Constipation with Dieting and Fasting – Causes and Remedies

Constipation is a common symptom that many people experience when dieting or fasting. There are several reasons why constipation may occur in these situations. However, it is important to note that constipation does not affect every person who is on a diet or fast. It is more likely to occur during periods of strict dieting or fasting, particularly when the diet or fast is prolonged.

Can dieting cause constipation?

This is a common question that many people ask when constipation arises after commencing with a diet or fast. The answer is yes, both dieting and fasting can lead to constipation. It is important to define these terms to understand how and why these changes in diet leads to alterations in bowel habit.

Dieting is the reduction in food intake for the purpose of weight loss. Fasting is largely the same but the reasons for this change in diet is for purposes other than weight loss. Irrespective of the underlying goal of these dietary changes, the common approach is to reduce and sometimes even completely stop food intake altogether.

Constipation is an abnormality in bowel habit where a person has less than three bowel movements in a week. Usually there is difficulty in passing the hard stools that are characteristic of constipation thereby requiring significant straining to have a bowel movement. Constipation can be acute where it lasts for only a week or two, or it can be chronic where it may persist for several weeks, months and even years.

It is important to note that constipation may occur for reasons other than the current diet or fast. Therefore it should be assessed by a medical professional. Sometimes a diet or fast aggravates a pre-existing condition that may be the cause of the constipation thereby making the constipation more obvious.

How does dieting cause constipation?

Stool is formed from the food that is consumed. Depending on individual bowel habit, it can take up to three days for food that is eaten to eventually be digested and absorbed and for the remaining material to form stool. In addition, water, wastes, digestive enzymes, bile, mucus and bacteria also make up the stool that is passed out.

Bowel motility is the movement through the bowels which helps with the formation of stool and pushes it out during defecation. The muscles in the bowel wall contract and relax in a coordinate manner to move the bowel contents. Furthermore physical activity also assists with this process. Therefore constipation arises when there is insufficient stool as well as reduced bowel motility.

Minimal Food Intake

When there is minimal intake of food due to fasting or dieting then there is not enough residual material to form stool. Essentially the bowels are “empty”. The little food that is being consumed may not comprise sufficient bulk to form a soft but solid stool which can then be pushed out. It also depends on the type of food that is consumed, if any food at all is eaten.

Insufficient Fiber

Dietary fiber is an important component of soft, solid and sausage-shaped stool which is considered as normal. One of the leading causes of constipation is insufficient fiber intake in foods and it may be more pronounced with strict dieting and fasting. Fiber cannot be absorbed by the human body so it remains in the gut where it absorbs water to keep stool soft. This helps with the easy passage of stool.


Depending on the type of diet or fast, dehydration may arise. Insufficient intake of fluids with suitable electrolytes can lead to dehydration within a day or even within hours in hot environments and with strenuous physical activity. Water is essential for soft but solid stool that can be easily passed out during defecation.

Underactive Thyroid Gland

Thyroid activity slows down during periods of starvation. This is also known as starvation-induced hypothyroidism. It is one of the reasons, along with low blood glucose (hypoglycaemia), why people who are fasting or dieting experience symptoms like fatigue and poor memory. Constipation is one of the symptoms of reduced thyroid activity.

Low Blood Glucose Levels

Hypoglycemia (low blood glucose levels) is a common consequence of strict dieting or fasting. It shifts the body into a low energy state. A person feels tired and lethargic. This leads to lower levels of physical activity, which slows down movement in the bowels. Furthermore the body shifts resources away from areas like the bowels to maintain life-sustaining process with its limited resources. It may further result is slower bowel activity.

Read more on exercise for constipation.

How to Prevent Constipation When Dieting

Fad diets that promise rapid weight loss are at times the same as starvation. Contrary to popular belief this does not lead to weight loss in the long term and a person may find there is significant weight gain after the diet is stopped. Constipation may be one sign that the choice of diet is unsuitable for maintaining health and wellbeing while losing weight, especially if a person did not previously suffer with constipation.

Read more on rapid weight loss dangers.

The following dietary and lifestyle remedies can prevent constipation during dieting and fasting:

  • Consult with a registered dietitian about a suitable diet to lose weight gradually without limiting the body’s resources to the point that side effects like constipation may arise.
  • Periods of starvation can lead to weight gain which ultimately defeats the purpose of a diet. Rather continue with a calorie restricted diet that is balanced to prevent any deficiencies.
  • Avoid high protein diets that eliminate fruits and vegetables which have essential micronutrients. These diets can be dangerous.
  • Consume enough fiber either through fruits, vegetables and wholegrains or with fiber supplements. Fiber cannot be absorbed and does not contribute to fat.
  • Drink sufficient water throughout the diet or fast. Water with electrolytes are a better option as it will prevent dehydration.
  • Increase physical activity which will allow for faster weight loss without having to severely restrict calorie intake. It also helps with movement through the bowels.
  • Eat small meals several times in a day to maintain energy levels. It also helps regulate blood glucose levels and minimize weight gain.
  • Do not use laxatives as a means of having a bowel movement during dieting. Some laxative can draw out water from the body and hasten dehydration.

Please note that any information or feedback on this website is not intended to replace a consultation with a health care professional and will not constitute a medical diagnosis. By using this website and the comment service you agree to abide by the comment terms and conditions as outlined on this page