Background: Pancreatic enzyme replacement therapy frequently fails to correct intestinal fat malabsorption completely in cystic fibrosis (CF) patients. The reason for this failure is unknown. Objective: We investigated whether fat malabsorption in CF patients treated with pancreatic enzymes is caused by insufficient lipolysis of triacylglycerols or by defective intestinal uptake of long-chain fatty acids. Design: Lipolysis was determined on the basis of breath 13 CO 2 recovery in 10 CF patients receiving pancreatic enzyme replacement therapy after they ingested 1,3-distearoyl,2[1- 13 Results: Fecal fat excretion ranged from 5.1 to 27.8 g/d (x -± SD: 11.1 ± 7.0 g/d) and fat absorption ranged from 79% to 93% (89 ± 5%). There was no relation between breath 13 CO 2 recovery and dietary fat absorption (r = 0.04) after ingestion of [ 13 C]MTG. In contrast, there was a strong relation between 8-h plasma [ 13 C]LA concentrations and dietary fat absorption (r = 0.88, P < 0.001). Conclusion: Our results suggest that continuing fat malabsorption in CF patients receiving enzyme replacement therapy is not likely due to insufficient lipolytic enzyme activity, but rather to incomplete intraluminal solubilization of long-chain fatty acids, reduced mucosal uptake of long-chain fatty acids, or both. Am J Clin Nutr 1999;69:127-34. KEY WORDSBreath test, mixed triacylglycerol, [13 C]linoleic acid, fat malabsorption, fat balance, lipolysis, stable isotopes, cystic fibrosis, recommended dietary allowance, children, long-chain fatty acids INTRODUCTIONIn humans, triacylglycerols composed of long-chain fatty acids constitute 92-96% of dietary fats (1). Absorption of these fats is by 2 main processes. Lipolysis, by lipolytic enzymes originating predominantly in the pancreas, leads to hydrolysis of triacylglycerols into fatty acids and 2-monoacylglycerols. Second, intestinal uptake involves the formation of mixed micelles composed of bile components and lipolytic products, followed by the disintegration of the mixed micelles in the unstirred water layer and the translocation of the lipolytic products across the intestinal epithelium (1-4).Most cystic fibrosis (CF) patients malabsorb dietary fats because of pancreatic insufficiency, which leads to impaired lipolysis (5, 6). The symptoms of pancreatic insufficiency, such as steatorrhea and poor growth, can be alleviated by oral supplementation with pancreatic enzymes. However, despite recent improvements in the pharmacokinetics of these supplements, many patients continue to experience a certain degree of steatorrhea (7-9), with 10-20% of the dietary fat they consume being malabsorbed. It has not been elucidated whether this malabsorption is due to insufficient pancreatic enzyme replacement therapy. This possibility is likely because decreased pancreatic bicarbonate secretion may negatively affect enzyme activity by sustaining a low pH in the duodenum (10, 11). At a low duodenal pH, the release of enzymes from the microcapsules is inhibited and denaturation of the enzymes is stimul...

Defects in lipolysis due to pancreatic insufficiency can be diagnosed by the mixed triglyceride (MTG) 13CO2 breath test. However, the effects of various test conditions on the 13CO2 response have only been partially elucidated. In healthy adults, we performed the 13CO2 mixed triglyceride breath test and we compared (a) the inter- and intra-individual variation in the 13CO2 response; (b) the effect of two different test meals; (c) the effect of an additional meal during the test; and (d) the effect of physical exercise during the test. Upon repeating the test in the same individual (test meal cream), repeatability coefficients were large, with respect to either time to maximum 13C excretion rate (3.8 h). maximum 13C excretion rate (4.9% 13C dose h-1) or cumulative recovery of 13C over the 9-h study period (22.7% 13C dose). The cumulative 13C expiration over 9 h obtained with the test meal composed of cream was quantitatively similar to that obtained with bread and butter: 42.2 +/- 8.4% and 47.7 +/- 6.3% respectively. Fasting for 9 h during the test resulted in similar 13C expiration rates and cumulative 13C expiration (43.4% +/- 7.2%) when compared with consumption of an additional meal 3 h after the start of the test (38.3 +/- 5.3%). The 13CO2 response increased in five out of seven subjects, but decreased in the other two, when moderate exercise was performed (bicycle ergometer, 50 W for 5 h). We conclude that the repeatability of the MTG test in healthy adults is low. The present results indicate that a solid and a liquid test meal, containing a similar amount of fats, give similar cumulative 13CO2 responses, and that stringent prolonged fasting during the test is unnecessary. Standardization of physical activity seems preferable, since the unequivocal effects of moderate exercise on the 13CO2 response were observed in the individuals studied.

Background: Classic fat balance studies detect fat malabsorption but do not discriminate between the potential causes of malabsorption, such as impaired intestinal lipolysis or reduced uptake of fatty acids. Objective: We aimed to validate a novel test for the specific, sensitive detection of impaired intestinal uptake of long-chain unesterified fatty acids in an appropriate rat model of fat malabsorption. Design: The absorption and appearance in plasma of [13 C]palmitic acid were determined in control rats and in rats with fat malabsorption due either to chronic bile deficiency (permanent bile diversion) or to oral administration of the lipase inhibitor orlistat (200 mg/kg diet). [13 C]Palmitic acid results were compared with the percentage absorption of ingested dietary fat determined by fat balance. Results: Between 1 and 6 h after intraduodenal administration, plasma [ 13 C]palmitate concentrations in control rats were 4-10-fold higher than in bile-deficient rats (P < 0.05) but were not significantly different between orlistat-supplemented rats and their controls. In control and bile-deficient rats, plasma [13 C]palmitate concentrations allowed complete discrimination between normal (> 92%) and reduced (< 92%) fat absorption, whereas the percentage absorption of [ 13 C]palmitate over 48 h appeared to be highly correlated with the percentage absorption of ingested dietary fat (r = 0.89, P < 0.001). Conclusions: The [13 C]palmitic acid absorption test detects impaired intestinal absorption of long-chain fatty acids selectively and sensitively in a rat model of fat malabsorption due to bile deficiency. Our data strongly support the use of the

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Digestion and absorption of lipids is a highly efficient process. From Western diets about 95% will be absorbed. This implies that together with lipids from endogenous sources 6-8 g of lipids will enter the colon daily. This input significantly increases during various lipid malabsorption syndromes. It has long been assumed that the biological fate of unabsorbed lipids is physiologically not relevant. However, significant microbial lipid metabolism occurs. Circumstantial evidence is arising which supports a role of unabsorbed lipid metabolites in the development of colonic diseases. Lipid metabolites may act as detergents in the colon, leading to mucosal injury and reactive hyperproliferation, which in its turn could promote tumour development. Lipid metabolites could also be transformed in biological active metabolites, which have a tumour promoting potency. More mechanistic information is needed on the colonic metabolic fate of lipids in order to develop strategies for manipulating colonic flora in the prevention of lipid related colonic diseases.