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This review found that a low residue diet with laxatives for a day before colonoscopy is just as safe and effective in clearing the bowel as the clear liquid diet with laxatives currently recommended. The low residue diet was also preferred by patients and better tolerated.

Good clearance of the bowel assists diagnosis and treatment of conditions such as early cancer. Many people show poor bowel preparation, mainly due to lack of compliance with instructions.

This review did not look at the optimal dietary components of the low residue diet or look at other aspects of successful bowel preparation such as laxative use. The findings do suggest that if practice changed compliance might improve leading to more successful colonoscopy procedures, earlier diagnoses and lower costs.

Why was this study needed?

Colonoscopy inspects the whole of the large bowel with a flexible camera. It is currently the best test for identifying and removing growths (polyps) which may be or could turn cancerous. Around 60-70% of colonoscopies in England are performed for suspected cancer. Another important use is in inflammatory bowel disease (15-20% of colonoscopies). Patients can be put off by the preparation needed to clear the bowel first, drinking only clear liquids the day before combined with laxatives. Yet failure to prepare the bowel properly can prevent an adequate colonoscopy. The procedure may be prolonged, or abandoned, to be repeated on another day. There are around 150 colonoscopies performed per 10,000 people in England a year, each costing the NHS £532. In a Scottish study, 17% of patients had poor bowel preparation, and only 36% of these people had a successful colonoscopy. The researchers reviewed studies to discover whether making preparation more acceptable to patients would increase the chance that instructions were followed and the success of colonoscopy.

What did this study do?

This systematic review included nine randomised controlled trials that assessed the adequacy of bowel preparation when people ate a low residue diet the day before colonoscopy, compared to when they drank clear liquids only. Low residue diet meant low fibre food such as noodle soup, energy bars or apple sauce. Both groups also took laxatives, though the preparations used varied between trials. The outcomes examined were patients’ reported tolerance of the dietary preparation, patients’ willingness to repeat the diet and laxative preparation and adverse effects.

The review was carried out to a high standard, including a thorough search for trials and detailed assessment of their quality. However, none of the included trials were performed in the UK and none of the trials reported all the outcomes of interest, which limits the applicability of the findings.

What did it find?

  • There was no significant difference in the proportion of patients whose bowel preparation was adequate to carry out colonoscopy (87% of patients in the low residue diet group compared with 83% in the clear liquid diet group, odds ratio [OR] 1.21, 95% confidence interval [CI] 0.64 to 2.28).
  • Significantly more patients in the low residue diet group (77.8%) reported being able to tolerate the dietary preparation than in the clear liquid diet group (68.1%) OR 1.92, 95% CI, 1.36 to 2.70.
  • Significantly more patients in the low residue diet group also reported being willing to repeat bowel preparation using the trial diet (79.9%) compared with the clear liquid diet group (68.5%) OR 1.86, 95% CI, 1.34 to 2.59.
  • Rates of adverse events (such as discomfort or disturbed sleep) were not significantly different in either group (OR 0.88, 95% CI, 0.58 to 1.35), though these were assessed in only three of the nine studies.

What does current guidance say on this issue?

There is no UK-wide guidance on dietary preparation for colonoscopy. Guidance published in 2015 from the American Society for Gastrointestinal Endoscopy emphasises the importance of bowel preparation and recommends a clear liquid diet as part of preparation for colonoscopy. Guidance from individual UK hospital trusts varies but common advice is to eat low fibre foods two days before the operation, switching to clear fluids 18 to 24 hours before the procedure.

What are the implications?

This review found that a low residue diet was just as adequate for successful colonoscopy, was preferred by patients and better tolerated. Compliance wasn’t directly assessed, and preference here was used as an indirect measure for willingness to follow instructions. The review only assessed dietary preparation before colonoscopy, and did not cover other aspects such as changes to existing medication, laxative preparations or timing of administration.

Given the lack of UK-wide dietary recommendations in preparation for colonoscopy, the potential impact will vary. Where clear fluids only are allowed at present, a change to low residue diet may increase patient satisfaction and save costs through reducing the proportion of unsuccessful colonoscopies resulting from lack of compliance. The trials in the review did not follow the same low residue diets, and so there is scope for further investigation into the most effective and preferable dietary option that is linked with colonoscopy success.


Nguyen DL, Jamal MM, Nguyen ET, et al. Low-residue versus clear liquid diet before colonoscopy: a meta-analysis of randomized controlled trials. Gastrointest Endosc. 2015. [Epub ahead of print].


Cairns SR, Scholefield JH, Steele RJ, et al. Guidelines for colorectal cancer screening and surveillance in moderate and high risk groups (update from 2002). Gut 2010;59(5):666-89.

Hendry P, Jenkins J, Diament R. The impact of poor bowel preparation on colonoscopy: a prospective single centre study of 10 571 colonoscopies. Colorectal Dis 2007;9(8):745-8.

National tariff payment system 2014/15. Monitor / NHS England; 2013.

NICE. Colorectal cancer: diagnosis and management. CG131. London: National Institute for Health and Care Excellence; 2011.

Right care. Map 9A: Rate of colonoscopy procedures and flexisigmoidoscopy procedures per population by PCT. Indirectly standardised rate, adjusted for age, sex and deprivation 2012/13. In: NHS atlas of variation. Right care; 2015:58-60.

Saltzman JR, Cash BD, Pasha SF, et al. Bowel preparation before colonoscopy. Gastrointest Endosc 2015;81(4):781.

Produced by the University of Southampton and Bazian on behalf of NIHR through the NIHR Dissemination Centre


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Typical instructions for patients preparing for colonoscopy at a UK hospital include:

Seven days before the procedure – stop taking all iron tablets but continue all other medication.

Four days before the procedure – stop taking any constipating agents, but continue all other medication.

Two days before the procedure – eat foods which are low in fibre. Avoid foods such as fruit, green, vegetables, cereals, baked beans and brown bread.

One day before the procedure -instructions vary from having a low fibre breakfast and lunch such as soup or yoghurt, to having clear liquids only for all the meals. Examples of clear liquid foods include Bovril or clear soups.

The laxative preparation (e.g. Picolax) is taken in more than one dose, with the recommendation to drink up to three litres of water during the day.


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