Constipation on Keto: Causes and What the NHS Advises

On this page

1. Can keto cause constipation?
2. Signs of constipation
3. What the NHS advises
4. Fibre on a low-carb diet
5. Sweeteners and the laxative label statement
6. Pharmacists and laxatives
7. When to see a GP
8. FAQs
9. Sources

This guide to keto constipation sets out what the NHS says about the signs and causes of constipation, the diet and lifestyle changes it suggests, how those fit with a low-carb way of eating, and when the NHS says you should speak to a pharmacist or see a GP. It isn't medical advice, and everything health-related here comes from the NHS and the BDA.

Can keto cause constipation?

The NHS constipation page doesn't mention keto, but its list of common causes overlaps with what changes when you start a ketogenic diet. The NHS says the most common causes include:

• not eating enough fibre, which is found in fruits, vegetables and cereals
• not drinking enough fluids
• not moving enough and spending long periods sitting or lying down
• being less active and not exercising
• often ignoring the urge to go to the toilet
• changing your diet or daily routine
• a side effect of medicine
• stress, anxiety or depression

Two of these are directly linked to diet. Starting keto is a change of diet in itself, and it cuts out many of the foods that contain fibre. The BDA's low carb diets fact sheet says that breakfast cereal, bread, potatoes, pasta and rice are generally avoided on a low carb diet, and it lists "eating less fibre" among its concerns about the dietary changes that come with low carb diets.

The NHS also notes that constipation sometimes has no obvious reason, and that it can rarely be caused by a medical condition such as diverticular disease or bowel cancer. That's why its advice on when to see a GP, below, is worth knowing.

Signs of constipation

According to the NHS, it's likely to be constipation if:

• you have not had a poo at least 3 times during the last week, or you're pooing less often than usual
• the poo is unusually large or small and is dry, hard or lumpy
• you are straining or in pain when you have a poo
• you feel like you haven't fully emptied your bowels

The NHS adds that you may also have a stomach ache and feel bloated or sick.

What the NHS advises

The NHS says simple changes to your diet and lifestyle can help with constipation, and that you may notice a difference within a few days, though it sometimes takes a few weeks. Its suggestions are:

• Diet: include fruits that contain sorbitol, such as apples, apricots, grapes (and raisins), raspberries and strawberries; drink plenty of water and other fluids and avoid alcohol; gradually increase the fibre in your diet; and add some wheat bran, oats or linseed.
• Toilet routine: keep to a regular time and place, give yourself plenty of time, and don't delay if you feel the urge to poo. Resting your feet on a low stool, with your knees above your hips if possible, can make it easier.
• Activity: the NHS says a daily walk or run can help you poo more regularly.

On fluids, the NHS quotes the government's Eatwell Guide, which says to aim for 6 to 8 cups or glasses of fluid a day, and says most people should drink enough that their pee is a clear pale yellow colour. You may need more if you're pregnant or breastfeeding, in a hot environment, active for long periods, or ill or recovering from illness.

The NHS also says these measures are safe to try when you're pregnant. Separately, the BDA's fact sheet says ketogenic diets should not be followed by women who are pregnant.

Fibre on a low-carb diet

Some of the foods on the NHS list carry far more carbohydrate than a ketogenic diet allows, so it helps to know the figures. This table shows the carbs and fibre in the foods the NHS names, from the UK's official food tables (CoFID):

CoFID 2021, food codes 14-375, 14-324, 14-025, 14-319, 14-350, 11-906, 14-393 and 11-788. Fibre is AOAC fibre. Carbohydrate doesn't include fibre, as on UK labels.

Of these eight, raspberries and strawberries have the fewest carbs. Linseed isn't in CoFID's main table. Our guide to flaxseed (linseed) on keto gives the carbs and fibre from pack labels.

For other sources of fibre, here's how some low-carb vegetables, avocado and almonds compare in CoFID:

CoFID 2021, food codes 14-896, 13-502, 13-630, 14-386, 13-509, 13-512, 13-505 and 13-627. Sorted by fibre. Fibre is AOAC fibre.

Of these eight foods, almonds have the most fibre per 100g and broccoli has the most of the vegetables. Our guides to broccoli and cauliflower give portion figures.

For context, the NHS says government guidelines are for 30g of fibre a day, and that most adults eat an average of about 20g. The BDA's fibre fact sheet adds two points:

• If you increase your fibre intake, do it gradually, to avoid gastrointestinal symptoms such as bloating and gas.
• Remember to drink a good amount of fluid as you increase your fibre.

That applies to fibre powders used in keto baking too. If you have IBS, our guide to keto and the low FODMAP diet explains how the two fit together.

Sweeteners and the laxative label statement

Many keto sweets and bakes use polyols, such as erythritol, xylitol or maltitol. UK labelling law says that foods containing more than 10% added polyols must carry the statement "excessive consumption may produce laxative effects". That's a labelling rule, not a remedy. Our guides to polyols and xylitol explain how they appear on labels.

Pharmacists and laxatives

The NHS says to speak to a pharmacist if diet and lifestyle changes are not helping. A pharmacist can suggest a suitable laxative. The NHS adds that:

• most laxatives work within 3 days, and they should only be used for a short time
• if you bought laxatives without a prescription, stop taking them when your constipation improves and do not take them for longer than 7 days
• if you've used laxatives for 3 days and they have not helped, speak to a doctor or pharmacist

Laxatives are medicines, so follow the pharmacist's advice and the leaflet that comes with them.

When to see a GP

The NHS says to see a GP if you:

• are constipated and it's not getting better with treatment
• are regularly constipated
• are regularly bloated
• have blood in your poo
• have lost weight without trying
• are constipated and feel tired all the time
• are taking medicine that's causing constipation, such as opioid painkillers
• notice sudden changes in how you poo (your bowel habits)
• have tummy pain

The NHS also says to speak to the GP before you stop taking any prescribed medicine.

Frequently asked questions

Sources

• NHS. Constipation (symptoms, common causes, diet and lifestyle changes, pharmacist advice, when to see a GP). Page last reviewed 26 October 2023; checked 3 November 2026.
• NHS. Laxatives (short-term use, 7 days without a prescription, 3 days without improvement). Page last reviewed 30 April 2026; checked 3 November 2026.
• NHS. How to get more fibre into your diet (30g a day guideline, average adult intake) and Water, drinks and your health (6 to 8 cups or glasses a day, pale yellow pee). Checked 3 November 2026.
• British Dietetic Association. Low carb diets food fact sheet (20–50g of carbohydrate a day on a ketogenic diet, foods generally avoided, concerns about eating less fibre, pregnancy) and Fibre food fact sheet (increasing fibre gradually, drinking fluid).
• Retained Regulation (EU) No 1169/2011 on food information to consumers, via legislation.gov.uk: Annex III, point 2.4 (laxative statement for foods with more than 10% added polyols).
• Public Health England / Food Standards Agency. Composition of Foods Integrated Dataset (CoFID) 2021: sheet 1.3 Proximates, food codes as listed under the tables.