eMedicine Specialties > Pediatrics: General Medicine > Gastroenterology

Malabsorption Syndromes

Author: Stefano Guandalini, MD, Director, University of Chicago Celiac Disease Program, Section Chief of Gastroenterology, Hepatology and Nutrition; Professor, Department of Pediatrics, University of Chicago Comer Children's Hospital
Coauthor(s): Richard E Frye, MD, PhD, Assistant Professor, Departments of Pediatrics and Neurology, University of Texas Health Science Center at Houston; M Akram Tamer, MD, Program Director, Professor, Department of Pediatrics, University of Miami
Contributor Information and Disclosures

Updated: Oct 23, 2009

Introduction

Background

Malabsorption syndromes encompass numerous clinical entities that result in chronic diarrhea, abdominal distention, and failure to thrive. Clinical malabsorption can be broken down into several distinct conditions, both congenital and acquired, that affect one or more of the different steps in the intestinal hydrolysis and subsequent transport of nutrients.

Pathophysiology

Carbohydrate, fat, or protein malabsorption is caused by a disorder in the intestinal processes of digestion, transport, or both of these nutrients across the intestinal mucosa into the systemic circulation. Either a congenital abnormality in the digestive or absorptive processes or, more commonly, a secondarily acquired disorder of such processes may result in malabsorption.

Carbohydrates

Of the carbohydrates most commonly present in the diet (starches, sucrose, lactose), only starches require preliminary luminal digestion by salivary and, more importantly, pancreatic amylases. Despite the slow development of pancreatic amylase, whose secretion reaches adult levels only during the second semester of life, cooked starch malabsorption is rare in infants because of the activity of the brush-border bound glucoamylase, an esoglycosidase that develops early in life.

The final products of amylase digestion include maltose, maltotriose, and higher residues of glucose polymers. The final hydrolysis of disaccharides and oligosaccharides occurs at the brush border of the enterocytes, where sucrase-isomaltase breaks down maltose, isomaltose (to glucose), and sucrose (to glucose and fructose); glucoamylase releases glucose from glucose polymers; and lactase splits lactose into glucose and galactose. Subsequent entry of the final monosaccharides (glucose, galactose, fructose) into the enterocytes through the brush border occurs via carrier molecules. Glucose and galactose share the same carrier, SGLT-1, which transports one molecule of the monosaccharide and one molecule of sodium (Na) in a secondarily active transport, energized by Na-activated and potassium (k)-activated adenosine triphosphatase (NaK ATPase). Instead, fructose uses a carrier that allows its entry only down a concentration gradient (facilitated diffusion).

Disorders of these processes can be congenital (cystic fibrosis and Shwachman-Diamond syndrome, which may cause amylase deficiency; the extremely rare congenital lactase deficiency; glucose-galactose malabsorption; sucrase-isomaltase deficiency; adult-type hypolactasia) or acquired: the most common being lactose intolerance, typically secondary to a damage of the mucosa, such as a viral enteritis or conditions that cause mucosal atrophy, such as celiac disease.

Protein

Proteins are first digested in the stomach, where pepsinogens, which are activated to pepsins by a pH of less than 4, hydrolyze them in large molecular weight peptides. Upon entering the duodenum, the pancreatic proteases (activated by trypsin, secreted by the pancreas as a proenzyme, trypsinogen, which is subsequently activated by the brush border–bound enterokinase) further split them into low molecular weight peptides and free amino acids.

Interestingly, the final breakdown products of intraluminal digestion of protein are composed of low molecular weight peptides (2-6 amino acid residues) for 70% and of free amino acids for 30%. Subsequently, brush border–bound peptidases further hydrolyze peptides to release a mixture of free amino acids and small peptides (2-3 amino acid residues). Finally, free amino acids are taken up by enterocytes through specific Na-linked carrier systems (5 carriers with selective affinities for groups of amino acids are described), whereas the small peptides are translocated into the absorptive epithelial cells by a system with a broad specificity, linked to H entry. Of interest, in the first few months of life, the latter system is much more active than those that transport amino acids and is thought to play a bigger physiological role.

Congenital disorders of protein digestion include conditions such as cystic fibrosis, Shwachman-Diamond syndrome, and enterokinase deficiency, which cause inadequate intraluminal digestion. No congenital defects have been described in any of the brush border–bound peptidases or in the peptide carrier.

Acquired disorders of protein digestion and/or absorption are nonspecific (ie, they also affect the absorption of carbohydrates and lipids) and are found in conditions that result in damage to the absorptive intestinal surface, such as extensive viral enteritis, milk protein allergy enteropathy, and celiac disease.

Lipids

A lingual lipase is responsible for the first partial hydrolysis of triglycerides; this enzyme becomes active in persons with low gastric pH levels and is active even in premature infants. However, the largest part of triglyceride digestion is accomplished in the duodenojejunal lumen because of a complex of pancreatic enzymes, the most important of which is the lipase-colipase complex. Like amylase, these enzymes also develop slowly, and this accounts for the known low capacity of babies to absorb lipids, termed physiologic steatorrhea of the newborn. Additionally, adequate concentrations of intraluminal conjugated bile salts are needed to form micelles, and the secretion of bile acids may also be partially inadequate in very young patients.

Disorders of these processes can be congenital (cystic fibrosis and Shwachman-Diamond syndrome, which cause lipase and colipase deficiency; the uncommon isolated deficiency of lipase and colipase; the extremely rare congenital primary bile acid malabsorption, which results in low bile acids concentrations) or acquired (secondary mostly to disorders of the liver and the biliary tract or to chronic pancreatitis). Clearly, any condition that results in the loss of small intestinal absorptive surface also causes steatorrhea.

Frequency

United States

Genetically determined syndromes

The prevalence of celiac disease in the United States is around 1% of the general population¹ and has increased over time.² Celiac disease in its entirety (ie, including the forms without overt malabsorption) is by far the most common inherited malabsorption syndrome. Cystic fibrosis is the second most common malabsorption syndrome. Other congenital disorders are rare, with the exception of adult-type hypolactasia, which has a prevalence that varies greatly among different ethnic groups.

Acquired syndromes

Cow's milk and soy milk protein allergies are common, especially in infants and young children. The prevalence of milk protein allergy, of which enteropathy is one of the presenting clinical symptoms, is estimated to be around 3%. A transient and common form of malabsorption in infants results from acute-onset enteritis (mostly viral, specifically rotaviral), which causes transient lactose intolerance. Although toddler's (or unspecific) diarrhea accounts for approximately 7.5% of referrals to pediatric gastroenterologists, it should not be considered a malabsorption syndrome because, by definition, no digestive or absorptive processes are impaired.

Secondary malabsorption syndromes that result from liver, pancreas, and intestinal diseases are uncommon. The manifestations vary according to the severity of each disease and the extent of intestinal mucosal injury.

Mortality/Morbidity

Although the morbidity can be severe, and aside from the single entity of cystic fibrosis, common malabsorption syndromes carry low mortality rates. Neonates and young infants, especially those with signs of malnutrition, are particularly at risk. In many of the congenital syndromes, morbidity varies with the particular syndrome and may be associated with systemic manifestations of the disease.

Race

Congenital sucrase-isomaltase deficiency is most common in Canadian Eskimos and natives of Greenland. Deficiency of trehalose, a sugar found almost exclusively in mushrooms, is rare, except in natives of Greenland. Adult-onset lactase deficiency is most common in persons of Asian, African, and Mediterranean descent.

Sex

Celiac disease is slightly more common in females. Autoimmune enteropathy is an X-linked disorder that only affects males in familial cases.

Age

Neonates and young infants with malabsorption syndromes are at particularly high risk for chronic diarrhea and malnutrition. Symptoms of a congenital disease are usually apparent shortly after birth (the exception being adult-onset lactase deficiency, which appears only after age 6-8 y) or after a short hiatus once a particular substance is ingested in substantial amounts. Protein sensitivity syndromes to milk or soy protein usually present in infants younger than 3 months, but solid food protein sensitivity syndromes are also known to occur in older patients.

Clinical

History

The following are important to assess in patients with possible malabsorption syndromes:

• Diet history: Obtain a complete history of the patient's diet, including the amount and type of fluids, solid foods, and formula ingested. Caregivers should keep a detailed journal of the patient's diet and symptoms for a minimum of 1 week.
  • The proper amount of fluid for most young children is around 100 mL/kg/d. Fluid intake that exceeds this amount may result in looser stools, which causes a referral for a suspected but nonexistent malabsorption.
  • Fat is important for slowing the movement of food through the intestine via hormonal mechanisms. Fat intake of less than 3 g/kg/d may contribute to toddler's diarrhea, especially in the setting of excessive free fluid and carbohydrate intake (eg, as occurs with large amounts of fruit juice intake). Thus, history is important in differentiating the common toddler's diarrhea from the rarer malabsorption syndromes.
  • In the United States, juice is commonly introduced into the diet in the latter portion of the first year of life. Purple grape juice has a high osmolarity, which can cause osmotic diarrhea. The transport of fructose into enterocytes is not active and relatively inefficient. Apple and pear juice contain a high fructose-to-glucose ratio, and consumption of these juices can result in fructose malabsorption and diarrhea. Because sorbitol and fructose compete for the same intestinal transporter, ingesting them together may result in the malabsorption of these sugars.
• GI tract symptoms: GI tract symptoms are common in patients with malabsorption syndromes, and symptoms range from mild abdominal gaseous distention to severe abdominal pain and vomiting. Chronic or recurrent diarrhea is by far the most common symptom.
  • Abdominal distention and watery diarrhea, with or without mild abdominal pain, associated with skin irritation in the perianal area due to acidic stools are characteristic of carbohydrate malabsorption syndromes.³
  • Periodic nausea, abdominal distention and pain, and diarrhea are common in patients with chronic Giardia infections.
  • Vomiting, with moderate-to-severe abdominal pain and bloody stools, is characteristic of protein sensitivity syndromes or other causes of intestinal injury (eg, inflammatory bowel disease).
  • Recurrent abdominal pain has been implicated as a symptom of dietary disorders, although psychological variables that relate to an elevated anxiety level have clouded the certainty of this relationship. Malabsorption syndromes can definitely cause abdominal pain or irritability (particularly seen in celiac disease).
  • Failure to identify the cause of malabsorption can result in the misdiagnosis of a physiologic syndrome as a behavioral disorder. Some dietary items may cause symptoms only when they are taken alone or with other specific dietary items.
  • Poor appetite is common in food sensitivity syndromes. The child becomes conditioned to refuse foods that cause inflammatory reactions of the intestine. However, this is not typically obvious in celiac disease. Malabsorption syndromes not associated with inflammatory reactions typically cause an increase in appetite (eg, cystic fibrosis), unless the associated abdominal gaseous distention hampers intake and induces early satiety.
• Stool characteristics
  • Patients with toddler's diarrhea often have loose stools with undigested food particles. This should not be taken to imply the presence of true malabsorption.
  • Frequent loose watery stools may indicate carbohydrate intolerance.
  • Pasty or loose foul-smelling stools indicate fat malabsorption, also termed steatorrhea. This symptom is commonly seen in Giardia infections, enterokinase deficiency, hepatic and pancreatic dysfunction, and protein sensitivity syndromes.
  • Bloody stools are seen in patients with protein sensitivity syndromes and not in disaccharidase and pancreatic enzyme deficiencies or in patients with giardiasis.
• Other symptoms
  • Systemic symptoms, including weakness, fatigue, and failure to thrive, are systemic consequences of chronically poor nutrient absorption. Malabsorption of carbohydrates, fats, or proteins can cause failure to thrive, whereas folate and B-12 malabsorption result in macrocytic anemia.
  • Patients with abetalipoproteinemia develop retinitis pigmentosa and ataxia because of chronic fat-soluble vitamin malabsorption and deficiency (vitamins A and E).

Physical

• In the absence of GI tract symptoms, malabsorption syndromes should be considered during the workup for failure to thrive, malnutrition, poor weight gain, or delayed puberty. Signs of malnutrition discovered during physical examination, such as reduced muscle and fat mass, atrophic tongue changes, or enlarged liver or spleen, have been reported in children with chronic malabsorption. Malabsorption syndromes should be suspected in infants with weight loss or little weight gain since birth and in infants with low weight and weight-for-height percentiles. By definition, toddler's diarrhea does not cause failure to thrive, unless anxious parents intervene by imposing unnecessary dietary restrictions that result in lower caloric intake.
• Dehydration caused by a diarrhea that is induced by a malabsorption syndrome is not common, but, when it occurs, dehydration can cause serious morbidity and mortality. Assess each patient for signs, symptoms, and severity of dehydration. Lethargy, depressed consciousness, sunken anterior fontanel, dry mucous membranes, sunken eyes, poor skin turgor, and delayed capillary refill are all signs of dehydration.
• Borborygmi, a significant increase in peristaltic activity, can be detected audibly and with tactile palpation. This is associated with decreased intestinal transient time.
• Large numbers of stools result in a constant wet diaper in young children. Failure to properly dry the buttocks and perianal area results in erythema, skin irritation, and skin breakdown, with evidence of bleeding seen in the diaper or in stool.
• Protein sensitivity may be associated with an eczematous rash.

Causes

• Carbohydrate malabsorption: Starch molecules are primarily digested by salivary and pancreatic amylase, but glucoamylase in the intestinal brush boarder also assists in digestion.
  • Pancreatic insufficiency impedes the digestion of large starch molecules.
  • Absence or reduction of the brush border disaccharidases causes selective carbohydrate malabsorption.
  • Transient reduction of these enzymes is common after an infection in the intestine, particularly a viral infection, because intestinal villi and microvilli may be damaged.
  • Glucoamylase and maltase are most resistant to the depleting effects of mucosal injury that result from infection, whereas lactase is the most sensitive because of its predominant distribution near the tips of the villi.
  • Lack of sucrase and isomaltase is, by far, the most frequent congenital enzyme deficiency. This enzyme deficiency is inherited in an autosomal recessive manner.
  • Congenital lactase deficiency is exceedingly rare, but adult-type lactase deficiency (also called adult-type hypolactasia) is very common in some ethnic groups.
  • A congenital deficiency in the glucose galactose transporter (SGLT-1) is inherited in an autosomal recessive manner.
  • Take care when diagnosing lactose or another complex carbohydrate intolerance because many complex carbohydrates are broken down into glucose.
  • Small bowel bacterial overgrowth of normal flora alters the intraluminal metabolism of carbohydrates and results in their malabsorption. This entity should also be suspected in children with diarrhea-predominant irritable bowel syndrome.⁴ Bacteria ferment carbohydrates into smaller osmotically active molecules and organic acids. Increased osmolarity causes fluid from systemic circulation to enter the intestinal lumen, resulting in diarrhea. Organic acids stimulate motility and may directly injure the intestinal mucosa. Fermentation eliminates the reducing substances and lowers the pH of the stool. The production of lactate and short-chain fatty acids in the human colon can result in systemic acidosis. In particular, a syndrome of D-lactic acidosis may develop when specific bacteria that are capable of producing this uncommon and poorly cleared D isomer of lactate exist in the intestinal flora.
  • Bile acids are usually recycled by enterohepatic circulation. Many factors can prevent this recirculation. Bacterial overgrowth of normal flora and growth of abnormal flora are the most common causes of altered intraluminal metabolism of bile acids. Anaerobes and Staphylococcus aureus deconjugate bile acids, which impedes their active reabsorption by the terminal ileum into the portal circulation for reuptake by the liver. A congenital deficiency in the sodium–bile acid cotransporter results in primary bile acid malabsorption. The resulting diminished transport of bile acids from the intestinal lumen allows intestinal flora to deconjugate bile acids.
  • Deconjugated bile acids directly inhibit the carbohydrate transporters, reduce intraluminal pH levels, and damage the enterocyte. They may also directly stimulate the colon to secrete fluid, contributing to diarrhea.
• Fat malabsorption
  • Increased delivery of fat to the colon results in diarrhea and soft, pasty, foul-smelling stools. However, the gas causes stools to float. Consequences include the malabsorption of fat-soluble vitamins A, D, E, and K and insufficient energy intake due to the high energy value of dietary lipids.
  • Exocrine pancreatic insufficiency is the principal condition that results in severe fat malabsorption.⁵ Pancreatitis, pancreatic cancer, pancreatic resection, cystic fibrosis, Shwachman-Diamond syndrome, Johnson-Blizzard syndrome, and Pearson syndrome can all result in pancreatic insufficiency. Significant obstructive biliary or cholestatic liver disease or extensive intestinal mucosal disease, such as occurs in celiac disease, may also result in severe steatorrhea.
  • Impaired bile production or secretion is seen in liver or biliary tract disease. Inflammation or resection of the ileum impedes enterohepatic circulation, which results in a reduced bile acid pool. Bacterial overgrowth in the small bowel deconjugates bile acids, thereby inactivating their ability to help lipids form a micelle. These syndromes result in moderate lipid malabsorption.
  • Abetalipoproteinemia is a rare disorder with autosomal recessive inheritance. Absence of the lipoproteins results in cytoplasmic lipid accumulation in the enterocyte. Lymphatic transport of long-chain fats is impaired in patients with abetalipoproteinemia, lymphangiectasia, and protein-losing enteropathy, resulting in moderate fat malabsorption.
• Protein malabsorption
  • Protein malabsorption is a fairly common result of exocrine pancreatic enzyme deficiency, as occurs in patients with cystic fibrosis.
  • Protein malabsorption that results from congenital enterokinase deficiency is well described but rare.
  • Creatorrhea, loss of protein in the stool (ie, protein-losing enteropathy), is often caused by the leakage of protein from the serum due to inflammation of the mucosa, as in Crohn disease, celiac disease, and protein sensitivity syndromes. Congenital lymphangiectasia, a developmental disorder in which dilation and dysfunction of intestinal lymphatics occurs, often in association with limb edema (Milroy disease), may present with severe protein-losing enteropathy without mucosal injury.
• Vitamin malabsorption
  • Malabsorption of vitamin B-12 and folate is associated with tropical spruce, a disorder that is acquired after travel to tropical areas.
  • Vitamin B-12 is absorbed in the ileum, and absorption requires an intrinsic factor made in the gastric parietal cell. Intrinsic factor deficiency that results from atrophic gastritis or absence (from resection) or disease of the terminal ileum (the predominant site of active B-12) results in vitamin B-12 malabsorption.

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