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Gastrointestinal Outcomes and Confounders in Cystic Fibrosis

2005; Lippincott Williams & Wilkins; Volume: 41; Issue: 3 Linguagem: Inglês

10.1097/01.mpg.0000178439.64675.8d

1536-4801

Drucy Borowitz, Peter R. Durie, Lane L. Clarke, Steven L. Werlin, Chris Taylor, John R. Semler, Robert C. De Lisle, Peter Lewindon, Steven Lichtman, M. Sinaasappel, Robert D. Baker, Susan S. Baker, Henkjan J. Verkade, Mark E. Lowe, Virginia A. Stallings, Morteza Janghorbani, Ross N. Butler, James E. Heubi,

Infant Nutrition and Health

Until the mid 20th century, children with cystic fibrosis (CF) died at a young age from a combination of malnutrition and suppurative lung disease. In the past three to four decades, coincident with new treatments for pulmonary complications and the use of high-calorie diets without restrictions on fat intake, there has been marked and progressive improvement in lung function, nutritional status and survival. However, treatment for pancreatic insufficiency, which affects most individuals with CF, remains largely unchanged, and there have been few advances in our understanding and management of gastrointestinal problems in this population. Pancreatic Enzyme Replacement Therapy (PERT), principally using enzyme extracts of porcine origin, has been the mainstay for treating maldigestion resulting from pancreatic insufficiency (PI) for over a century. Powdered PERT was instituted before the 1931 creation of the United States Food and Drug Administration (FDA) and the subsequent Federal Food, Drug and Cosmetic Act of 1938 that required proof of safety and efficacy before approval of drugs for marketing. Enteric-coated microcapsules, the most commonly used PERT, have never been FDA approved. After the epidemic of fibrosing colonopathy in the early 1990s, a condition that was clearly related to high doses of exogenous pancreatic enzymes (1), the FDA initiated a process of re-evaluating the safety and efficacy of PERT. In 2004, the FDA released a draft of a new Guidance Document for public comment (2) that stated that within the next 4 years, all manufacturers of current enzyme products will need to submit a New Drug Application to the FDA with proof of safety in manufacturing, stability and efficacy. The coefficient of fat absorption (CFA) is the traditional method of testing the clinical efficacy of PERT. CF clinicians have long recognized that there is a very wide range of CFA in patients with CF. Less well known is that there is marked variability in the actual amount of enzyme in each capsule. Capsules are overfilled so that the stated dosage reflects the least amount of enzyme activity present after a six-month shelf life. There is little correlation between CFA and enzyme dose when subjects are receiving a dose of PERT considered optimal by the caregiver and/or patient (3). Furthermore, a recent cross-sectional study showed no correlation between enzyme dose and growth or self-reported symptoms (4). Consequently, the common belief that PERT dose can be titrated to correct malabsorption and/or maldigestion using patient-reported relief of abdominal symptoms or stool bulk and consistency appears to be questionable. PERT may not completely correct PI even if given in adequate doses because the release and onset of activity is not synchronous with the presence of food in the proximal intestine, the site for optimal absorption. Furthermore, as discussed below, it is becoming evident that poorly understood nonpancreatic intestinal and/or hepatobiliary factors likely contribute to incomplete and variable nutrient assimilation in patients with CF. However, we lack tools to distinguish PERT-related factors that might contribute to treatment failure from other confounding factors. Ironically, as mouse models of CF were developed in the late 20th century, these animals did not develop lung or severe pancreatic manifestations. However, the mice have characteristic CF-like bowel obstruction and are proving to be excellent models for study of nonpancreatic gastrointestinal manifestations of CF. The combination of new ways to study gastrointestinal disease in CF that further our understanding of the pathophysiology of CF and the need to evaluate novel methods to study pancreatic and nonpancreatic causes of maldigestion and malabsorption led the CF Foundation to sponsor a workshop, which was held in Baltimore in May 2005. The goals of this workshop were to evaluate what is known about gastrointestinal issues leading to malabsorption, to consider other factors that may contribute to malabsorption and to identify ways to improve care of patients with CF. We set an agenda for further clinical and laboratory-based investigations that would help to elucidate the spectrum of causes of maldigestion and malabsorption. The workshop participants included clinicians and investigators with a variety of interests, members of the US and European regulatory agencies and individuals from industry who were involved in producing traditional PERT or developing novel PERT or diagnostic tests. PATHOGENESIS OF PANCREATIC, HEPATIC AND INTESTINAL DISEASE IN CF Classic CF reflects two loss-of-function CFTR mutations whereas patients with nonclassic CF carry at least one copy of a mutant gene that confers partial function of the CFTR protein (5). Patients with “classic” and “nonclassic” CF exhibit wide heterogeneity in the severity of lung disease. However, in statistical terms, the overall rate of progression of lung disease among the nonclassic CF patients is slower and median survival is almost twice that of patients with “classic” CF. Among the various gastrointestinal organs affected by CF disease, the exocrine pancreas shows the strongest association between genotype and phenotype. Loss-of-function mutations on both alleles almost always confer pancreatic insufficiency (PI). In most cases, considerable obstructive destruction of the pancreas arises in utero and functional loss of the exocrine pancreas develops at birth or in early infancy. These patients need PERT with meals and snacks. In patients with the pancreatic sufficient phenotype the presence of at least one mutation with some residual CFTR function protects the pancreas from complete destruction. However, the functional capacity of the exocrine pancreas in the patients who are pancreatic sufficient varies widely, from values just above the threshold for developing PI to values that are within the reference range for healthy controls (6). Approximately 20% of pancreatic sufficient patients are at risk of developing symptoms of recurrent, acute pancreatitis or chronic pancreatitis. In contrast to the exocrine pancreas, expression of disease in other gastrointestinal organs is less clearly associated with CFTR genotype. However, patients with classic CF are more vulnerable than their nonclassic counterparts to meconium ileus (MI), distal intestinal obstruction syndrome (DIOS) and clinically significant liver disease (CFLD). As disease penetrance is incomplete, disease expression seems to depend on two loss-of-function CFTR mutations as well as the influence of modifier genes and/or environmental factors. Approximately 25% of neonates with classic CF develop MI. A higher incidence of MI is seen among subsequent born siblings with CF within families in which the first-born child had MI. This suggests a genetic contribution to MI. Linkage and association analysis of different murine strains of CF led to the identification of two candidate regions for modifier genes of MI. The strongest linkage (CFM1) was detected on proximal murine chromosome 7. Subsequently, a modifier locus for CFM1 was detected in the syntenic region of human chromosome 19q13 using DNA from CF siblings who were concordant or discordant for MI. As CFM1 showed no association with pulmonary function, a gene in the CFM1 region probably acts solely within the small intestine and together with two severe CFTR alleles increases the likelihood of MI (7). Most patients with CF develop mild liver abnormalities including slightly elevated liver biochemical tests, hepatosteatosis and focal areas of portal tract disease (biliary plugging with eosinophilic material, bile duct proliferation and cholangitis). Only a small proportion of CF patients, most of whom have classic CF, develop CFLD with multilobular cirrhosis leading to portal hypertension and hypersplenism. The median age at diagnosis of CFLD is 9-10 years and most cases of CFLD are diagnosed by midadolescence. Because the ability of the liver to synthesize proteins, metabolize toxins and secrete bile tends to remain intact for many years and even decades, most complications of CFLD are attributable to the consequences of portal hypertension. A number of putative modifier genes of CFLD have been reported in recent years. However, in a recent study of a larger number of patients with severe CFLD, only two candidate genes (the Z allele for alpha-1-antitrypsin deficiency and polymorphisms of TGFβ1 with increased expression) were shown to be statistically associated with CFLD (8). Taken together, the CFTR genotype, modifier genes and environmental factors appear to explain the variability in the severity of the CF phenotype in different organs (Fig. 1). However, the relative contributions of these factors vary from organ to organ. In the case of the exocrine pancreas, CFTR genotype seems to dictate the severity of disease. In contrast, expression of disease in other organs, such as the small intestine and the liver, requires the influence of severe CFTR genotypes on both alleles as well as modifier gene and/or potential environmental influences.FIG. 1: The relative contribution of modifier genes, CFTR, and environment on phenotype.ANIMAL MODELS OF GASTROINTESTINAL DISEASE IN CF CF has not been reported as a spontaneous disease entity in any species other than humans. This has led to the development of 13 genetically-manipulated mouse models including seven with gene-targeted disruption of the murine CFTR (knockouts), three mutant models expressing the ΔF508 mutation and mouse models with the G551D, R117H or G480C mutations (Table 1). The major manifestation of CF in the knockout mouse is obstruction of the distal intestine, which commonly results in bowel perforation, peritonitis and death. Intraluminal obstruction occurs at two stages; the immediate perinatal period (recapitulating MI) and after weaning to solid chow (recapitulating DIOS). Bowel obstruction in mouse models with severe disease occurs with high frequency (>90%), significantly exceeding the incidence of MI or DIOS in humans. The incidence of obstructive bowel disease in CF mouse models is closely associated with abnormalities in electrolyte transport across the epithelium, which in turn leads to goblet cell hyperplasia and dehydration of secreted mucus and debris in the crypts. Obstructing impactions can be prevented by several dietary methods such as including osmotic laxatives in the drinking water (polyethylene glycol-based), use of a liquid diet (Peptamen® or Liquidiet®) and, more recently, by providing a calorie-dense diet (9-11).TABLE 1: Mouse models of cystic fibrosisIn both mouse and human, CFTR is the predominant apical anion channel in the intestinal epithelium (9,10). Loss of CFTR greatly diminishes basal and stimulated anion secretion, including the loss of transepithelial bicarbonate secretion in the upper small intestine (12-14). Furthermore, the absence of CFTR function in the villus epithelium of the proximal bowel results in deficient cyclic adenosine monophosphatase regulation of intestinal Na+ absorption, which further exacerbates luminal dehydration (15). The severity of gastrointestinal obstructive disease in the CF mice varies with the genotype (targeting strategy) and with the genetic background of the mouse strain. Knockout or ΔF508 CFTR mouse models generated by replacement of a portion of the endogenous gene by vector sequences typically have very low levels of CFTR gene transcription, significantly impaired anion secretion and severe intestinal disease (e.g., Cftrtm1Unc or Cftrtm1Cam models). In contrast, mouse models generated by insertion or “hit and run” gene-targeting (no replacement of the endogenous CFTR sequence) have residual expression of ∼10% wild-type CFTR by alternate splicing in the knockout (Cftrtm1Hgu mice) or sufficiently high levels of ΔF508 CFTR that allow “escape” of the mutant protein from the processing defect (e.g., Cftrtm1Eur mice). These models typically have significant anion secretion and mild intestinal disease (9). The severity of intestinal disease also varies with the genetic background of the CF mice. The importance of background was elegantly demonstrated by Rozmahel and coworkers, who showed that only a small percentage of CFTR knockout mice with mixed background had long-term survival in the absence of palliative treatment (16). When the responsible mutation was inbred to a BALB/cJ background, the percentage of long-term surviving mice increased and this was subsequently correlated with an increased capability for non-CFTR-mediated anion secretion via alternate Cl− conductance pathways (17). Despite the varying influences of genetic background, diet and targeting strategy, the accumulated data on CF mouse models show two important correlations. First, the survival of CF mice is a positive linear function of the magnitude of residual anion secretion. Second, the degree of growth retardation of the CF mice correlates in a linear fashion with the magnitude of stimulated anion secretion. Because the expression of CF disease in the airways and pancreas of CF mice remains subclinical, these animal models can be used to isolate deficiencies in mucosal digestion and intestine absorption. Although significant hepatic disease is not evident in young CF mice, one study of long-lived CF mice found progressive hepatobiliary changes that mimic the disease of adult CF patients (18). CF mouse models display several other intestinal features in common with CF patients that may be of value for evaluating factors that contribute to mucosal malabsorption. First, CF mice with severe disease phenotypes have acidic, weakly sulfated surface intestinal mucus that may alter its functional properties and affect nutrient diffusion or negatively influence the juxtamucosal environment. Second, consistent with recent studies of human intestine, CF mice have low-grade intestinal inflammation that might be secondary to deficiencies in host defense or microbial intestinal interactions (19). Postsecretory Paneth cell granules, which are rich in α-defensins (cryptdins) and other antimicrobial peptides, do not undergo normal dissolution in the intestinal crypts. This increases the propensity for bacterial colonization of the small intestine (20). CFTR acts as a HCO3− unloading mechanism for the epithelial cell. Abnormalities of intracellular pH regulation in CFTR-null intestinal epithelia have been associated with increased enterocyte proliferation in the upper intestine that may alter nutrient transporter expression along the crypt-villus axis (21). The experimental conditions for evaluating nutrient maldigestion and malabsorption in CF mouse models must be carefully designed with regard to background strain and diet. For example, early investigations of CFTR knockout mice with a mixed background and consuming a liquid diet (Peptamen®) were shown to have an essential fatty acid imbalance (22). However, an essential fatty acid deficiency was not detected in congenic CFTR knockout mice consuming either a conventional diet or Peptamen® (23). In a follow-up to the latter study, it was shown that CFTR knockout mice have steatorrhea and reduced mucosal lipolysis that was indirectly related to a deficiency in epithelial bicarbonate secretion (11). Earlier investigations of Na+-coupled glucose transport in CFTR knockout mice with varying backgrounds and diets did not find differences from wild-type littermates (10), whereas a more recent study of ΔF508 CFTR mice with congenic background report reduced glucose absorption (24). The latter study contrasts with previous investigations of human CF intestine that show either increased or unchanged glucose absorption. However, these studies report large differences in the absolute rates of Na+-coupled glucose transport between the two species. Although amino acid and peptide absorption in CF mice has received little attention, neonatal CFTR knockout mice have been reported to show decreased Na+-coupled alanine transport (10). Thus there is a need to evaluate the array of amino acid transport systems (currently 17 systems) and H+-coupled peptide transporters. Accessory proteins that support the transport process also need to be considered. The apical membrane Na+/H+ exchanger isoform NHE3 is activated during Na+-coupled glucose and H+-coupled dipeptide absorption, ostensibly to maintain intracellular pH (25,26). A recent study has shown that intestinal NHE3 activity is decreased in CF mice with severe disease phenotypes (27). Whether this contributes to carbohydrate and peptide malabsorption remains unknown. Recommendations for investigations of mucosal maldigestion and malabsorption using CF mouse models can be offered. First, to accentuate the role of CFTR in nutrient absorption, mice with severe intestinal disease are recommended (CFTR knockout, ΔF508 CFTR by replacement gene-targeting strategies). Second, it is necessary to use CF mice bred in congenic backgrounds to avoid variability of disease phenotype. Most studies using a C57BL6/J background report a severe intestinal disease phenotype, whereas a BALB/cJ background might accentuate the role of the alternate Cl− conductance. Third, a strong case can be made against the use of liquid diets, which do not provide the beneficial effects of dietary bulk or food antigens. Some liquid diets, such as Peptamen® are not appropriate for rodent metabolism because a large volume of fluid (∼75% of body wt.) must be consumed in a 24-hour cycle and amino acid containing diets may not adequately stimulate CCK release. Special conditions are also required to prevent malocclusion of continuously growing rodent incisors. In contrast to liquid diets, CF mice on calorie dense diets without palliative treatment may have confounding pathology related to impending impaction. Thus, optimal studies of mouse digestion and absorption may require the use of osmotic agents, such as polyethylene glycol containing solutions, which offset luminal dehydration and allow conventional mouse diets to be used. Only age-matched siblings [CFTR (+/+) or (+/−)] should be used as controls because age-related changes in intestinal transport properties can be detected even in congenic strains. Finally, it should be recognized that all animal models have intrinsic limitations resulting from differences between strains and species. POTENTIAL FACTORS PERTURBING THE EFFECTIVENESS OF PANCREATIC ENZYME THERAPY Although mouse models can help us understand basic aspects of gastrointestinal pathophysiology in CF, clinicians are faced with a number of pressing, poorly understood clinical problems that adversely affect patients and limit our ability to conduct meaningful clinical studies. One such challenge has been our inability to dissect out the factors that may contribute to the prevalence of gastrointestinal complaints and the wide range of severity of nutrient maldigestion and malabsorption, as well as factors that perturb the effectiveness of current PERT. A number of factors might contribute to these difficulties. Gastric Emptying Gastric emptying of food and enzymes may not be synchronous. Although a variety of methods for assessing gastric emptying have been described, the most widely used is serial scanning of radiolabeled solid and/or liquid meals. The patterns of gastric motility and the rates of gastric emptying of viscous nonnutrient meals (e.g., inert cellulose) differ from nutrient meals (e.g., casein, glucose, or oleic acid). The rate of delivery of inert liquids into the duodenum is a constant fraction of the volume of liquid within the stomach. For example, 250 mL of saline will empty twice as fast as 125 mL. Nutrients within the stomach slow gastric emptying because of feedback control from the small intestine. The rate of emptying of liquid meals is directly related to the caloric content of the meal within a given range. Digestible solids must be ground to 1-2 mm particles before emptying occurs. Smaller particles are emptied first. The particle size of most PERT is smaller than 2 mm so, in theory, they would be expected to empty mixed with a meal. Powdered enzymes would be expected to empty faster with the liquid phase of the meal. There have been eight studies of gastric emptying in CF (28-35). No two studies used the same test meal, and five different test methods were used. No study evaluated both solid and liquid phase emptying. The results are difficult to interpret because three studies showed rapid emptying, four showed normal emptying and one showed delayed emptying. Most studies evaluated a small number of individuals and the reference measures were often incompletely defined. The degree of intra-patient variability in gastric emptying was not evaluated. This is important because the degree of intra- and inter-patient variability in gastric emptying is considerable even in healthy subjects and is influenced by multiple factors including physical or emotional stress, composition and volume of gastric contents, vagal reflexes, propulsive motility of the antral pump, compliance of the gastric reservoir, feedback signals from the duodenum and the ileal brake phenomenon. One study of patients with CF evaluated simultaneous emptying of food and PERT using a double-labeled radionuclide technique. There was wide inter-patient variability in the rate of gastric emptying and intestinal transit of food. Furthermore there was considerable asynchrony between emptying of PERT and nutrients, although enzyme pellets generally emptied faster than food from the stomach (36). Another study found a significant negative correlation between the rate of gastric emptying and lipolysis as measured by 13C-mixed triglyceride breath testing (34). A third study showed improved recovery of 13CO2 from 13C-mixed triglyceride when enzymes were taken during or after a meal rather than before the meal. However, roughly half of subjects did not normalize recovery of 13CO2 with any dosing regimen (37). Intestinal Acidification Low intestinal pH and gastric pepsin may inhibit both endogenous and exogenous pancreatic enzyme activity. Duodenal pH is regulated by bicarbonate-rich pancreatic secretions and intestinal bicarbonate secretion. CFTR is known to play an important role in both routes of bicarbonate secretion. Duodenal bicarbonate secretion by the epithelium involves two pathways: electroneutral secretion via a CFTR-assisted Cl−/ HCO3− exchange process and an electrogenic secretion of HCO3− via a CFTR conductance pathway (12). Exposure of the duodenal mucosa to an acidic pH triggers HCO3− secretion via pathways that include prostaglandin release and neural activity. Basal HCO3− secretion is reduced in the CF duodenal mucosa, and, in contrast to normal controls, cyclic adenosine monophosphatase-stimulated HCO3− secretion is absent (13). Although Cl−/HCO3− exchange provides basal HCO3− secretion in the CF intestine, the magnitude of secretion is lessened by simultaneous activity of a Na+/H+ exchanger. During cyclic adenosine monophosphatase stimulation of the CF duodenum, a small net increase in base secretion can be measured as a result of cyclic adenosine monophosphatase inhibition of Na+/H+ exchange activity rather than increased HCO3− secretion (14). Most commercially available PERT are coated with an acid-resistant film to prevent the pancreatic enzymes from being denatured within the acidic environment of the stomach. Reduced pancreatic and duodenal bicarbonate secretion may fail to neutralize gastric acid and thereby prevent or delay dissolution of the enteric coating until the microspheres have passed the major absorptive surface area in the duodenum and jejunum. Even if the coating dissolves, the activity of most pancreatic enzymes, particularly pancreatic lipase/colipase, is greatly impaired in an acidic intraluminal environment. In theory, duodenal pH can be made more alkaline by inhibiting gastric acid secretion with the use of H2-receptor antagonists or proton pump inhibitors (PPI) (38). PPIs can decrease malabsorption in patients with CF (39). Treatment with a PPI partially corrected fat malabsorption that was present in CF mice lacking a lipolytic defect (11). Abnormal intestinal acidification may contribute to fat malabsorption in CF irrespective of its effect on pancreatic enzyme activity. In vivo monitoring of intestinal pH in an ambulatory clinical setting may be available in the near future. The SmartPill™ gastrointestinal monitoring system (SmartPill, Buffalo, NY) uses an ingestible radio telemetry capsule to characterize pressure and pH of the entire gastrointestinal tract. The system includes a small radio frequency receiver, docking station and data processing software along with the 1 in. × 1.5 in. ingestible capsule. The SmartPill™ contains microelectronic pH, pressure and temperature sensors and an internal power supply, transmitter and microprocessor. The device is capable of recording gastrointestinal pressure, pH and temperature for more than 3 days. Once approved by the Food and Drug Administration, this device may make it easier to define intestinal acidification profiles in patients with CF. INTRALUMINAL SOLUBILIZATION AND MUCOSAL ABSORPTION Dietary fats are essential for health. They are an important source of calories, supply precursors for cellular membranes, prostaglandins, thromboxanes and leukotrienes, present a vehicle for fat-soluble vitamins and improve the palatability of foods. Triglycerides comprise approximately 95% of the fats in a typical Western diet. Phospholipids and cholesterol account for the majority of the remaining fats. In addition to the diet, intraluminal fats come from biliary lipids and mixed membrane lipids from desquamated intestinal cells and dead bacteria. The majority of ingested lipids are absorbed by intestinal enterocytes and less than 5% are normally excreted in the stool. Before enterocytes can absorb fats, the lipids must be digested into their component parts and made soluble. Triglycerides are digested into fatty acids and monoacylglycerols whereas phospholipids are digested into fatty acids and lysophosphatides. Digestion proceeds at the surface of multilamellar emulsion particles through the action of lipases. First, gastric lipase releases approximately 15% of the fatty acids during digestion in the stomach. Then pancreatic lipase, in conjunction with colipase, preferentially cleaves at the sn-1 and sn-3 positions, thereby completing the process in the upper small intestine. As digestion proceeds, fatty acids, monoacylglycerides, lysophosphatides and cholesterol form unilamellar vesicles and intestinal mixed micelles by mixing with bile salts. Most fat absorption occurs from the micelles, which increase aqueous solubility of lipids up to 1,000,000-fold compared with monomer concentrations (39-41). The increased solubility enhances intestinal absorption of the digested lipids by facilitating diffusion across the unstirred water layer that is contiguous with the intestinal membranes. The unstirred water layer provides an aqueous barrier to the diffusion of monomeric fatty acids but is readily traversed by mixed micelles. Once the mixed micelles approach the intestinal membrane they enter an acidic microclimate where the fatty acids are protonated, destabilizing the micelle and facilitating translocation of the fatty acids across the plasma membrane of the enterocyte. It is still unclear whether entry into the intestinal cells occurs through passive diffusion or carrier-mediated transport. Fatty acid binding proteins inside the cell may act as a sink to remove the fatty acids from the plasma membrane and increase the diffusion gradient and thereby transport across the membrane. Once inside the cells, acyl synthetases re-esterify the fatty acids into triglycerides and phospholipids before being packaged into lipoprotein particles for distribution throughout the body. Defects in CFTR can theoretically affect the postlipolytic process. For example, there is evidence that long chain monomeric fats and phospholipids are malabsorbed in CF (42,43). The clinical consequences of phospholipid malabsorption in CF are unknown. Animal models suggest a role for deranged bile salt pharmacokinetics in CF. Fat absorption in two murine CF models was determined by measuring fat excretion in stool. Lipolysis and postlipolytic long chain fatty acid uptake was assessed by comparing uptake of 3H-triolein-derived lipid and 14C-oleate (11). Biliary function was investigated by assessing bile salt secretion rate and composition and kinetics of the enterohepatic bile salt circulation using a stable isotope dilution method. Both CFTR null mice and homozygous ΔF508 mice had increased fecal bile salt losses and increased bile salt secretion rates compared with controls. Uptake of 14C-labeled dietary lipid in intestinal mucosa and appearance in plasma was significantly reduced, indicating that postlipolytic events were impaired. Only the CFTR null mice had steatorrhea, although there was no evidence of reduced pancreatic secretion. Of note, this maldigestion could be partially restored by suppression of gastric acid. Although both the CFTR null and homozygous ΔF508 mice had abnormal bile salt pharmacokinetics, the fact that the homozygous ΔF508 mice did not have steatorrhea suggests that the contribution of abnormal fat solubilization by bile salts to fat malabsorption is minimal. Likewise, humans with CF have been found to have high fecal bile acid excretion, but this is poorly correlated with fecal fat losses (44). It is possible that the excessive fecal bile acid losses seen in both humans and CF mice are a clue to the abnormal terminal ileal function that must be a part of both MI and DIOS, which bears further exploration. Epithelial damage and/or dysfunction may contribute to fat malabsorption in humans, similar to findi