Hypochromic microcytic anemia characterizes iron deficiency. Since malassimilation may result in folate or B₁₂ deficiency (producing megaloblastic red cells), the microcytosis of iron deficiency may be obscured with automated cell counters; a dimorphic picture is present. Rarely accompanying the development of anemia may be symptoms of pica and dysphagia. Pica originally referred to the eating of clay or soil; however, the commonest "pica" in North America is the eating of ice. Dysphagia may be due to the Plummer-Vinson (Paterson-Kelly) syndrome (with atrophic papillae of the tongue and postcricoid esophageal webs), and/or cheilosis (reddened lips with angular fissures, also known as cheilitis or angular stomatitis). Weakness, fatigue, dyspnea and edema also can occur. Physical examination often reveals pallor, an atrophic tongue and koilonychia (brittle, flat or spoon-shaped fingernails).

The clinical picture of vitamin B₁₂ and folic acid deficiency includes the nonspecific manifestations of megaloblastic anemia and its sequelae - i.e., anemia, glossitis, megaloblastosis, and elevated serum lactate dehydrogenase (LDH). In addition, deficiency of B₁₂ may induce neurologic abnormalities consisting of symmetrical paresthesias in the feet and fingers, with associated disturbances of vibration sense and proprioception, progressing to ataxia with subacute combined degeneration of the spinal cord. This subacute combined spinal cord degeneration includes cortisospinal as well as dorsal column damage. Neurologic manifestations are not part of folic acid deficiency alone.

Impaired absorption of calcium, magnesium and vitamin D may lead to bone pain, fractures, paresthesias, tetany, Chvostek's sign and Trousseau's sign. Osteomalacia resulting from vitamin D deficiency principally affects the spine, rib cage and long bones with or without fractures (Milkman's fractures), and may cause extreme pain, particularly in the spine, pelvis and leg bones. A child with calcium or vitamin D malabsorption will present with classical rickets. Hypomagnesemia may cause seizures and symptoms identical to those of hypocalcemia. In addition, hypomagnesemia may reduce the responsiveness of the parathyroids to calcium and impair parathyroid regulation of calcium homeostasis.

To avoid embarking on the shotgun approach to investigation, there are several questions one must ask. First, does malassimilation exist? And second, if so, is it due to a disorder of intraluminal digestion or a disorder of intramural absorption? Physicians should attempt to restrict the use of laboratory tests to those that establish the presence of malassimilation and the cause of the malassimilation (Figure 14).

To determine if malassimilation exists, one begins, as always, with the simplest, least invasive tests. A complete blood count (CBC) and differential might reveal a macrocytic or microcytic anemia. A peripheral smear may demonstrate megaloblastosis, microcytosis and/or lymphopenia. Serum calcium, phosphorus and alkaline phosphatase will detect the presence of osteomalacia. Serum albumin can assess protein stores. Serum cholesterol, carotene and prothrombin time (vitamin K) indirectly assess fat assimilation. Iron stores from serum iron, total iron binding capacity (TIBC) and ferritin assess proximal intestinal integrity. Serum B₁₂ is an index of ileal integrity. Red cell folate measures folate stores.

If their results are abnormal, the above tests suggest the presence of malassimilation and may indicate the deficient nutrient(s). Steatorrhea is the most important feature in the diagnosis of generalized malassimilation. Accurate measurement of fecal fat is important. Qualitative Sudan stain for fat globules on suspension of stool will give an indication of possible steatorrhea. However, this test cannot substitute for a quantitative fecal fat determination for a definitive diagnosis.

Quantitative fecal fat determination is the most reliable measure of steatorrhea. In the normal individual, the amount of fat appearing in the stool is relatively constant despite small changes in the quantity of dietary fat. Even when the daily fat intake is zero, the fecal fat output equals about 2.9 g/day. Presumably this is the amount of fat that is derived from endogenous sources, such as sloughed mucosal cells, excreted bile lipids (cholesterol and bile acids) and bacterial lipids. As the dietary intake of fat is increased, the fecal fat will increase to about 5 g/day on a 100 g fat diet. Fecal fat (FF) bears some relation to dietary fat: normally, the fecal fat loss is usually less than 5% of dietary intake. A defect at one of the four steps in the overall process of fat assimilation dramatically increases this fat loss. In disease the absolute value of the quantitative fecal fat output may depend on the dietary load, which must be carefully assessed prior to adequate evaluation of the test. Thus a number of conditions should be met in order to obtain a reliable quantitative stool fat output. The patient should be on a steady dietary intake of a known 60-100 g fat, there must be a regular pattern of stooling, and all stool must be collected for 72 hours.

There are numerous possible artifacts for this test. Poor food intake, interrupted food intake, constipation or incomplete stool collection all give rise to a spuriously low value for the 24-hour fecal fat output. An artifactually high value will be seen when castor oil or nut oils have been consumed, but not petroleum mineral oils. The Van de Kamer method is the most commonly used procedure to chemically determine fecal fat output. This method, however, may lead to incomplete extraction and quantitation of medium-chain triglycerides (MCT) and thus may underestimate (by 10%) the quantity of fecal fats in patients whose diet has been supplemented with MCT. Steatorrhea does not indicate into which category of malassimilation the patient falls. An elevated fecal fat may be due to intraluminal maldigestion or intramural malabsorption. Therefore, further investigations are required to fully characterize the problem (Figure 15).

Intraluminal maldigestion will occur with (1) inadequate mixing; (2) pancreatic insufficiency; and (3) reduced bile salt concentration. If the D-xylose absorption test and small bowel x-rays are normal, then it is likely but not absolutely certain that malassimilation is due to an intraluminal disorder. Pancreatic function is assessed by the secretin stimulation test used to measure secretory capacity of the exocrine pancreas. A tube placed in the duodenum adjacent to the ampullae of Vater collects pancreatic juice to measure volume, bicarbonate and enzyme (amylase) output. Maximal secretion should reach 2 mL/min at 90 minutes after injection of 2 units of secretin/kg of body weight, and bicarbonate concentration is normally 90 mEq/L. Both enzyme and bicarbonate secretion are diminished in chronic pancreatitis. Partial duct obstruction resulting from pancreatic cancer often reduces the volume of secretion without reducing bicarbonate concentration. The test is cumbersome and not very sensitive.

Assessment of biliary disease includes liver biochemistry, abdominal ultrasound and, where indicated, transhepatic or endoscopic cholangiography to ensure patency of the ductal system. In the bacterial overgrowth syndrome, bile acids are deconjugated and rapidly absorbed in the small intestine, and are not available or active for micellar solubilization. With bacterial overgrowth, the Schilling test for vitamin B₁₂ is abnormal, even with the addition of intrinsic factor. Bile salt concentration may be diminished as a result of failure of reabsorption in a diseased ileum, adding to the malabsorption from gut loss. A ¹⁴C-labeled bile acid breath test detects bile acid deconjugation in the bacterial overgrowth syndrome; an isotope scan with a radiolabeled bile acid analogue measures bile acid absorption.

The bile acid breath test is used if one suspects bile acid malabsorption due to ileal dysfunction (decreased absorption) or bacterial overgrowth (deconjugation and thus diminished absorption). The basis of the test is that the amino acid (glycine) part of the bile salt is labeled with ¹⁴C-glycine. Bacteria deconjugate the glycine and metabolize this amide to ¹⁴CO₂, which is then exhaled. With ileal dysfunction, an excess of bile salts reaches the colon, where colonic bacteria split off the glycine, producing ¹⁴CO₂. With bacterial overgrowth, the excess coliform bacteria in the jejunum metabolize these bile salts to ¹⁴CO₂. The bile acid breath test is beginning to be replaced by the hydrogen breath test, which assesses the presence of an abnormal increase in the concentration of breath H₂ after the subject has ingested a nonabsorbable sugar such as lactulose. This is an inexpensive and sensitive test for bacterial overgrowth. The type of sugar used in the breath test may be changed to test for malabsorption of various carbohydrates. The H₂ breath test is not used to test for bile salt malabsorption.

Disordered mixing of ingested food with endogenous enzymes results from the rapid transit in postgastrectomy syndromes. Small bowel x-rays with transit times sometimes help in the diagnosis.

Malabsorption from an intramural defect occurs as a result of (1) inadequate absorptive surface - e.g., the short gut syndrome; (2) mucosal absorptive defects - e.g., celiac disease; or (3) lymphatic obstruction. Since the pentose sugar D-xylose does not require intraluminal digestion, D-xylose absorption tests serve to separate patients with intramural malabsorption from those with intraluminal maldigestion. An abnormal result points toward intramural malabsorption, while a normal result points toward intraluminal maldigestion. Remember that patients with renal impairment (who cannot excrete the sugar), delayed gastric emptying, bacterial overgrowth (who metabolize the sugar in the lumen) and advanced age may have spuriously low measurements of D-xylose absorption. Aspirin (ASA) or nonsteroidal anti-inflammatory drugs (NSAIDs) inhibit the renal excretion of xylose and may lead to a false-negative test. The presence of ascites may also lead to a false-negative test by increasing the volume of distribution of the sugar and thereby decreasing its renal clearance. Measurement of the serum level of xylose may be a better test of xylose absorption because it avoids these issues of renal function. Indeed, clinicians may elect to omit a xylose test and advance to specific intramural or intraluminal investigations (Figure 14).

Following an abnormal D-xylose test, the next evaluation should be barium contrast x-rays of the small intestine. They may demonstrate structurally abnormal bowel patterns, dilation of bowel lumen, segmentation of barium or a dilution of barium because of increased intraluminal fluid. Although segmentation and flocculation of barium have been used as indications of malassimilation in the past, the use of nondispersible barium sulfate in recent years rarely allows us to observe such signs. Additionally, x-ray films may demonstrate diverticula as sites of bacterial overgrowth or thickening of folds resulting from infiltration or edema.

Following the x-ray, endoscopic or suction biopsy of the small intestine will identify evidence of specific mucosal disease. If there is a high clinical suspicion of a small bowel disorder such as celiac disease, the patient may be referred at a much earlier stage for a small bowel biopsy to be performed. Once the position and/or etiology of the intramural disease is known, further tests can be carried out to determine the extent of functional derangement. Tests can help define unabsorbed carbohydrate as a result of disaccharidase deficiency, generalized mucosal damage, inadequate surface area or bacterial overgrowth. Unabsorbed carbohydrate produces stool with an acid pH, easily tested on pH paper strip. The hydrogen breath test will detect an increase in exhaled hydrogen, which might result when an ingested carbohydrate is not absorbed. This test is used to diagnose suspected disaccharidase deficiencies or bacterial overgrowth. The basis of the test is that bacteria ferment sugars to fatty acids and H₂, which is exhaled. Normally, sugars are absorbed as monosaccharides in the small intestine and no H₂ is exhaled. In lactase deficiency, exhaled H₂ is elevated after ingestion of the test sugar lactose, since the unabsorbed sugar reaches colonic bacteria and is catabolized. In bacterial overgrowth, bacteria in the jejunum ferment the sugar before it can be absorbed and H₂ is exhaled. The hydrogen breath test is reliable, noninvasive and helpful in establishing the diagnosis of carbohydrate malabsorption and/or the bacterial overgrowth syndrome.

Vitamin B₁₂ absorption is tested by the Schilling test. Radiolabeled vitamin B₁₂ with intrinsic factor (labeled with ⁵⁸Co) and without intrinsic factor (labeled with ⁵⁷Co) are simultaneously administered orally. The excretion of both compounds is then measured in the urine over a 24-hour period. Excretion of both radiolabeled compounds is normal. Failure to excrete ⁵⁷Co-labeled vitamin B₁₂ indicates absent gastric intrinsic factor - e.g., pernicious anemia or gastrectomy, while failure to excrete ⁵⁸Co- and ⁵⁷Co-labeled vitamin B₁₂ indicates ileal disease or loss, or absent ileal B₁₂ receptors.

The specific treatments for malabsorption or maldigestion are given in Table 6. The nutritional therapies necessary for any associated deficiencies are given in Table 7.