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Currently participants in the colorectal cancer screening program follow a 3 days low residue diet before colonoscopy. This is the active comparator arm of this study.

Participants assigned to this arm are NOT instructed to follow any kind of restriction in the diet before colonoscopy.

To follow the regular dietary habits with no restriction nor modification.

Free diet

To follow the three days before colonoscopy a diet with low content in fiber or residues.

Three days low residue diet

Eva Martínez, PhD

One tricky aspect of the recommendations for colonoscopy prep is diet. This has a significant impact on the experience of the patient or participant in the screening program and, on the other hand, low adherence has been found in some studies despite a potential Hawthorne effect . It is noteworthy that despite its impact on patient experience, it is an area for which little evidence is available, which is why the guidelines give low-quality recommendations and there is probably considerable variability in clinical practice .

In the early days of colonoscopy, a liquid diet for 48 hours was mainly recommended, although some centers indicated a low-residue diet or even the commercially available NASA astronaut diet. Later, the indication for a liquid diet was consolidated until finally numerous studies were published in favor of a low-residue diet, managing to increase tolerance and the quality of the preparation . A limitation of the preparation studies must be borne in mind that the colon cleansing rating scales were not introduced until 1999 when the Aronchick scale was published.

Although there is solid evidence in favor of a low-residue diet versus a liquid diet, the investigators do not have evidence on how many days of a low-residue diet should be recommended, and this is reflected in the ESGE (European Society of Gastrointestinal Endoscopy) and ASGE (American Society of Gastrointestinal Endoscopy) guidelines . A randomized clinical trial comparing 3 days versus 1 day of a low residue diet has recently been published . There were no statistically significant differences in the rate of adequate preparations (82.7% vs. 85.6% OR 1.2 95% IC 0.72 to 2.15). However, this study has limited statistical power and a design that allows a non-inferiority analysis has not been followed. In relation to this, our research group is finalizing a non-inferiority clinical trial in whose intermediate analysis, with 421 participants, the non-inferiority of 1 day of diet is fulfilled (rate of poor preparation in 1 day 0.95% vs. 4.74% in 3 days; d + 5%, difference -3.78% IC -6.88% to -1.12%) (38).

It is likely, taking into account the available evidence and its evolution, that diet plays a secondary role in preparation. Although no studies designed to directly assess this have been conducted, the research group has indirect data.

Walter et al, under the hypothesis that the impact of the fractional preparation and the new preparations on the preparation diminished the importance of the diet, conducted a non-inferiority clinical trial between 2012 and 2013 in which they randomized the patients to follow a diet liquid versus low residue for one day and fractional preparation with Moviprep (39). They established a non-inferiority margin of -13.5%. Their results show a rate of good preparation (Boston\> 5) in 68/72 (94.4%) in a liquid diet compared to 60/68 (88.2%) in a low-residue diet (p = 0.04) with a difference of -5.08% demonstrating non-inferiority of the low residue diet.

Efficacy of Free Versus Low Residue Diet as Preparation for Screening Colonoscopy

Efficacy of Free Versus Low Residue Diet as Preparation for Screening Colonoscopy: CriLi Study, a Pilot Trial

Bowel cleansing has a rating using the Boston Bowel Preparation Scale above 1 in each segment . This score ranges fron 0 to 3 in each one of the three segments. Zero is the worst outcome and 3 is the best cleansing

Tolerance to the diet assigned to the participant measured with a likert scale from 0 to 5, 0 the worst ad 5 the best.

Tolerance to the cleansing solution used for bowel cleansing measured with a likert scale from 0 to 5, 0 the worst ad 5 the best.

An adenoma was detected, resected and confirmed by the histology report in the colonoscopy.

Polyp detection in the colonoscopy as reported by the endoscopist.

Time expended in the withdrawal from cecum till the anus.

Colonoscopy achieving a Boston Bowel Preparation Scale (BBPS) of at least 8 points or above. BBPS global score ranges from 0 to 9, being 9 the best cleansing outcome.

Perceived quality of the instructions and education received for the colonoscopy. preparation by the participant. Assessed using a likert scale from 0 to 5. Five is the best outcome and 0 the worst.

Boston Bowel Preparation Score in each segment, it ranges from 0 to 3 being 3 the best cleansing outcome and 0 the worst.

Study title	NCT ID	Conditions	Interventions	Eligibility	Organization	Link
Efficacy of Free Versus Low Residue Diet as Preparation for Screening Colonoscopy	NCT04664543	Colorectal Canc... Colon Polyp Colon Adenoma	Free diet Three days low ...	50 Years - 69 Years	Parc Taulí Hospital Universitari
Efficacy of Free Versus Low Residue Diet as Preparation for Screening Colonoscopy	NCT04664543	Colorectal Canc... Colon Polyp Colon Adenoma	Free diet Three days low ...	50 Years - 69 Years	Parc Taulí Hospital Universitari

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