Substantial numbers of patients with intestinal malabsorption present initially with symptoms or laboratory abnormalities that point to other organ systems in the absence of or overshadowing symptoms referable to the gastrointestinal tract. For example, there is increasing epidemiologic evidence that more patients with coeliac disease present with anemia and osteopenia in the absence of significant classic gastrointestinal symptoms. Microcytic, macrocytic, or dimorphic anemia may reflect impaired iron, folate, or vitamin B12 absorption. Purpura, subconjunctival hemorrhage, or even frank bleeding may reflect hypoprothrombinemia secondary to vitamin K malabsorption. Osteopenia is common, especially in the presence of steatorrhea. Impaired calcium and vitamin D absorption and chelation of calcium by unabsorbed fatty acids resulting in fecal loss of calcium may all contribute. If calcium deficiency is prolonged, secondary hyperparathyroidism may develop. Prolonged malnutrition may induce amenorrhea, infertility, and impotence. Edema and even ascites may reflect hypoproteinemia associated with protein losing enteropathy caused by lymphatic obstruction or extensive mucosal inflammation. Dermatitis and peripheral neuropathy may be caused by malabsorption of specific vitamins or micronutrients and essential fatty acids.

Depending on the nature of the disease process causing malabsorption and its extent, gastrointestinal symptoms may range from severe to subtle or may even be totally absent. Diarrhea, weight loss, flatulence, abdominal bloating, abdominal cramps, and pain may be present. Although diarrhea is a common complaint, the character and frequency of stools may vary considerably ranging from over 10 watery stools per day to less than one voluminous putty-like stool, the latter causing some patients to complain of constipation. On the other hand, stool mass is invariably increased in patients with steatorrhea and generalized malabsorption above the normal with 150–200 g/day. Not only do unabsorbed nutrients contribute to stool mass but mucosal fluid and electrolyte secretion is also increased in diseases associated with mucosal inflammation such as coeliac disease. In addition, unabsorbed fatty acids, converted to hydroxy-fatty acids by colonic flora, as well as unabsorbed bile acids both impair absorption and induce secretion of water and electrolytes by the colon adding to stool mass. Weight loss is common among patients with significant intestinal malabsorption but must be evaluated in the context of caloric intake. Some patients compensate for fecal wastage of unabsorbed nutrients by significantly increasing their oral intake. Eliciting a careful dietary history from patients with suspected malabsorption is therefore crucial. Excessive flatus and abdominal bloating may reflect excessive gas production due to fermentation of unabsorbed carbohydrate, especially among patients with primary or secondary disaccharidase deficiency. Malabsorption of dietary nutrients and excessive fluid secretion by inflamed small intestine also contribute to abdominal distention and bloating. Prevalence, severity, and character of abdominal pain vary considerably among the various disease processes associated with intestinal malabsorption. For example, pain is common in patients with chronic pancreatitis or pancreatic cancer and Crohn disease, but it is absent in many patients with coeliac disease or postgastrectomy malabsorption.

Bile acid malabsorption, known also as bile acid diarrhea, is a cause of several gut-related problems, the main one being chronic diarrhea. It has also been called bile acid-induced diarrhea, cholerheic or choleretic enteropathy and bile salt malabsorption. It can result from malabsorption secondary to gastrointestinal disease, or be a primary disorder, associated with excessive bile acid production. Treatment with bile acid sequestrants is often effective.

Bile acid malabsorption was first recognized in patients with ileal disease. When other causes were recognized, and an idiopathic, primary form described, a classification into three types was proposed:

- Type 1: Bile acid malabsorption, secondary to ileal resection, or ileal inflammation (e.g. in Crohn's disease)

- Type 2: Idiopathic bile acid malabsorption, Primary bile acid diarrhea

- Type 3: Secondary to various gastrointestinal diseases including cholecystectomy, vagotomy, small intestinal bacterial overgrowth, radiation enteropathy, celiac disease, chronic pancreatitis, etc.

Intestinal failure is decreased intestinal function such that nutrients, water, and electrolytes are not sufficiently absorbed. Short bowel syndrome is when there is less than of working bowel and is the most common cause of intestinal failure.

The symptoms of short bowel syndrome can include:

- Abdominal pain

- Diarrhea and steatorrhea (oily, bulky stool, which can be malodorous)

- Fluid depletion

- Weight loss and malnutrition

- Fatigue

Persons with short bowel syndrome may have complications caused by malabsorption of vitamins and minerals, such as deficiencies in vitamins A, D, E, K, B (folic acid), and B, calcium, magnesium, iron, and zinc. These may appear as anemia, hyperkeratosis (scaling of the skin), easy bruising, muscle spasms, poor blood clotting, and bone pain.

Irrespective of the cause, achlorhydria can result as known complications of bacterial overgrowth and intestinal metaplasia and symptoms are often consistent with those diseases:

- gastroesophageal reflux disease (source needed)

- abdominal discomfort

- early satiety

- weight loss

- diarrhea

- constipation

- abdominal bloating

- anemia

- stomach infection

- malabsorption of food.

- carcinoma of stomach.

Since acidic pH facilitates the absorption of iron, achlorhydric patients often develop iron deficiency anemia.

Acidic environment of stomach helps conversion of pepsinogen into pepsin which is most important to digest the protein into smaller component like complex protein into simple peptides and amino acids inside stomach which is later absorbs by gastro intestinal tract.

Bacterial overgrowth and B12 deficiency (pernicious anemia) can cause micronutrient deficiencies that result in various clinical neurological manifestations, including visual changes, paresthesias, ataxia, limb weakness, gait disturbance, memory defects, hallucinations and personality and mood changes.

Risk of particular infections, such as "Vibrio vulnificus" (commonly from seafood) is increased. Even without bacterial overgrowth, low stomach acid (high pH) can lead to nutritional deficiencies through decreased absorption of basic electrolytes (magnesium, zinc, etc.) and vitamins (including vitamin C, vitamin K, and the B complex of vitamins). Such deficiencies may be involved in the development of a wide range of pathologies, from fairly benign neuromuscular issues to life-threatening diseases.

Achlorhydria or hypochlorhydria refers to states where the production of hydrochloric acid in gastric secretions of the stomach and other digestive organs is absent or low, respectively. It is associated with various other medical problems.

Most of the symptoms of BLS are non specific but nevertheless warrant the utmost attention. These include:

- Loss of appetite

- Nausea

- Flatulence

- Diarrhea

- Fullness after a meal

- Fatty stools (steatorrhea)

- Unintentional weight loss

- Generalised weakness

As a result of the concomitant vitamin and mineral deficiencies that occur as a result of the malabsorption associated with BLS patients with advanced cases should be investigated for:

- Vitamin B12 deficiency

- Folate deficiency

- Iron deficiency

- Vitamin E deficiency

The small intestine consists of the duodenum, jejunum and ileum. Inflammation of the small intestine is called enteritis, which if localised to just part is called duodenitis, jejunitis and ileitis, respectively. Peptic ulcers are also common in the duodenum.

Chronic diseases of malabsorption may affect the small intestine, including the autoimmune coeliac disease, infective Tropical sprue, and congenital or surgical short bowel syndrome. Other rarer diseases affecting the small intestine include Curling's ulcer, blind loop syndrome, Milroy disease and Whipple's disease. Tumours of the small intestine include gastrointestinal stromal tumours, lipomas, hamartomas and carcinoid syndromes.

Diseases of the small intestine may present with symptoms such as diarrhoea, malnutrition, fatigue and weight loss. Investigations pursued may include blood tests to monitor nutrition, such as iron levels, folate and calcium, endoscopy and biopsy of the duodenum, and barium swallow. Treatments may include renutrition, and antibiotics for infections.

Diseases that affect the large intestine may affect it in whole or in part. Appendicitis is one such disease, caused by inflammation of the appendix. Generalised inflammation of the large intestine is referred to as colitis, which when caused be the bacteria "Clostridium difficile" is referred to as pseudomembranous colitis. Diverticulitis is a common cause of abdominal pain resulting from outpouchings that particularly affects the colon. Functional colonic diseases refer to disorders without a known cause, and include irritable bowel syndrome and intestinal pseudoobstruction. Constipation may result from lifestyle factors, impaction of a rigid stool in the rectum, or in neonates, Hirschprung's disease.

Diseases affecting the large intestine may cause blood to be passed with stool, may cause constipation, or may result in abdominal pain or a fever. Tests that specifically examine the function of the large intestine include barium swallows, abdominal x-rays, and colonoscopy.

Blind loop syndrome (BLS), commonly referred to in the literature as small intestinal bacterial overgrowth (SIBO) or bacterial overgrowth syndrome (BOS), is a state that occurs when the normal bacterial flora of the small intestine proliferates to numbers that cause significant derangement to the normal physiological processes of digestion and absorption. In some cases of blind loop syndrome, overgrowth of pathogenic non-commensal bacteria has also been noted. It has long been understood that from birth, and throughout life, large amounts of bacteria reside symbiotically within animal gastrointestinal tracts such as the human gastrointestinal tract. The understanding of this gut flora has even led to novel treatments for bowel irregularity that utilize so called "probiotics" or good bacteria that aid in normal digestion.

The problem of BLS arises when the bacterial colonies residing in the upper gastrointestinal tract begin to grow out of control or are altered in their makeup thereby creating a burden on the normal physiological processes occurring in the small intestine. This results in problems inclusive of but not restricted to vitamin B12 deficiency, fat malabsorption and steatorrhea, fat-soluble vitamin deficiencies and intestinal wall injury.

Fructose malabsorption may cause gastrointestinal symptoms such as abdominal pain, bloating, flatulence or diarrhea.

Gastrinoma causes the following symptoms:

- Hypergastrinemia

- Ulcers of the duodenum, stomach, and small intestine.

- Severe diarrhea.

- Generalized cancer symptoms.

Fructose malabsorption, formerly named "dietary fructose intolerance" (DFI), is a digestive disorder in which absorption of fructose is impaired by deficient fructose carriers in the small intestine's enterocytes. This results in an increased concentration of fructose in the entire intestine. Intolerance to fructose was first identified and reported in 1956.

Occurrence in patients identified to be suffering symptoms of irritable bowel syndrome is not higher than occurrence in the normal population. However, due to the similarity in symptoms, patients with fructose malabsorption often fit the profile of those with irritable bowel syndrome. In some cases, fructose malabsorption may be caused by several diseases which cause an intestinal damage, such as celiac disease.

Fructose malabsorption is not to be confused with hereditary fructose intolerance, a potentially fatal condition in which the liver enzymes that break up fructose are deficient.

Postcholecystectomy syndrome describes the presence of abdominal symptoms after surgical removal of the gallbladder (cholecystectomy), 2 years after the surgery.

Symptoms of postcholecystectomy syndrome may include:

- Dyspepsia, nausea, and vomiting.

- Flatulence, bloating, and diarrhea.

- Persistent pain in the upper right abdomen.

Symptoms occur in about 5 to 40 percent of patients who undergo cholecystectomy, and can be transient, persistent or lifelong. The chronic condition is diagnosed in approximately 10% of postcholecystectomy cases.

The pain associated with postcholecystectomy syndrome is usually ascribed to either sphincter of Oddi dysfunction or to post-surgical adhesions. A recent study shows that postcholecystectomy syndrome can be caused by biliary microlithiasis.

Approximately 50% of cases are due to biliary causes such as remaining stone, biliary injury, dysmotility, and choledococyst. The remaining 50% are due to non-biliary causes. This is because upper abdominal pain and gallstones are both common but are not always related.

Chronic diarrhea in postcholecystectomy syndrome is a type of bile acid diarrhea (type 3). This can be treated with a bile acid sequestrant like cholestyramine, colestipol or colesevelam, which may be better tolerated.

Bacterial overgrowth can cause a variety of symptoms, many of which are also found in other conditions, making the diagnosis challenging at times. Many of the symptoms are due to malabsorption of nutrients due to the effects of bacteria which either metabolize nutrients or cause inflammation of the small bowel, impairing absorption. The symptoms of bacterial overgrowth include nausea, flatus, constipation, bloating, abdominal distension, abdominal pain or discomfort, diarrhea, fatigue, and weakness. SIBO also causes an increased permeability of the small intestine. Some patients may lose weight. Children with bacterial overgrowth may develop malnutrition and have difficulty attaining proper growth. Steatorrhea, a sticky type of diarrhea where fats are not properly absorbed and spill into the stool, may also occur.

Patients with bacterial overgrowth that is longstanding can develop complications of their illness as a result of malabsorption of nutrients. Anemia may occur from a variety of mechanisms, as many of the nutrients involved in production of red blood cells are absorbed in the affected small bowel. Iron is absorbed in the more proximal parts of the small bowel, the duodenum and jejunum, and patients with malabsorption of iron can develop a microcytic anemia, with small red blood cells. Vitamin B is absorbed in the last part of the small bowel, the ileum, and patients who malabsorb vitamin B can develop a megaloblastic anemia with large red blood cells.

In older adults, small bowel bacterial overgrowth is associated with a higher frequency of diarrhea, a lower body mass index, and a significantly lower serum albumin concentration.

A gastrinoma is a tumor in the pancreas or duodenum that secretes excess of gastrin leading to ulceration in the duodenum, stomach and the small intestine. There is hypersecretion of HCl acid into the duodenum, which causes the ulcers. Excessive HCl acid production also causes hyperperistalsis, and inhibits the activity of lipase, causing severe diarrhea.

It is frequently the source of the gastrin in Zollinger-Ellison syndrome.

It is usually found in the duodenum, although it may arise in the stomach or pancreas. Those occurring in the pancreas have a greater potential for malignancy. Most gastrinomas are found in the gastrinoma triangle; this is bound by the junction of cystic and common bile ducts, junction of the second and third parts of the duodenum, and the junction of the neck and body of the pancreas.

The primary symptoms of IBS are abdominal pain or discomfort in association with frequent diarrhea or constipation and a change in bowel habits. Symptoms usually are experienced as acute attacks that subside within one day, but recurrent attacks are likely. There may also be urgency for bowel movements, a feeling of incomplete evacuation (tenesmus), bloating, or abdominal distension. In some cases, the symptoms are relieved by bowel movements. People with IBS, more commonly than others, have gastroesophageal reflux, symptoms relating to the genitourinary system, chronic fatigue syndrome, fibromyalgia, headache, backache, and psychiatric symptoms such as depression and anxiety. About a third of men and women who have IBS also report sexual dysfunction typically in the form of a reduction in libido.

Steatorrhea (or steatorrhoea) is the presence of excess fat in feces. Stools may be bulky and difficult to flush, have a pale and oily appearance and can be especially foul-smelling. An oily anal leakage or some level of fecal incontinence may occur. There is increased fat excretion, which can be measured by determining the fecal fat level. The definition of how much fecal fat constitutes steatorrhea has not been standardized.

Several medical conditions, or comorbidities, appear with greater frequency in patients diagnosed with IBS.

- Neurological/psychiatric: A study of 97,593 individuals with IBS identified comorbidities such as headache, fibromyalgia, and depression. IBS occurs in 51% of chronic fatigue syndrome patients and 49% of fibromyalgia patients, and psychiatric disorders occur in 94% of IBS patients.

- Inflammatory bowel disease: IBS may be a type of low-grade inflammatory bowel disease. Researchers have suggested IBS and IBD are interrelated diseases, noting that patients with IBD experience IBS-like symptoms when their IBD is in remission. A three-year study found that patients diagnosed with IBS were 16.3 times more likely to be diagnosed with IBD during the study period. Serum markers associated with inflammation have also been found in patients with IBS.

- Abdominal surgery: IBS patients were at increased risk of having unnecessary gall bladder removal surgery not due to an increased risk of gallstones, but rather to abdominal pain, awareness of having gallstones, and inappropriate surgical indications. These patients also are 87% more likely to undergo abdominal and pelvic surgery and three times more likely to undergo gallbladder surgery. Also, IBS patients were twice as likely to undergo hysterectomy.

- Endometriosis: One study reported a statistically significant link between migraine headaches, IBS, and endometriosis.

- Other chronic disorders: Interstitial cystitis may be associated with other chronic pain syndromes, such as irritable bowel syndrome and fibromyalgia. The connection between these syndromes is unknown.

The diagnosis of bacterial overgrowth can be made by physicians in various ways. Malabsorption can be detected by a test called the "D-xylose" test. Xylose is a sugar that does not require enzymes to be digested. The D-xylose test involves having a patient drink a certain quantity of D-xylose, and measuring levels in the urine and blood; if there is no evidence of D-xylose in the urine and blood, it suggests that the small bowel is not absorbing properly (as opposed to problems with enzymes required for digestion).

The gold standard for detection of bacterial overgrowth is the aspiration of more than 10 bacteria per millilitre from the small bowel. The normal small bowel has less than 10 bacteria per millilitre. Some experts however, consider aspiration of more than 10 positive if the flora is predominately colonic type bacteria as these types of bacteria are considered pathological in excessive numbers in the small intestine. The reliability of aspiration in the diagnosis of SIBO has been questioned as SIBO can be patchy and the reproducibility can be as low as 38 percent. Breath tests have their own reliability problems with a high rate of false positive. Some doctors factor in a patients' response to treatment as part of the diagnosis.

Breath tests have been developed to test for bacterial overgrowth, based on bacterial metabolism of carbohydrates to hydrogen and/or methane, or based on the detection of by-products of digestion of carbohydrates that are not usually metabolized. The hydrogen breath test involves having the patient fast for a minimum of 12 hours then having them drink a substrate usually glucose or lactulose, then measuring expired hydrogen and methane concentrations typically over a period of 2–3 hours. It compares well to jejunal aspirates in making the diagnosis of bacterial overgrowth. C and C based tests have also been developed based on the bacterial metabolism of D-xylose. Increased bacterial concentrations are also involved in the deconjugation of bile acids. The glycocholic acid breath test involves the administration of the bile acid C glychocholic acid, and the detection of CO, which would be elevated in bacterial overgrowth.

Some patients with symptoms of bacterial overgrowth will undergo gastroscopy, or visualization of the stomach and duodenum with an endoscopic camera. Biopsies of the small bowel in bacterial overgrowth can mimic those of celiac disease, making the diagnosis more challenging. Findings include blunting of villi, hyperplasia of crypts and an increased number of lymphocytes in the lamina propria.

However, some physicians suggest that if the suspicion of bacterial overgrowth is high enough, the best diagnostic test is a trial of treatment. If the symptoms improve, an empiric diagnosis of bacterial overgrowth can be made.

Most damage to the pyloric valve occurs as a complication of gastric surgery. Other causes of biliary reflux may be:

- Peptic ulcer

- Gallbladder surgery (cholecystectomy)

A significant fraction of cases are idiopathic, with no identified specific etiology.

Impaired digestion or absorption can result in fatty stools.

Possible causes include exocrine pancreatic insufficiency, with poor digestion from lack of lipases, loss of bile salts, which reduces micelle formation, and small intestinal disease producing malabsorption. Various other causes include certain medicines that block fat absorption, or indigestible or excess oil/fat in diet.

The absence of bile secretion can cause the feces to turn gray or pale. Other features of fat malabsorption may also occur such as reduced bone density, difficulty with vision under low light levels, bleeding, bruising and slow blood clotting times.

Congenital tufting enteropathy is an inherited disorder of the small intestine that presents with intractable diarrhea in young children.