What Is a Diarrhea?

Diarrhea (from Greek ‘dia’ - through, ‘rhein’ - flowing) means having more than three bowel movements, or passing more than 300g of watery stool daily (1).

What Is Not a Diarrhea?

Ten diapers a day are usual in a 14 days old infant. Three soft bowel movements a day may be considered normal for adult on a fibre-rich diet. Stool soiling in children who are already toilet trained may be due to defective anus. Stool incontinence or mucus seeping in adults may be due to rectal inflammation, rectal prolapse, hemorrhoids, uncoordinated pelvic floor muscles, or anal muscle or nerve damage (2). In all mentioned cases, bowel movements tend to be of normal volume and consistency.

Occasional single loose stool still isn’t a diarrhea. Unripe fruits, green potatoes, spicy or hot food may all irritate the bowel. Insufficiently cooked or chewed food, a heavy sugary or fatty meal may be hard to digest. Wrong food combinations, like meat with sugar, may result in a loose stool. Food which is psychically rejected, after ingesting, might flow through the intestine quickly. Caffeine stimulates peristalsis, as can strong emotions like fear.

How a Healthy Bowel Works?

Bowel anatomy. Stomach lies right below the bottom end of the breastbone. The small intestine (duodenum, jejunum and ileum) fills up the most of the abdominal cavity. The large intestine starts in the lower right abdomen with the appendix, runs up as the ascending colon toward the liver, then as the transverse colon below the stomach, then on the left side down as the descending colon, in the pelvis it widens into the sigmoid colon, and ends as the rectum and anal opening.

Gut motility. Swallowed food excites the nerves in the gut wall and triggers rhythmic contractions of the smooth gut muscles, which slowly push the food toward the anus; this process is called peristalsis. Food factors which stimulate peristalsis: food volume, temperature, fiber and liquid content, and low calorie value (4). The gut and the brain are connected with nerves (’the brain- gut axis’), so the ingested food may influence a person’s mood; on the other hand the pain, thoughts, and emotions, which act through the brain, may stimulate or depress peristalsis (5). Defecation can be consciously controlled by the muscle, which encircles the anus (external anal sphincter).

Liver, gallbladder and pancreas. In the liver, bile is produced, temporarily stored in the gallbladder, and then delivered into duodenum via cystic and common hepatobiliary duct. In the pancreas, digestive enzymes are produced and delivered into duodenum via pancreatic duct. The common hepatobiliary and the pancreatic duct join before they open into the duodenum (3). This opening, called papilla Vateri, is embraced by a smooth muscle - sphincter of Oddi.

Digestion. Proteins are broken down in the stomach by enzyme pepsin, and in the duodenum by the pancreatic enzymes trypsin and chymotrypsin, into amino acids. Carbohydrates are broken down by pancreatic enzyme amylase, and by enzymes from the small intestinal mucosa: sucrase break down sucrose (table sugar); lactase breaks down lactose (milk sugar) into glucose and galactose. Fats are broken down by pancreatic enzyme lipase into fatty acids and glycerol. Bile acids are needed to emulsify fats for digestion.
 
Intestinal absorption. Most nutrients, water and electrolytes (Na, K, Ca, Mg, Fe) are absorbed in the duodenum and jejunum, while vitamin B12 is absorbed mainly in the terminal ileum. Bile acids are absorbed through the entire small intestine, but mainly in the terminal ileum; after absorption they enter the liver and may be used again. Fats can only be absorbed in the presence of bile, while vitamins A, D, E and K may only be absorbed together with fats. Additional sodium and water are absorbed in the colon. From the intestinal hollow, nutrients enter intestinal mucosal cells and from there into venous blood and lymph. From the intestinal mucosa many small finger-like projections (villi) bulge toward the intestinal hollow, and thus greatly increase absorptive surface. 

Gut Flora (Intestinal Bacteria)

Many bacteria (and some yeasts and protozoa) normally live in a healthy human gut; mostly in the colon, fewer in the small intestine, but none in the stomach. Newborn gets its first bacteria by swallowing them during parturition.
 
Normal (beneficial) intestinal bacteria can:

• prevent the growth of harmful bacteria;
• prevent intestinal inflammation;
• chemically change bile acids so they can be absorbed when they reach the distal small intestine;
• break down fibers, lactose and other carbohydrates into short chain fatty acids, which are absorbed in the colon, enhancing colonic water absorption;
• synthesize some vitamin K and vitamin B12. 

The composition of gut flora differs from person to person and depends on age, diet, environment and use of antibiotics (6). Bacteria in the proximal small intestine are mostly aerobic lactobacilli and enterococci, while those in the distal small intestine and colon are mostly anaerobes (Bacteroides, Bifidobacterium bifidum, E. coli, enterococci, clostridia, anaerobic lactobacilli) (6). In breast-fed infants Bifidobacteria predominate.

Normal Stool

Stool frequency. A newborn passes its first stool in the first two days. During the first month, breast-fed babies usually have 8-10 stools per day, at one month 4 per day, at four months 2 per day, and a child at four years usually has 1 stool per day (7). Three stools per day down to 3 stools per week may be normal for children and adults on solid food.

Stool quantity depends on the amount of ingested food and its fiber content. Two liters of mixed food yields about 200g of stool. The more fiber in the diet, the bulkier the stool.

Stool consistency. A normal stool is semi-solid. Food fibers make stools soft as they tend to bind water. If not enough water is consumed, stools will be hard; on the other hand a lot (up to 20 liters/day) of water consumption will not result in a softer stool, since most of the water is absorbed in the intestine.

The color of the stool in a healthy adult is any shade of brown, or even green. A green stool may originate from green vegetables, fruit juices, or iron supplements. A newborn’s first stool (meconium) is greenish black. A black stool may come from licorice, iron supplements, or Pepto-Bismol. A red stool may originate from beetroot, tomato sauce, red Jelly-O, etc.

Stool composition: 60-90% of water, the rest are fibers and other undigested substances, bacteria, shed intestinal cells, bile pigments, and minerals.

Mechanisms of Diarrhea

Osmotic diarrhea. When a particular nutrient (solute) is not absorbed, it attracts water from blood vessels in the intestinal wall (where it is found in lower concentrations) into the intestinal hollow (with high concentrations). This process is called osmosis and occurs after ingestion of large amounts of unabsorbable solutes (e.g. sorbitol), or when nutrients stay within the intestine, because they can’t be digested (in lack of digestive enzymes), or absorbed (in inflammation or surgical resection of a part of intestine).

Secretory diarrhea. Unabsorbed fatty or bile acids trigger water secretion from colonic mucosa; toxins from some bacteria (E. coli, V. cholerae) or some drugs (quinine) have the same effect on the small intestinal mucosa. Secretory diarrhea is watery.

Exudative diarrhea. From ulcerated intestinal mucosa (in shigellosis, amebiasis, ulcerative colitis), the blood, proteins and pus may exudate and appear in the stool. Exudative diarrhea is often of low volume.

Motility diarrhea. In increased gut motility (psychic stress, irritant food, bacterial toxins, laxatives, hyperthyroidism) there is not enough time for adequate water and nutrient absorption, thus resulting in motility diarrhea.

More than one mechanism is usually involved in each diarrheal event.

Diarrheal Stool

The frequency of diarrheal stool may vary from three a day to twenty a day or more in extreme cases. Over 20 liters of water with electrolytes (potassium, sodium, magnesium) may be lost in one day in severe diarrhea. Diarrheal stool may be anything from clear liquid to soft formed mass.
Water in diarrheal stools originates from food, unabsorbed digestive juices, or increased intestinal secretion. White diarrheal stool is from unabsorbed fats (>6g fats/day is abnormal), and yellow stool from lack of bile acids. Green diarrhea is from unabsorbed bile acids. Bloody diarrhea is from ulcerated colonic, or (rarely) small intestinal mucosa. Black colored diarrheal stool is from bleeding from the mouth, nose, throat, lungs, esophagus or stomach, or from antidiarrheal drug Pepto-Bismol. Other components of diarrheal stool: undigested substances, mucus, sugars (e.g. lactose), and microorganisms.

Is Diarrhea Harmful?

In many cases, diarrhea is only an unpleasant event. However, a few liters of body water lost during diarrhea may lead to dehydration within 24 hours, may severely affect metabolism, muscles, nerves, heart, or consciousness, and may cause permanent damage of affected organs. About 2.2 million children die from diarrhea (mostly from dehydration) every year, mostly in countries where medical help is not easily accessible; malnourished children with lowered immunity or chronic diseases are at greatest risk (8). Repeating acute or chronic diarrhea may lead to malnutrition.

Causes of Diarrhea

1. gastrointestinal infections: viruses (mostly Rotavirus), bacteria (e.g. Escherichia coli, Campylobacter, Salmonella, Shigella), parasites (e.g. Giardia, intestinal worms); primarily non-gastrointestinal infections (e.g. measles, tuberculosis);
2. inappropriate food (artificial sweeteners, overfeeding);
3. psychic factors;
4. food intolerance: lactose intolerance, celiac disease, food allergies;
5. medication: antibiotics;
6. toxins: pesticides, poisonous plants;
7. intestinal disease: e.g. inflammatory bowel disease, lymphoma;
8. other abdominal disease: liver, pancreatic, gallbladder disease;
9. other causes: AIDS, hyperthyroidism, cystic fibrosis, competitive running, etc.

Causes of acute diarrhea and chronic (constant) diarrhea.

References:

1. http://cmaj.ca/cgi/content/abstract/116/7/737 DEFINITION OF DIARRHEA
2. http://digestive.niddk.nih.gov/ddiseases/pubs/fecalincontinence/ STOOL INCONTINENCE
3. http://www2.healthtalk.com/reference/graphics/images/en/19261.jpg BILIARY TREE
4. http://www.biomedcentral.com/1471-230X/3/12/#IDA3E2MF FOOD FACTORS INFLUENCING GUT MOTILITY
5. http://ibs-research-update.org.uk/ibs/brain1ie4.html BRAIN-GUT AXIS
6. http://textbookofbacteriology.net/normalflora.html GUT FLORA
7. http://cme.med.umich.edu/pdf/guideline/peds03.pdf STOOL FREQUENCY
8. http://rehydrate.org/diarrhoea/index.html DIARRHEA MORTALITY