Preparing for a colonoscopy: what to eat and how to ensure your bowels are properly cleansed
Bowel preparation has a direct impact on the quality of the colonoscopy. In order to examine the inner surface of the colon accurately and detect small lesions, the bowel must be clean and free of any residue that might obstruct the view.
‘In a sense, the colonoscopy begins before the patient even arrives at the hospital. Proper preparation creates the conditions necessary to examine the entire colon clearly and to fully benefit from the diagnostic and preventive value of the test,” explains Dr Malena García Arredondo, head of the Gastroenterology Department at the Memorial Publio Cordón Hospital.
Preparing correctly involves temporarily adjusting one’s diet, taking the prescribed laxative solution and adhering to the scheduled times for food, fluids, medication and fasting.
These guidelines may vary depending on the product used, the time of the examination, the patient’s usual bowel habits and their clinical characteristics. Therefore, the instructions provided by specialists should always take precedence over any general recommendations.
Why preparation affects the outcome of a colonoscopy
During the examination, the colonoscope transmits high-definition images of the intestinal mucosa. If any solid or liquid residues remain that impair visibility, some areas may be partially obscured and small lesions, such as certain polyps, could go unnoticed.
The clinical significance of this preparation has been analysed in recent studies. A study published in 2024 in *The American Journal of Gastroenterology* examined more than 335,000 colonoscopies carried out as part of Austria’s national colorectal cancer screening programme. The results showed that poorer bowel preparation was associated with a lower detection rate of adenomas, serrated lesions and other significant polyps.
Preparation, therefore, is not merely a preliminary formality, but a necessary condition for the colonoscopy to fulfil its full diagnostic and preventive potential. When bowel preparation is insufficient, the examination may prove more difficult and, in some cases, may need to be repeated.
Why you need to temporarily adjust your diet
In the days leading up to the procedure, it is recommended to reduce your intake of high-fibre foods and those that may leave small residues in the bowel.
Fibre is beneficial as part of a normal diet, but before a colonoscopy it can make bowel preparation more difficult, particularly when it comes from fruit, vegetables, seeds, nuts, pulses or whole grains.
The diet alone does not cleanse the colon. Its purpose is to reduce the amount of waste and facilitate the subsequent effect of the bowel preparation.
“Eating less does not necessarily mean preparing better. The important thing is to follow a well-organised plan: choose low-fibre foods, stay adequately hydrated whilst permitted to do so, and take the bowel preparation solution at the specified time,” says Dr García Arredondo.
There is no need to fast for several days or restrict your diet beyond what is specified. It is simply a matter of following a simple, low-fibre diet for a short period, in line with the instructions provided.
What you can eat before a colonoscopy
Although guidelines may vary, they usually allow foods that are low in fibre.
Common options include meat, fish, eggs, cheese, rice and pasta (not wholemeal), and boiled potatoes without their skins.
It is best to choose simple preparations, such as boiled, grilled or baked foods. During this time, it is best to avoid heavy meals, fried foods, rich sauces and dishes that are difficult to digest.
For example, you could have non-wholemeal bread with cooked ham for breakfast, rice with chicken or fish for lunch, and a French omelette or a thoroughly strained stock for dinner, provided that solid foods are still permitted.
These examples are for guidance only. The number of days on the diet and the exact time at which you should stop eating solid foods depend on the prescribed guidelines.
Can you eat cooked ham before a colonoscopy?
Cooked ham or York ham can be eaten on many low-residue diets because it contains virtually no fibre.
You should choose a plain product, without pepper, herbs, vegetables, nuts or other added ingredients. Once the liquid-only phase has begun, it must be stopped along with all other solid foods.
Which foods should be avoided
During the recommended period, foods with a high fibre content or those containing skins, seeds and small particles that are difficult to pass are usually avoided.
This includes fruit, vegetables, salads, pulses, nuts, seeds and whole grains, as well as wholemeal bread, pasta or rice, bran, popcorn and products containing small grains or seeds.
Blending food does not remove its fibre. Therefore, a vegetable purée still leaves behind residue and should not be confused with a clear liquid.
During the final phase, you may also be advised to avoid red, purple or very dark-coloured drinks, as their colour may make it difficult to identify certain residues during the colonoscopy.
The day before: solid foods and clear liquids
The day before usually marks the transition from the low-residue diet to the bowel preparation itself.
Depending on the time of the colonoscopy, a light meal may be permitted before you stop eating solid foods. From the time specified by the hospital, only expressly authorised liquids may be consumed.
Clear liquids are those that contain no particles and are transparent enough to see through. These include water, herbal teas without milk, some fully strained broths and certain clear drinks.
Milk, milkshakes, purées, soups containing solids and juices with pulp do not fall into this category. The stock must be skimmed and thoroughly strained, with no rice, pasta, vegetables or food particles.
Coffee or tea may be permitted in some protocols, provided they are served without milk and only until the specified time. The hospital’s instructions should specify which liquids may be consumed and when they should be stopped.
How to take the bowel preparation
Effective cleansing of the colon is achieved using a laxative solution that induces repeated, liquid bowel movements.
There are various products available, and each requires a specific amount of liquid, a particular preparation method and a set schedule. Therefore, the instructions for Citrafleet, Pleinvue, polyethylene glycol or other solutions must not be interchanged.
Many colonoscopies follow a split regimen: part of the preparation is taken the day before and the second part several hours before the procedure. This short interval between the final dose and the examination usually results in a more thorough cleanse.
When the colonoscopy is scheduled for the afternoon, the preparation should be carried out on the morning of the same day. In any case, you must follow the timetable provided by the hospital and not base your calculations solely on the time of your appointment.
You must not reduce the prescribed amount just because your stools already appear clear, nor should you add another laxative on your own initiative.
How to make the preparation more bearable
The bowel-emptying phase is usually the least pleasant part, but a few simple measures can make it easier to tolerate.
Many people find it easier to drink the solution when it is chilled, provided the product instructions allow it to be refrigerated. It can also help to drink it at regular intervals, without rushing, and to alternate it with the permitted clear liquids.
Once the effect sets in, it is advisable to stay at home and close to a toilet. Bowel movements will become increasingly frequent and watery. Applying a protective cream to the anal area and practising gentle hygiene can reduce irritation.
You may experience a feeling of fullness, wind, cramps or mild nausea. In such cases, you can take a short break and resume taking the solution more slowly, provided it is still possible to complete it within the scheduled time.
“If you experience repeated vomiting, severe pain, significant dizziness or find it impossible to finish the solution, it is best to consult the hospital before making any changes on your own or taking other laxatives,” warns Dr García Arredondo.
How to tell if the preparation has been carried out correctly
At first, stools may be solid or semi-liquid. They then gradually lose consistency and colour until they become a yellowish liquid that becomes progressively clearer.
By the end, it is normal to pass a clear or yellow liquid, similar to clear urine or a very clear broth, with no significant solid fragments.
This appearance indicates that the preparation is progressing as it should, although it does not in itself guarantee that the entire colon is clean.
If, after completing the regimen, dark stools, thick contents or large amounts of solid matter continue to appear, you should contact your specialist before taking any further medication.
Inadequate cleansing may hinder the examination, force the specialist to spend more time flushing and suctioning out residues, or prevent certain areas from being examined properly. Depending on the degree of cleansing and the reason for the colonoscopy, it may be necessary to repeat the procedure using a different regimen.
Chronic constipation, poor prior preparation, reduced mobility, certain medicines and specific medical conditions can make thorough cleansing difficult. Informing the specialist of these factors in advance allows the preparation to be adapted accordingly.
Medication and situations requiring a special regimen
Preparing for a colonoscopy does not automatically mean stopping all your usual treatments.
Some medicines can be continued, others need to be taken at different times, and certain treatments require an individual assessment. It is particularly important to inform the medical team if you are taking anticoagulants, antiplatelet agents, oral iron, diuretics, diabetes medicines or drugs that may slow down gastric emptying.
“Patients should not decide for themselves which medication to stop taking. Adjustments are sometimes necessary, but it is always important to consider why the treatment is being taken and what procedures might be required during the colonoscopy,” emphasises the specialist.
Anticoagulants and antiplatelet agents must not be stopped without specific instructions. The decision depends on the individual’s risk and the possibility of taking biopsies or removing a polyp during the procedure.
People with diabetes also require specific instructions. Changes to diet, fasting and the timing of the colonoscopy can affect blood glucose levels and may require adjustments to insulin or other medication.
Elderly patients, or those with kidney or heart failure, difficulty drinking large volumes of fluid, or a tendency towards dehydration, may also require preparation and guidelines tailored to their clinical situation.
When to stop drinking
The cut-off time for drinking fluids does not only affect bowel cleansing. It is also a safety measure relating to sedation.
There is no single cut-off time that applies to all patients. The cut-off time depends on the product used, the scheduled time of the colonoscopy and the centre’s anaesthetic protocol.
Continuing to drink after the specified time may result in the examination being delayed or cancelled. You must therefore follow the instructions you have been given exactly and ask any questions you may have before you start.
Frequently asked questions about preparation
Do I need to finish the entire preparation?
Yes. The fact that your stools appear clear before you have finished does not guarantee that the entire colon is properly cleansed. You must complete the prescribed amount, unless a problem arises that requires consultation with the medical team.
What should I do if I start vomiting?
If you experience mild nausea, you can take a short break and then resume drinking the solution more slowly. If the nausea persists, you may take an anti-sickness medicine provided there are no contraindications. If you are vomiting repeatedly or are unable to complete the preparation, you should contact the hospital.
What happens if the laxative does not work?
The time it takes to work may vary. If the specified time has elapsed without bowel movements beginning, you must not take an additional dose or another laxative on your own initiative.
Can I go to work whilst taking the preparation?
It is not usually practical. Bowel movements may be frequent and urgent, so it is advisable to organise your preparation so that you can stay at home and close to a toilet.
What should I do if there are still traces of residue?
If your stools remain dark, thick or contain a large number of solid fragments, you should consult the hospital before taking any more solution or changing the dosage schedule.
Proper preparation is also an important part of the colonoscopy
Proper preparation ensures that the colonoscopy can be carried out under the best possible conditions. Following the recommended diet, completing the bowel preparation and sticking to the timetable facilitates a more accurate examination and reduces the likelihood of it having to be repeated.
If you have any queries about food, dosages, fasting or your usual medication, you should consult the team carrying out the procedure. The hospital’s personalised instructions should always take precedence over any general guidelines found online.
The information in this article is provided for general awareness and does not replace an individualised medical assessment.
