Thin Stools (Bowel Movement) – What Causes It?

Defining normal stool is not always easy as there are variations among individuals and can the size, consistency and even color can vary between bowel movements. However, there are stools that can be clearly identified as being abnormal. For example, very watery stool or green stool are usually considered to be abnormal due to the presence of some disease. Similarly, stool that is thin is also considered to be abnormal although there is no clear measure of just how thick or thin normal stool should be.

Why are stools thin?

The thickness of stool is determined by the size of the anal canal and anus as stool passes through it. The rectum also plays some role in determining whether stool is thick or thin. Therefore narrowing within the rectum or anus will lead to thin stools. It can also occur when the stool is too watery or the movement through the bowel is too fast for the feces to firm up to the proper consistency. Similarly dry stool as is seen in constipation can become thin.

However, a narrowing in the rectum or anus is by far the most common cause. This may be caused by a mass that occupies space within the rectum or colon, or it can be caused by swelling in the lower bowel walls. Since there are muscles in the walls that can change the diameter of the lumen, any abnormal constriction of the bowel walls can lead to thinning stool. Similarly the highly muscular anal sphincter which has to open to let stool out may not relax fully and stool is forced through the smaller opening leading to thin stools.

The degree to how thin stools may be varies by condition and also depends on the severity of the underlying cause. Very thin stools may sometimes be described as pencil thin stools or worm-like stools. However, each person’s perspective differs and it should be compared to prior ‘normal size’ stools before determining whether it is thin or not.

Causes of Thin Stool

It can be difficult to identify the exact cause of thin stools without an assessment by a medical professional. Various diagnostic investigations may also be required, like a colonoscopy, before the exact cause can be identified. Other symptoms that accompany thin stools need to also be taken into account when attempting to diagnose the underlying cause.


Any diarrheal illness can lead to thin stools. Most of the time we think of watery stools when we think of diarrhea. While this is often the case, diarrhea can also be defined by having more than three bowel movements in a day. The rapid movement of stool through the bowels coupled with reduced bowel contents from not eating as normal can contribute to the thinner than normal stools. This may be seen with gastroenteritis, inflammatory bowel disease (IBD), irritable bowel syndrome (IBS) and other conditions where diarrhea occurs.


Thin stool s can also occur in constipation. In some cases the anal sphincter does not relax fully to open wide enough for stool to pass out with ease and the stool may be thinner than normal. Similarly hard stool can take on a thin form at times. Thin stool is more likely with a complication of constipation known as fecal impaction. Here the stool becomes hard and obstructs the rectum. Any soft stool will have to squeeze through the limited space and may therefore be expelled as thin stool.


Polyps are growths that extend from the bowel wall. This can lead to thin stool when it occurs near the end of the colon. The growth causes a physical obstruction and thin stool is more likely to occur when the mass is larger. Colon polyps may be non-cancerous (benign) or pre-cancerous meaning that it has the potential to become a cancer.

Foreign Object

A foreign object in the gut is any indigestible substance that is not part of food. This is usually a problem with children who may swallow small objects like toys. However, sometimes the object is produced within the body as is the case with a gallstone ileus. Here a large stone released from the gallbladder can cause an obstruction within the bowels.


Thin stools may also occur with colorectal cancer, one of the most common cancers seen in developed nations.In this case the pencil thin stools persists as other symptoms of cancer become apparent, such as weight loss and rectal bleeding. It can also occur with anal cancer but anal cancer is far less common than colorectal cancer.


A pseudo-obstruction means that there is no physical blockage caused by an abnormal mass within the bowels. This may be seen in conditions where the bowel wall stretches excessively. When this occurs the stool cannot be pushed through as normal and then collects at the area where the abnormal distension is present.


A hernia is where a portion of the abdominal wall is weakened and may protrude outwards. Hernias are common and may not contribute to thin stools on its own. However, if a portion of the bowel is trapped within the hernia then the bowel becomes narrowed. In severe cases the trapped bowel may become strangulated and blood supply is cut off to the section of the bowel wall.

Twisting and Telescoping

The bowel can become twisted or slide into itself (telescoping) which may then narrow the lumen. Stool then has very little space to move through and thin stools may arise. This twisting is known as a volvulus and telescoping is referred to as intussusception. These are not common conditions and are more likely to be seen in children.


Strictures refer to narrowing of the cavity without a mass within the cavity. In terms of thin stools this may be seen with an anorectal stricture where there is narrowing within the rectum, the anal canal or anus. Compression of these structures by a mass outside of the rectum or anal canal that presses against it. This can occur with an enlarged prostate or bladder cancers.

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