What Causes Diarrhea After Eating?

Most of us do not consider any link between eating and diarrhea unless we have a common diarrheal illness like the stomach flu (viral gastroenteritis). Howevever, for some people diarrhea after eating can be an almost daily occurrence. There are several possible reasons for this eating and diarrhea link. Some of these causes may be acute while others can be chronic.

It is important to understand some of the basics of how food travels from the mouth to the anus. Contrary to what may people may think, the food that has just been eaten is not the stool that is passed out if there is diarrhea immediately after eating. Even when the gut is irritated and the movement of food and waste through the bowels is faster than normal, it can still take several hours for food that is eaten to reach the rectum.

Time for Food to Exit the Bowels

The time it takes for food to travel from the mouth, be digested and passed out as a waste (feces) is known as the bowel transit time. This can vary from person to person but it is within 12 to 48 hours. It can be, however, take as long as 72 hours. Some people experience a bowel movement just after eating but this is not the same food that has been consumed.

The process of defecation is controlled by certain nerve reflexes which are triggered by surrounding parts of the bowel. When the lower parts of the colon is stretched with a large quantity of feces, it triggers nerve reflexes that lead to the defecation process. A person feels the urge to pass stool and if the setting is appropriate, the anal sphincters will relax and a bowel movement will occur.

Diarrhea After a Meal

Having a bowel movement does not constitute diarrhea if the stool is firm, well formed but soft, and there are less than three bowel movements within a 24 hour period. Sometimes there may be the need to pass stool shortly after having a meal and it can occur repeatedly with almost every significant meal for a number of different reasons. The causes of bowel movement after eating can overlap with the cause of diarrhea after eating.

When a person is suffering from diarrhea due to any number of causes, the bowel and nerves controlling bowel activity are usually irritated. The bowels are essentially hyperactive. Although the nerves for each part of the gut have specific functions for that area, there are some nerve reflexes in one part of the gut that can also affect another part of the gut. In the case of bowel movements, this is known as defecation reflexes.

Most defecation reflexes are isolated to the lower bowel. However, some reflexes can be triggered as high as the stomach and then stimulate the defecation process.The two important ones in this case are the :

  • Gastrocolic reflex which is initiated by stretching of the stomach while eating or immediately after a meal.
  • Duodenocolic reflex which is triggered by stretching of the duodenum (first part of the small intestine) after eating.

Both these reflexes then stimulate the lower bowels (colon). The urge to have a bowel movement is triggered almost immediately after eating or within a 20 to 45 minute period on average.

Picture of the stomach and bowels.

Source : Wikimedia Commons

The causes of diarrhea after eating can be considered in terms of when it arose and how long it has been persisting. Therefore it may either be acute where it started suddenly and only occurred a few times without a previous history of diarrhea after eating of most meals. Alternatively it may be chronic where diarrhea may arise with almost every meal and has been ongoing for weeks, months or even years.

Acute Causes

A single attack (or few attacks) of diarrhea occurring few minutes to few hours after eating may be due to :

  • Gastroenteritis which is an infection of the stomach and bowels often caused by viruses but may also be caused by bacteria and protozoa. Enterocolitis is another condition which involves only the bowels that can also result in diarrhea.
  • Combination of certain foods – some foods do not mix well in the stomach and small intestine and can irritate the gut thereby leading to diarrhea.
  • Magnesium which may be in some mineral waters, sport drinks, antacids and supplements may draw water out of the bowels or prevent reabsorption of water in the colon thereby resulting in diarrhea.
  • Food poisoning with toxins released from bacteria, like Staphylococcus aureus. The symptoms of food poisoning such as diarrhea may appear from 20 minutes to 8 hours after ingesting contaminated food.

Chronic Causes

Diarrhea after a meal, occurring on a long term basis with repeated attacks, may be due to :

  • Food allergy which causes tingling around the mouth or in the throat, hot facial flushes, hives with skin itch and diarrhea may appear in the first few minutes or several hours after eating. Diarrhea can also arise particularly in milder cases.
  • Lactose intolerance is an inability to digest lactose found in certain foods like dairy there resulting in bloating and/or diarrhea.
  • Rapid gastric emptying – dumping syndrome where the contents of the stomach quickly pass into the intestines without adequate digestion.
  • Irritable bowel syndrome (IBS) (diarrhea-predominant IBS) where a person’s bowel activity is exaggerated for no known reason.
  • Toddler’s diarrhea is due to overfeeding fruit juices to toddlers which irritates the gut but otherwise the child is healthy.
  • Fructose malabsorption– bloating and/or diarrhea may appear in few hours after ingestion fruits, honey, or other foods containing fructose, sorbitol, xylitol, or HFCS.
  • Inflammatory bowel disease (IBD) where there is inflammation of the wall of the bowels which may be related an immune disorder.
  • Short bowel syndrome (SBS) is a condition where food is not properly digested in the small intestine either because a part of it is diseases or has been surgically removed.

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