The results of the current study showed that the fecal behenate method which measures fecal lauric acid/behenic acid ratio after a test meal containing lauric acid mixed with other fatty acids, correlates with the “gold standard”, the 72 hour quantitative fecal fat measurement. Unfortunately, the correlation is not sufficiently robust that we could confidently suggest that the behenate method could replace the 72 hour fat balance method for the management of pancreatic enzyme treatment of patients with cystic fibrosis. In our preliminary studies, fat absorption does not appear to be impacted by whether enzymes are taken preceding or during the meal in cystic fibrosis. To definitively answer this question an extremely large sample (n=329 with 80% power and alpha of 0.05) would have to be studied to definitively indicate that there are differences in absorption resulting from the timing of enzyme administration.
The goal of this study was to determine if we could find a facile way to estimate fat absorption in patients with cystic fibrosis that could be used to monitor responses to interventions directed at correcting fat absorption. Given the complications that can result from fat malabsorption, accurate and timely identification is critical in the diagnosis and management of these patients. Although other tests of fat malabsorption have been proposed, these tests are limited in application, accuracy and feasibility. Several other diagnostic tests to identify fat malabsorption have been proposed. In 1961, Drummey et al. reintroduced the technique of microscopic examination of stool for fat globules and outlined a scale for grading this steatorrhea (7
). Although the sensitivity of this method has been reported as high as 96% (8
), these results have not been reproducible (9
). Additionally, this test is only a semiquantitative measure with relatively poor specificity (11
), thus patients with fat malabsorption identified by this method still need a 72-hour fat balance study to quantify and confirm this steatorrhea. Another method, the triolein breath test, measures exhaled 14
after ingestion of a known amount of triglyceride labeled with 14
C or 13
C. Reports have been mixed on the sensitivity and specificity of this test, with sensitivity varying from 64%–100% (12
). Unfortunately, several other factors other than triolein absorption affect the rate of conversion to carbon dioxide and this test may be inaccurate in many diseases that lead to fat malabsorption such as cholestatic liver disease and cystic fibrosis (12
). Due to these limitations, the triolein breath test is not commercially available for clinical use. Finally, in 1981, Phuapradit et al. introduced the steatocrit, a new screening method to estimate fecal fat that required only a small volume of stool, making it ideal for use in the pediatric population (14
). This procedure was modified by Tran et al., who found that acidification of the stool prior to performing the steatocrit improved the fat separation and increased the sensitivity of this method (15
). Unfortunately, the two initial studies by Tran et al. using this method were quite small and larger studies using the acid steatocrit have not been as promising, with a lower sensitivity (17
) and a weak correlation between the acid steatocrit and the 72 hour fat-balance method (18
). Recently, work using dysprosium chloride as a nonabsorbable marker with stably-labeled triglycerides as a method to assess fat absorption has been reported (19
). This work has demonstrated that a single marked stool with brilliant blue in which the fractional excretion of a non-absorbable marker, dysprosium, is assessed can be compared to the presence of a labeled fatty acid. A high correlation was demonstrated between the fractional excretion of Dy and 13(C) in stool indicating that this was a promising method to assess fat absorption on a single marked stool. Unfortunately, the measurement of Dy and 13(C) require mass spectrometry, which is not routinely available, and material costs are expensive.
There are little data examining the effects of timing of pancreatic enzyme supplementation in relationship to meals. There is anecdotal evidence that administering enzyme supplements during the meal rather than before the meal may reduce gastrointestinal symptoms and potentially enhance fat absorption (personal communication, Preston Campbell). There has been no controlled study examining this question. Our results do not suggest a relationship between timing of PERT administration. Based upon the small differences observed in fat absorption in relationship to timing of PERT administration, the calculated large sample size needed to assess this question makes the performance of such a study in the future unlikely.
The reason for the suboptimal correlation between the fecal behenate method and the 72 hour fat balance method are not clear. There are a number of possibilities: 1) Absorption from a single meal, on which the fecal behenate method is based, may not be representative of the average fat absorption over the 3 day period over which the 72 hour fat balance method is based. There are little data on meal-to-meal and day-to-day variation in fat excretion in normal or diseased humans even though assays for fecal fat have been reported for over a century (20
). Early work on fat absorption does suggest there is day-to-day variation in fat excretion in adults with jaundice reported secondary to cirrhosis or hepatitis (21
). The results of additional studies that we have performed examining the effect of Xenical on fat absorption in 10 participants comparing the fecal behenate method with the 72 hour fat balance method using NMR analysis had correlations between 0.51 and 0.80 (unpublished results, JEH, RJ). 2) There may be intrinsic variability in fat absorption in CF as suggested by the failure to show robust correlations between the 13
C- triolein breath test and the 72 hour fat balance method (22
); however, studies by Jongorbani et al suggest that the technique which utilizes stable-labelled triglyceride with a non-absorbable marker, dysprosium given as a single meal has a strong correlation with the 72 hour fat balance method in CF patients (18
). 3) Although the 72 hour fecal fat method is considered the “gold standard” for assessing fat absorption, there is considerable variability of results in normal subjects and those with CF. In normals, the variability on test-retest can be −8.1 to+5.9% while in CF it may be larger with test-retest values of −19.7% to 42.8% (23
) Additional unpublished results indicate large test-retest variability in CF patients on PERT (−30%to +58%) or on placebo (+42 to−42%) (personal communication, D. Borowitz). Additional factors may play a role although likely to be minor. Conditions causing interruption of the enterohepatic circulation of bile acids, such as ileal resection, lead to intraluminal bile acid concentrations falling below the critical micellar concentration as the day progresses (24
). Although patients with CF have mild perturbations in the enterohepatic circulation and none of our participants had a previous ileal resection, it seems unlikely that they have sufficient reductions in intraluminal bile acids with meals during the day to explain the discrepencies (25
). 3) It is possible that dietary fat containing lauric acid might have compromised the results since the fecal behenate method is dependent upon knowing the dietary lauric at and around the time of the test meal. This seems unlikely since lauric acid is contained in only a limited number of foods and the participants diets were carefully controlled in a hospital setting. 4) Half of the subjects were on some type of acid suppression therapy. This might have had some impact on their absorption of dietary fats especially if there was any variability in their compliance with the medication. 5) Additional factors in CF may have some impact on fat absorption that have been incompletely explored including bacterial overgrowth, variability in release of exogenous pancreatic enzymes during the course of the day, and recently recognized mucosal abnormalities in absorption.
Despite the somewhat discouraging results of the present study, there are some encouraging findings. The behenate test does correlate with fat absorption and, with modification, may ultimately prove useful in this regard because of its ease of sample collection and analytical requirements. Investigators should be encouraged to continue to pursue development of facile methods for assessing fat malabsorption that may be used in adult and children with diarrhea for diagnosis and management of their diseases.