Colonoscopy Prep: Low-Residue Diet and How to Take the Bowel Prep

The endoscope shows only what is visible on the surface of the bowel lining. Stool residue, fluid or air bubbles can cover a small polyp or a flat lesion, and the endoscopist cannot assess it. This is why the quality of bowel cleansing is treated in guidelines as an indicator of the quality of the whole examination: a prep is considered adequate when it allows standard intervals to the next check to be assigned, and the target proportion of adequately prepared patients in a given unit should exceed 90%.

This study has limitations: it included patients in whom polyps had already been found, in one country, and the confidence intervals are wide. The direction of the relationship is, however, consistent with clinical reasoning and with the guidelines: the poorer the cleansing, the greater the chance that something will be missed. The authors conclude that after an examination with inadequate preparation it is reasonable to shorten the interval to the next colonoscopy; in practice the doctor may decide to repeat the examination earlier than the report alone would suggest.

The most common question is whether the whole prep can be drunk in the evening to 'get it over with'. The data say this is inferior to splitting the dose. The mechanism is simple: the large bowel produces mucus and fluid during the night too, and residual contents from the small bowel gradually move into the colon. A prep drunk in the evening cleans the bowel well at the time it is taken, but after a few hours secretions appear again and obscure the view, particularly in the right colon, where most fluid collects. The second dose taken in the morning, shortly before the examination, 'washes away' what has accumulated.

The conclusions of the meta-analysis are confirmed by the guidelines. ESGE recommends a split dose for elective colonoscopy (strong recommendation, high-quality evidence) and specifies the timing: the last dose should be started within 5 hours of the examination and finished at least 2 hours before it begins. The second condition is probably related to the safety of sedation, in which fluid in the stomach is undesirable; the schedule should therefore be adapted to the time of the examination, not the other way round. If the examination is scheduled for early morning, the second dose may be taken very early — this is normal and consistent with the recommendations.

Yes, and here the guidelines make an exception. For a colonoscopy scheduled for the afternoon, ESGE considers taking the whole prep on the day of the examination (so-called same-day) an acceptable alternative to split dosing, also with a strong recommendation and high-quality evidence. The American consensus recommendations from 2025 go in the same direction: a same-day regimen is acceptable for afternoon examinations, but it is suggested to be an inferior alternative to a split dose for morning examinations. In comparative studies, same-day preparation performed similarly to split dosing, provided the interval to the procedure was suitably short.

For years the standard was a liquid diet for the whole day before the examination, often preceded by several days of avoiding seeds and fibre. Current guidelines are more relaxed. ESGE recommends a low-fibre diet on the day before colonoscopy (strong recommendation, moderate-quality evidence), and the American consensus from 2025 recommends limiting dietary restrictions to the day before the examination, with a choice between liquids and a low-fibre diet for the early and midday meals. This means that long, multi-day elimination diets have no basis in current recommendations, although some centres still use them and it is worth following their instructions.

A later meta-analysis from 2021, covering 16 trials, reached the same conclusion: the quality of preparation did not differ significantly between the groups (OR 1.19; 95% CI 0.79–1.81), but people on the low-residue diet tolerated it better, while hunger, nausea and vomiting were more common in the liquid-diet group. For the patient this has practical importance: if cleansing is similar and wellbeing is better, a low-residue diet is a reasonable choice, provided the centre allows it. The evidence is of good quality for the comparison of preparation quality, but the diets in individual trials differed, so the exact menu is not standardised.

The exact list of foods depends on the centre, but the principle is shared: choose what leaves little residue and avoid anything containing seeds, skins, plant fibres and dyes. A low-residue diet is not a starvation diet — it allows an early and a midday meal, after which you switch to liquids while waiting for the prep.

Preparations differ in volume and composition. Classic polyethylene glycol (PEG) kits require drinking about 4 litres; low-volume preparations (for example 2 litres of PEG with ascorbic acid, kits with sodium picosulfate and magnesium citrate, or sulfate preparations) need less fluid, but usually additional clear fluids. The ESGE guideline accepts both high- and low-volume PEG regimens as well as non-PEG preparations, provided they have been clinically validated. The American consensus goes further, suggesting that a 2-litre regimen may be sufficient.

It follows that for most people the choice between preparations is not a matter of 'better cleansing' but of tolerability and fit with the person's health. The choice is made by the doctor: people with kidney disease, heart failure, electrolyte disturbances or advanced age may need a preparation of a different composition (ESGE recommends individualising the choice in people at risk of fluid and electrolyte disturbances). We write about the role of electrolytes in the entry on sodium, potassium and chlorides. Do not change the preparation on your own or combine two kits.

The taste and volume of the preparation are a frequent reason for not finishing it, which directly worsens the quality of the examination; the Bucci meta-analysis showed that compliance was significantly higher in the split-dose group, which translates into better cleansing. It therefore makes sense to plan the drinking itself.

Both guidelines also mention adding simethicone, which reduces foam bubbles in the bowel and improves visibility: ESGE suggests adding it (weak recommendation, moderate-quality evidence), and the American consensus also suggests it. Centres often include it in the kit; if not, there is no need to buy it on your own without asking. 'Enhanced instructions' are also recommended — written, understandable materials and sometimes telephone reminders — because they improve cleansing quality (ESGE recommendation: strong, moderate-quality evidence).

There is no single universal list of medicines to stop, and decisions made alone can be dangerous. Three groups are discussed most often. Anticoagulant and antiplatelet drugs (among them warfarin, newer oral anticoagulants, clopidogrel) — the decision to stop them temporarily or continue depends on the thrombotic risk and on whether polyp removal is planned; it should be discussed with the treating doctor or endoscopist before the examination, not during the preparation. Iron preparations — some centres ask for them to be stopped beforehand because they darken stool and can leave a dark residue on the lining; the specific recommendation comes from the centre. Diabetes medicines (especially insulin and sulfonylureas) — with restricted eating there is a risk of hypoglycaemia, and on a liquid diet the doses need adjusting; people with diabetes should agree the plan with their doctor.

If the stool is still a dark suspension or contains lumps and the preparation has already been drunk, it is worth calling the centre before arriving for the examination. The unit may suggest an additional portion of fluid, moving the examination or another solution. If the examination goes ahead despite poor preparation, the endoscopist will grade the cleansing on a scale (Boston, Ottawa or Aronchick) and recommend in the report whether it needs repeating sooner than usual. This is not the patient's failure — some people have objectively more difficult conditions.

In the Bucci meta-analysis poorer cleansing was associated with male sex, the use of 4 litres of PEG and a long interval between the last dose and the examination. The guidelines also single out people at risk of inadequate preparation; commonly listed among them are chronic constipation, diabetes, obesity, earlier bowel surgery, older age and drugs that slow motility (for example opioids), although the strength of the evidence differs for individual factors. In such cases the doctor may recommend a longer diet, an additional agent or a preparation of larger volume — these decisions should be taken together with the centre, not alone.

A well-prepared colonoscopy does not require heroic sacrifices, only precision: splitting the dose, planning the times well and following the centre's instructions. The meta-analysis data clearly indicate that the quality of the examination is determined by simple organisational elements, not the length of the fast. Someone who prepares properly once usually does not have to repeat the examination — which is a real saving of time, stress and exposure to another sedation.