Constipation During Weight Loss: Causes and When to Get Help
Constipation During Weight Loss: Causes and When to Get Help
Constipation during weight loss is usually related to changes that accompany a weight loss plan, rather than the loss of body fat itself. Eating less can mean there is less material moving through the bowel. A new diet may also contain less fibre, fluid or food variety than before. Changes in activity, routine, supplements and prescribed treatment can contribute too.
Constipation is common and often improves with simple measures. However, it should not automatically be dismissed as a normal part of losing weight. Persistent symptoms, blood in the stool, unexplained weight loss, severe or worsening pain, vomiting, a very swollen abdomen, or being unable to pass stool or wind need medical attention.
This guide explains what constipation means, why it may appear during weight loss, what you can try safely and when to get help.
The short answer
Constipation means a change from your normal bowel habit, such as going less often, passing hard or lumpy stools, straining, or feeling that the bowel has not fully emptied.
Losing fat does not directly block the bowel. Changes in food quantity, fibre, fluid, movement or routine are more likely explanations.
A very restrictive diet can remove useful sources of fibre, even when calorie intake falls.
Increasing fibre suddenly can make wind and bloating worse. Add it gradually and take enough fluid.
Some supplements and prescribed treatments can cause constipation. Speak to the clinician who manages them before making any change.
A pharmacist can advise on short term treatment if diet and lifestyle measures are not enough.
Seek medical advice if constipation persists, keeps returning, causes pain or occurs with bleeding, unusual tiredness or weight loss that you did not intend.
Severe pain, repeated vomiting, marked swelling, black or dark red stool, heavy bleeding, or an inability to pass stool or wind requires urgent assessment.
What counts as constipation?
There is no single healthy number of bowel movements that suits everyone. Some people go more than once a day, while others naturally go only a few times a week. A change from what is usual for you can matter more than the number alone.
According to the NHS, constipation is likely if you have not passed a stool at least three times in the previous week, or if you are going less often than usual. Other signs include:
dry, hard or lumpy stools
unusual straining or pain when passing a stool
feeling that the bowel has not completely emptied
abdominal discomfort
bloating
feeling sick
A single day without a bowel movement does not necessarily mean you are constipated. Look at the whole pattern, how difficult the stool is to pass and how you feel.
Can losing weight cause constipation?
Losing body fat is not usually the direct cause. The more useful question is what changed when the weight loss plan began.
If you are eating smaller portions, there may be less food residue available to form stool. This can reduce stool volume and sometimes reduce how often you need to go. That is not automatically a problem if the stool remains soft, comfortable to pass and consistent with your new normal.
Constipation is more likely when smaller portions are also accompanied by lower fibre intake, less fluid, a major change in routine, reduced movement or a treatment side effect. Several factors can occur at the same time.
Common causes of constipation during weight loss
Eating less food overall
When you eat less, the bowel has less material to process. The result may be a smaller or less frequent stool. Some people interpret any reduction in frequency as constipation, even when there is no straining, hard stool or discomfort.
The amount of food is only part of the picture. A smaller diet still needs enough fibre, fluid and variety to support bowel function and overall nutrition.
Cutting out fibre rich foods
Some weight loss diets reduce bread, cereals, potatoes, fruit, pulses or other carbohydrate foods. This may lower calorie intake, but it can also remove a large amount of fibre.
Fibre adds bulk to stool, holds water and supports movement through the bowel. It is found in plant foods such as:
fruit and vegetables
oats and other wholegrain cereals
wholemeal bread
brown rice and wholewheat pasta
beans, lentils and chickpeas
nuts and seeds
potatoes with their skins, where suitable
A diet does not need to be high in carbohydrate to include fibre, but it does need a deliberate source of plant foods. Meat, fish, eggs and cheese do not contain fibre.
Increasing fibre too quickly
Fibre can help, but more is not always better straight away. A sudden large increase may cause wind, cramps and bloating. It can also be unhelpful if there is not enough fluid available.
UK guidance recommends that adults aim for about 30 grams of fibre each day. If your present intake is much lower, build towards that amount gradually. Add one change at a time so you can see what suits you.
People with certain digestive conditions, a history of bowel narrowing, recent bowel surgery or specific dietary restrictions should ask a clinician or dietitian before making a major fibre change.
Not drinking enough
The large bowel absorbs water from its contents. If fluid intake is low, stool can become harder and more difficult to pass.
The NHS generally advises six to eight glasses of fluid a day. Your needs can be higher in warm weather or during physical activity. Water, lower fat milk and lower sugar drinks can all contribute. Tea and coffee also count for most people.
This is general guidance, not a fixed target for everyone. If you have heart failure, kidney disease or another condition that requires fluid restriction, follow the plan given by your clinical team.
A change in activity
Regular movement can support bowel function. Constipation may become more likely if a new diet is accompanied by long periods of sitting, illness, fatigue or a sudden fall in activity.
You do not need a demanding exercise programme. A daily walk, standing up regularly and returning gradually to your usual activity may help. Choose movement that is safe for your health and mobility.
A change in routine
The bowel often responds to regular habits. Travel, shift work, stress, a new meal pattern or rushing in the morning can interrupt them. Repeatedly ignoring the urge to pass stool can also make constipation worse because the stool has more time to dry in the bowel.
Allow enough time to use the toilet without rushing. Many people find the bowel is more active after breakfast or another meal.
Supplements
Some supplements can contribute to constipation. Iron and calcium products are common examples, although the effect varies between people.
Check why the supplement was recommended before changing it. If it was prescribed or advised for a diagnosed deficiency, speak to a pharmacist, GP or the clinician managing your care. A different formulation or treatment plan may be more appropriate than simply stopping it.
Prescribed treatment
Constipation can be a side effect of several types of medicine. These include some pain medicines and some treatments used as part of medical weight management.
If constipation began after starting treatment or after a planned treatment change, tell the prescriber. Give them useful details such as when symptoms began, how often you are passing stool, whether you can pass wind, any abdominal pain or vomiting, and what you have already tried.
Do not independently stop, restart or alter a prescribed treatment. The prescriber needs to consider your symptoms, medical history, other medicines and the risks of changing the plan.
An unrelated health condition
Constipation that begins during a weight loss attempt may be unrelated to the plan. Possible causes include an underactive thyroid, irritable bowel syndrome, pregnancy, pelvic floor problems and other bowel conditions.
Most constipation is not caused by a serious disease. Even so, new or persistent symptoms deserve assessment when they do not improve or occur with warning signs.
What can help constipation during weight loss?
Start with gentle, practical changes unless you have warning symptoms or have been told to follow a different medical plan.
Review what changed
Think back to the week or two before symptoms began. Ask yourself:
Did I sharply reduce the amount I eat?
Did I remove bread, cereals, fruit, pulses or vegetables?
Am I drinking less because my appetite is smaller?
Have I been less active?
Am I ignoring the urge to use the toilet?
Did I start a supplement or prescribed treatment?
Do I have pain, vomiting, bleeding or unusual swelling?
A simple diary for a few days can help you spot a pattern. Record meals, drinks, activity, bowel movements and symptoms. You do not need to track every calorie for this purpose.
Increase fibre gradually
Choose one or two manageable additions rather than changing the whole diet overnight. For example:
add oats or a wholegrain cereal at breakfast
choose wholemeal bread instead of a lower fibre option
add beans, lentils or chickpeas to soup, curry or salad
include vegetables at lunch and dinner
choose whole fruit rather than relying only on juice
add a small portion of nuts or seeds if they are suitable for you
Some fruits contain sorbitol, a natural carbohydrate that can help soften stool in some people. NHS examples include apples, apricots, grapes, raisins, raspberries and strawberries.
If an increase causes significant discomfort, slow down and seek advice rather than forcing ever larger amounts. Persistent bloating or pain can have causes that need assessment.
Pair fibre with fluid
Fibre works best when fluid intake is adequate. Spread drinks through the day rather than trying to catch up all at once.
A glass of water with meals, a drink between meals and soup or milk as part of the day can all help. If nausea or a reduced appetite makes drinking difficult, frequent small sips may be easier.
Alcohol is not a good way to meet fluid needs. It can contribute calories and can affect hydration, sleep and food choices.
Keep a regular toilet routine
Go when you feel the urge rather than delaying. Give yourself privacy and enough time, but avoid prolonged straining.
The NHS suggests resting the feet on a low stool so the knees are above the hips. This position can make passing stool easier for some people. Keep the stool stable and make sure the position is safe for your balance and mobility.
Move regularly
A daily walk can help some people become more regular. If a long walk is not practical, several short periods of movement still break up sitting time.
Activity should be appropriate for your health. If you have severe abdominal pain, feel acutely unwell or have symptoms suggesting an obstruction, do not try to exercise the problem away. Seek medical help.
Keep the diet balanced when appetite is smaller
A reduced appetite can make it difficult to fit enough nutrition into the day. Try to make smaller meals count by including a source of fibre and a source of protein where practical.
Examples might include:
porridge with fruit and plain yoghurt
vegetable and lentil soup
beans on wholemeal toast
a baked potato with beans or another suitable filling
wholegrain crackers with hummus and vegetables
a smaller main meal with vegetables, protein and a wholegrain food
These are general ideas, not a personal treatment plan. People with allergies, digestive conditions, diabetes, kidney disease or another medical requirement may need tailored advice.
Should you use a laxative?
If diet, fluid, activity and routine changes are not helping, ask a pharmacist for advice. Laxatives are medicines that help with constipation, but different types work in different ways and not every product is suitable for every person.
The NHS advises that most adults try lifestyle measures first. Laxatives bought without a prescription should usually be used for a short time and stopped when constipation improves. NHS guidance says not to continue an over the counter laxative for longer than seven days without advice.
Check with a pharmacist or doctor before using one if you are pregnant, have heart or kidney problems, difficulty swallowing, inflammatory bowel disease, or take medicines that could interact. Always follow the label and never take extra doses to make a laxative work faster.
Stimulant laxatives do not cause loss of body fat. Using them to try to change the number on the scale can cause dehydration, disturb salts in the body and harm bowel function. If laxative use is connected with fear of weight gain, binge eating, purging or an eating disorder, speak to a GP or eating disorder service for confidential support.
Do not attempt to remove impacted stool yourself. Faecal impaction needs professional assessment and a planned treatment approach.
Can constipation make the scale go up?
Constipation can temporarily increase body weight because stool and fluid remain in the digestive system. Day to day changes on the scale can also reflect hydration, salt intake, carbohydrate storage, menstrual cycle changes and the time of weighing.
This is not the same as suddenly gaining body fat. Avoid responding to one higher reading by sharply restricting food, taking laxatives or altering prescribed treatment. Look at the trend over several weeks under consistent weighing conditions.
Once constipation improves, the scale may change. That change mainly reflects bowel contents and fluid, not a burst of fat loss.
When should you contact a GP?
Arrange a GP appointment if:
constipation is not improving with treatment
it keeps returning
you are regularly bloated
you have blood in the stool
you have persistent abdominal pain
you feel unusually tired
your bowel habit has changed suddenly
a medicine or supplement may be causing the problem
you are losing weight without trying
The phrase unexplained weight loss is important here. If you are deliberately following a supervised weight management plan, some loss is expected. Contact your GP if the amount or speed of loss is not explained by the plan, if you are struggling to eat, or if weight loss occurs with bleeding, persistent pain, unusual tiredness or a lasting bowel change.
Your GP may ask about the timing of symptoms, your usual bowel habit, diet, fluid intake, medicines, supplements and family history. Depending on the situation, examination or tests may be needed.
When is constipation urgent?
Get urgent medical advice through NHS 111 or an urgent clinical service if you have constipation with vomiting, abdominal pain, fever, a swollen abdomen, or an inability to pass stool or wind.
Call 999 or go to Accident and Emergency if:
abdominal pain is sudden or severe
your abdomen is very swollen and painful
you are vomiting blood or material that looks like coffee grounds
the stool is black and sticky, or there is heavy bleeding
you collapse, become confused or have severe difficulty breathing
An inability to pass stool or wind with pain, vomiting and marked swelling can be a sign of bowel obstruction. That needs urgent assessment. Do not take more fibre or laxatives in an attempt to manage possible obstruction at home unless a clinician has specifically told you to do so.
Constipation and medical weight management
If you receive prescribed weight management treatment, side effect monitoring is part of safe care. Tell your clinical team about constipation that is persistent, worsening or accompanied by pain, vomiting, reduced fluid intake or other concerning symptoms.
It helps to provide clear information:
when the constipation began
the date of the last comfortable bowel movement
whether you can pass wind
whether the abdomen is swollen
the presence and severity of pain or vomiting
what you can eat and drink
all medicines and supplements you take
any laxative or home measure already tried
Your prescriber can then decide whether the symptom can be managed with supportive measures or whether assessment or a treatment review is needed.
Keltoi's online onboarding questionnaire provides a snapshot of your health and circumstances. It does not diagnose the cause of constipation and it does not make clinical decisions. Dr Waqar Ahmed, Keltoi Clinical Lead, is the clinician responsible for individual prescribing and treatment decisions.
Contact Keltoi if constipation develops or changes during treatment, but do not wait for routine online contact when symptoms are urgent. Use NHS 111, Accident and Emergency or 999 as appropriate.
A practical daily checklist
If there are no warning symptoms and no reason to follow a different medical plan, this checklist may help:
Include fibre rich plant food across the day.
Increase fibre gradually rather than suddenly.
Drink regularly and follow any personal fluid restriction.
Move in a way that is safe for you.
Allow time for the toilet and do not ignore the urge.
Use a stable foot stool if it makes passing stool easier.
Check whether a new supplement or treatment could be contributing.
Ask a pharmacist if simple measures are not working.
Track the bowel pattern, not only the scale.
Seek help promptly if warning symptoms appear.
Frequently asked questions
Is constipation normal when losing weight?
It can happen, but it is not something everyone should expect or ignore. Eating less, reducing fibre, low fluid intake, routine changes, reduced activity and treatment side effects can all contribute. Mild symptoms may improve with simple measures. Persistent or painful constipation needs advice.
Can eating less make me poo less often?
Yes. Eating less can produce less stool, so you may go less often. This is not necessarily constipation if the stool is soft, easy to pass and you feel well. Hard stool, straining, pain, bloating or incomplete emptying suggests constipation.
How much fibre should adults have?
UK guidance recommends about 30 grams of fibre a day for adults. Increase towards this gradually and drink enough fluid. Individual needs can differ, especially if you have a digestive condition or have been given specialist dietary advice.
What are easy fibre sources during weight loss?
Useful choices include oats, wholemeal bread, wholegrain cereals, vegetables, whole fruit, beans, lentils, chickpeas, nuts and seeds. Choose portions that fit your overall plan and any medical or dietary requirements.
How much should I drink?
The NHS generally recommends six to eight glasses of fluid a day, with more sometimes needed in hot weather or during activity. People with a clinical fluid restriction should follow their own plan rather than general advice.
How long should constipation last before I seek help?
There is no single time limit that suits every situation. Speak to a pharmacist if lifestyle changes are not helping. Arrange GP advice if symptoms persist, keep returning, cause pain, or occur with bleeding, unusual tiredness, unexplained weight loss or a sudden bowel habit change. Seek urgent help immediately for severe symptoms.
Can I take a laxative while trying to lose weight?
A pharmacist or doctor can advise whether a laxative is suitable. Laxatives treat constipation, not body fat. Do not use them to influence weight and do not exceed the label or continue over the counter use beyond the recommended period.
Should I stop weight management treatment if I am constipated?
Do not make that decision alone. Contact the prescriber so they can assess the severity, other symptoms, your health history and the risks of any change. Get urgent help rather than waiting for the prescriber if you have severe pain, vomiting, marked swelling or cannot pass stool or wind.
Can constipation cause actual weight gain?
It can temporarily increase scale weight because of retained stool and fluid. This is not the same as gaining body fat. Follow longer term trends rather than reacting to one reading.
When should constipation be treated as an emergency?
Seek urgent help for constipation with severe or worsening abdominal pain, repeated vomiting, marked swelling, fever, black or dark red stool, heavy bleeding, collapse, or an inability to pass stool or wind. Follow NHS 111, Accident and Emergency or 999 guidance according to severity.
The bottom line
Constipation during weight loss is usually linked to changes in food quantity, fibre, fluids, activity, routine or treatment. A smaller amount of stool can be normal when you eat less, but hard stool, straining, discomfort and incomplete emptying are signs that need attention.
Gradual fibre changes, adequate fluid, regular movement and a calm toilet routine can help many people. A pharmacist can advise if these measures are not enough. Do not use laxatives for weight loss and do not independently alter prescribed treatment.
Persistent constipation, blood in the stool, unexplained weight loss, unusual tiredness or a lasting change in bowel habit should be checked. Severe pain, vomiting, marked abdominal swelling or an inability to pass stool or wind requires urgent medical assessment.
Sources and further reading
NHS: Constipation
NHS: Laxatives
NHS: Eating a balanced diet
NHS: Bloating
NHS: Stomach ache
NHS: Symptoms of bowel cancer
MHRA: Safe use of stimulant laxatives
Guy's and St Thomas' NHS Foundation Trust: Dietary advice for people taking weight loss medicines
Last updated: 18 August 2026
Medical disclaimer: This article provides general information and is not a diagnosis or a substitute for individual medical advice. Speak to a qualified healthcare professional about symptoms, medicines and treatment decisions. Use NHS 111, Accident and Emergency or 999 when symptoms are urgent.