There is an increasing emphasis on the importance of high-quality bowel preparation for colonoscopy. Inadequate bowel preparation can have consequences such as low detection rates of adenomas or polyps, incomplete colonoscopies, challenging intubations, and an increased risk of complications. These outcomes increase the workload on healthcare professionals while burdening patients with the expense and risk of more frequent surveillance intervals [1, 2].

Challenges in predicting and managing inadequate bowel preparation include factors such as age, inpatient status, diabetes mellitus, hypertension, cirrhosis, narcotic use, stroke, and tricyclic antidepressant use, in addition to the patient’s pre-existing bowel habits (e.g., chronic constipation) [3]. Nevertheless, the pathogenesis of constipation is multifactorial, requiring comprehensive assessment through careful history taking and the judicious use of clinical testing. The Rome IV guidelines provide diagnostic criteria for functional constipation, including infrequent bowel movements (< 3 per week), moderate straining (> 25%), hard stools with a Bristol Stool Form Scale (BSFS) rating of 1–2, sensation of incomplete evacuation or anorectal obstruction, and a history of manual maneuvers to facilitate defecation [4].

A major factor contributing to constipation is a slow colonic transit time [5]. Studies have focused on the relationships of slow colonic transit time with factors such as bowel movement frequency, stool consistency, and bowel preparation quality [6, 7]. Another study examined these relationships by directly measuring colonic transit time [8]. In a study involving 411 consecutive patients, Malhotra et al. [6] found that patients with a BSFS rating of 1–2 had a higher rate of inadequate bowel preparation (28.3% [15/53] vs. 18.0% [74/411]) among all patients. Multivariate regression revealed that the BSFS score was significantly associated with the adequacy of bowel preparation (odds ratio [OR], 1.48; 95% confidence interval [CI], 1.20–1.82). Lee et al. [7] analyzed the results of a personal bowel habit questionnaire and the quality of bowel preparation in 404 patients who underwent colonoscopy. Their multivariate analysis showed that infrequent bowel movements (< 3 per week) were associated with a > fivefold increase in the risk of inadequate bowel preparation compared with patients who had normal bowel movement form and frequency (OR, 5.20; 95% CI 1.79–15.2). The BSFS score of 1–2 was correlated with inadequate bowel preparation using univariate analysis (OR, 2.38; 95% CI 0.90–6.33), although the relationship was not statistically significant. Park et al. [8] directly measured colon transit time using multiple radiopaque markers (Kolomark TM tablets; M. I. Tech, Pyeongtaek, Gyeonggi, Korea) and serial X-rays, classifying colonic transit time as normal (≤ 30 h) or slow (> 30 h). The results showed a significantly higher rate of inadequate bowel preparation in the slow transit group than in the normal transit group (26.0% vs. 9.4%; P = 0.027).

Studies analyzing constipation symptoms, including straining, are rare. Paik et al. [9] conducted a study in which 90 patients completed a questionnaire regarding personal bowel habits. The results showed that infrequent bowel movement and moderate straining (> 25% of defecations) were significantly associated with inadequate bowel preparation. Using multivariate analysis, only moderate straining remained a significant factor for inadequate bowel preparation (OR, 3.99; 95% CI 1.26–12.65; P < 0.05).

In this issue of Digestive Diseases and Sciences, Higashimori et al.[10] present their analysis of bowel habit factors that can predict inadequate bowel preparation. According to the results of multivariate analysis, the risk of inadequate bowel preparation increased with constipation severity (P = 0.01), which was assessed using the Constipation Scoring System (CSS; range 0–30) [11]; among personal bowel habits, frequent straining (> 25% of defecations) (OR, 2.09; 95% CI 1.33–3.28) and chronic use of stimulant laxatives (OR, 2.57; 95% CI 1.59–4.17) were significant predictors of inadequate bowel preparation. In contrast, a BSFS score of 1–2, infrequent bowel movements (< 3 per week), and a sensation of incomplete evacuation (> 25%) were not statistically significant predictors.

Their study has several strengths[10], such as its relatively large prospective cohort of 1054 patients combined with its analysis of the relationships between the quality of bowel preparation and bowel movement frequency and stool consistency (indicators of colon transit time) and anatomical or behavioral issues such as straining and the sensation of incomplete evacuation. Moreover, they considered differences in constipation severity using questionnaire based on a comprehensive constipation scoring system [11], with over 100 symptoms and ranging from 0 (normal) to 30 (severe) and the chronic use of stimulant laxatives. Nevertheless, the weaknesses of the study include the use of same-day dose rather than the more commonly used split-dose bowel preparation, along with the atypical practice of using senna, an adjunctive stimulant laxative, on the day before colonoscopy. Moreover, the study did not investigate or consider tests intended to differentiate secondary constipation causes, such as dyssynergic defecation, which involves anatomical or behavioral issues that can lead to inadequate evacuation and retention.

Thus far, research regarding bowel habits and bowel preparation quality has encompassed two major areas of investigation: predicting inadequate bowel preparation and understanding the factors that contribute to its occurrence combined with finding appropriate methods to overcome inadequate bowel preparation. The analysis of these methods includes examination and optimization of bowel preparation regimens, selection of suitable laxatives, and exploration of rescue therapies aimed at improving the overall quality of bowel preparation.

Previous studies explored the relationship between changes in underlying bowel habits, such as constipation, and the quality of bowel preparation [5,6,7,8,9]. Their limitations were related to the subjective nature of the constipation diagnosis, its association with several underlying causes, the evolving criteria used to define constipation, and the frequent use of yes/no questions rather than specific diagnostic criteria. Despite the lack of consistent results regarding the relationship between bowel habits and inadequate bowel preparation, further research in this area holds great potential for efforts to advance the overall understanding of and ability to address constipation-related issues; these aspects can improve patient care and outcomes.

In conclusion, when preparing for a colonoscopy, risk factors for inadequate bowel preparation related to underlying bowel habits, such as infrequent bowel movement, hard stool consistency, and straining during defecation, should be carefully considered, although they may be inconsistent in terms of predicting inadequate bowel preparation. Furthermore, the severity of constipation can be taken into account. These approaches will enable healthcare professionals to make more informed decisions and consider more effective bowel preparation strategies that ensure optimal outcomes during colonoscopy procedures.