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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.

Inclusion Criteria:

• Men and women between 50 and 69 years old.
• Participants in the Program for the Early Detection of Colorectal Cancer with a positive result in the test for detecting occult blood in feces
• That they accept a colonoscopy and that they agree to participate in the study.

Exclusion Criteria:

• Contraindication to performing a colonoscopy.
• Severe renal insufficiency (<30 ml / min).
• Known hypersensitivity or allergy to polyethylene glycol, ascorbic acid or sulfate.
• Known glucose-6-phosphate dehydrogenase deficiency.
• Known phenylketonuria.
• Known dyselectrolytemia: hyper / hyponatremia, hyperphosphatemia, hypermagnesemia, hyper / hypokalemia, hypocalcemia.
• Gastric emptying disorders: Known gastroparesis.
• Known hypoalbuminemia less than 3.4 g / dl.
• Crohn's disease or known ulcerative colitis.
• Participants with difficult-to-control hypertension (SBP> 170mmHg or TAD> 100mmHg) or NYHA grade III or IV heart failure.
• Ascites of any etiology
• People with cognitive impairment or mental illness that makes it difficult to adhere to instructions.
• People who do not understand Catalan or Spanish.
• Factors of poor preparation: liver cirrhosis, diabetes mellitus, treatment with tricyclic antidepressants, opioids or neuroleptics, limited mobility, chronic constipation, history of colon or intestinal resection (appendectomy is not an exclusion criterion), poor preparation in previous colonoscopy Parkinson's disease, multiple sclerosis.