understanding, even in a small study, because it fits a growing body of separate research showing that mechanical bowel preparation measurably alters the gut microbiome in colorectal surgery patients, a plausible first domino in the mechanism this pilot study proposes.

A Small, Carefully Randomized Pilot, Not a Definitive Trial

This was explicitly designed as a pilot study, a small trial meant to generate preliminary data and test feasibility, not to definitively prove an effect. The 40-patients-per-arm sample size was chosen for feasibility, not calculated to achieve statistical power, which the authors acknowledge up front. Participants were men aged 45 to 80 in reasonably good baseline health (ASA class II-III, a standard anesthesia-risk classification), all at a single center. Cognitive testing happened at three points: before surgery, day 15 after surgery, and day 90 after surgery, using the MoCA and a secondary test (SMMSE). Researchers also drew blood at each timepoint to measure tau protein (a marker of neuronal injury), BDNF (brain-derived neurotrophic factor, a protein that supports neuron health), and LBP (lipopolysaccharide-binding protein, a marker that rises when bacterial components leak across a compromised gut wall). The idea that bowel prep itself can reshape gut bacteria composition after colorectal surgery has separate support: a pilot study of surgical bowel preparation and intestinal microbiota found measurable shifts in bacterial composition following colon and rectal surgery.

A binary logistic regression, a statistical method for estimating how strongly a factor predicts a yes/no outcome, found that receiving mechanical bowel prep was associated with roughly a 15-fold higher odds of developing POCD (odds ratio 15.1, 95% CI 3.98 to 57.07, P<.001). At the 90-day mark, patients who skipped bowel prep had significantly higher BDNF (a pattern the researchers interpret as protective, P<.001), while LBP was significantly higher in the bowel-prep group at both day 90 and before surgery (P=.001), consistent with greater gut-barrier disruption in that group. Tau protein was higher in patients who developed POCD regardless of which group they were in (P=.007 and P=.012 at the two follow-up points), and BDNF was lower in patients with POCD.

What This Means (and Doesn't Mean) for Patients

This study is not a reason to refuse mechanical bowel prep before surgery. Bowel prep exists for reasons independent of this question, including reducing stool burden in the surgical field and lowering infection risk, and the decision to use it is a surgical one that belongs to a patient's care team, not something to self-direct based on a single 80-person pilot study. What this study does support is a reasonable question to raise with a surgeon ahead of colorectal surgery: whether bowel prep is necessary for your specific procedure, and whether your surgical team has looked at newer research on this tradeoff. The authors' own interpretation is that MBP may disrupt gut bacteria, allowing bacterial components to cross a weakened gut barrier and trigger inflammation that reaches the brain via the gut-brain axis (the bidirectional communication network between gut and brain, increasingly implicated in perioperative neurocognitive disorders), but that remains a proposed mechanism, not an established one.

Why the Effect Size Should Be Read With Caution

The headline numbers here, an over sevenfold difference in POCD incidence and a 15-fold adjusted odds ratio, are large enough to warrant real skepticism rather than excitement alone. Several limitations matter. First, this was a single-center, male-only study of 80 total patients; the authors excluded women specifically to reduce variability, which means the findings may not generalize to women or to more diverse patient populations. Second, no direct gut microbiome sequencing was performed. LBP is only an indirect marker of gut barrier permeability, not direct proof that gut bacteria composition changed, so the "gut-brain" mechanism proposed by the authors is inferred, not directly demonstrated. Third, the bowel-prep group had a higher rate of colostomy (a surgically created opening for waste elimination), a potential confounder, though the authors report a sensitivity analysis suggesting colostomy status alone wasn't independently linked to POCD. Most importantly, the authors state plainly in their own paper that this pilot design "does not allow causal inferences" and describe their results as hypothesis-generating, meaning intended to justify a larger, properly powered trial, not to change practice on their own.

What Comes Next

The researchers call for confirmation in larger, multi-center trials that could establish whether this association holds up, include female patients, and ideally pair cognitive and blood-marker testing with direct microbiome sequencing to test the gut-brain mechanism they propose more rigorously. Until a larger trial replicates this finding, it remains a single pilot result rather than a basis for changing surgical bowel-prep protocols. The trial reports no competing interests among the authors and was funded by the Turkish Health Institutes Presidency.

Common Questions

Should I ask my surgeon to skip bowel prep before my colorectal surgery? Not based on this study alone. It's reasonable to ask your surgical team whether bowel prep is necessary for your procedure, since practices vary, but the decision should be made with your care team, not unilaterally, and this pilot alone doesn't establish that skipping it is safe or beneficial for your specific case.

What is postoperative cognitive dysfunction, exactly? It's a measurable decline in memory, attention, or processing speed that can persist for weeks to months after surgery, distinct from the short-term confusion (delirium) some patients experience right after anesthesia.

Does this study prove gut bacteria cause brain fog after surgery? No. It found an association between bowel prep, a gut-permeability marker, and cognitive outcomes, and the authors themselves say their design cannot establish causation. No direct microbiome sequencing was done.

Why did the study only include men? The researchers restricted enrollment to men specifically to reduce variability in a small feasibility study; they acknowledge this limits how well the findings generalize, and note that confirming the effect in women is a needed next step.

References

Ayşe Ülgey, Gamze Talih, Tutkun Talih, et al. (2026). Association Between Preoperative Mechanical Bowel Preparation and Postoperative Cognitive Dysfunction in Patients Undergoing Colorectal Surgery: A Pilot Randomized Controlled Trial. Therapeutics and Clinical Risk Management. https://doi.org/10.2147/tcrm.s613539