Serum Unconjugated Bile Acids as a Test for Intestinal Bacterial Overgrowth in Dogs TONATIUH MELGAREJO, PhD, DAVID A. WILLIAMS, PhD, NANCY C. O’CONNELL, MS, and KENNETH D.R. SETCHELL, PhD Small intestinal bacterial overgrowth (SIBO) has a high incidence in dogs and, as in humans, is difficult to diagnose. The aim of this study was to determine the diagnostic significance of serum unconjugated bile acid concentrations in dogs with bacterial overgrowth. Fasting sera were obtained from 23 dogs: 10 with culture-proven SIBO, 8 with indirectly diagnosed SIBO (normal pancreatic function but small intestinal disease associated with subnormal serum cobalamin and supranormal folate concentrations), and 5 healthy controls. Unconjugated bile acids were determined using gas chromatography–mass spectrometry after isolation by liquid–solid extraction and anion-exchange chromatography. Mean serum unconjugated bile acid concentrations were significantly elevated in dogs with SIBO (mean  SD: 0.91  1.03 mol/liter), and in dogs with indirectly diagnosed SIBO (2.11  2.20 mol/liter) compared to clinically healthy dogs (0.015  0.015 mol/liter, P  0.005). Cholic acid was the predominant unconjugated bile acid in the serum of dogs with SIBO. In conclusion serum unconjugated bile acid concentrations of healthy dogs are significantly lower than reported values for humans, and this fraction represents a relatively small proportion (0 –2.3%; mean 0.8%) of the total bile acids in dogs. Unconjugated bile acids increased 10- to 20-fold in dogs with SIBO indicating the clinical utility of serum unconjugated bile acids for diagnosis of intestinal bacterial overgrowth in dogs. KEY WORDS: bile acids; bacterial overgrowth; intestinal disease. Primary bile acids once synthesized from cholesterol are converted to the CoA esters and conjugated pri- marily with the amino acids glycine and taurine (1), a reaction catalyzed by two enzymes, a bile acid-CoA ligase and a bile acid-CoA N-acyl amino acid trans- ferase (2, 3). The capacity of the liver for conjugation is vast and, as a consequence, bile acids are secreted in bile predominantly as glycine and taurine conju- gates (4). There are major species differences in the extent of the specificity of bile acid conjugation (5). In humans, the glycine/taurine ratio is normally 3:1 (4), while in early life there is a predominance of taurine conjugation (6). The rat (7), cat (8), and dog (9) conjugate bile acids with mainly taurine. However, irrespective of these differences, unconjugated bile acids constitute a relatively small proportion of the total bile acids secreted in bile (10), even after oral administration of large amounts of unconjugated bile acids (11). The primary bile acid pool undergoes efficient en- terohepatic circulation, during which time approxi- mately one third undergoes intestinal bacterial de- conjugation (12). In the terminal ileum, cecum, and colon the majority of bile acids are consequently Manuscript received September 28, 1998; revised manuscript received May 25, 1999; accepted June 30, 1999. From the Gastrointestinal Laboratory, Department of Small Animal Medicine and Surgery, School of Veterinary Medicine, Texas A&M University College Station, Texas; and Clinical Mass Spectrometry Center, Department of Pediatrics, Children’s Hospi- tal Medical Center, Cincinnati, Ohio. This work was supported by the IAMS Company, Dayton, Ohio, USA; and the Purdue Research Foundation. Address for reprint requests: Dr. Tonatiuh Melgarejo, Veteri- nary Hospital, University of Pennsylvania, 3850 Spruce Street, Philadelphia, Pennsylvania 19104-6010. Digestive Diseases and Sciences, Vol. 45, No. 2 (February 2000), pp. 407– 414 407 Digestive Diseases and Sciences, Vol. 45, No. 2 (February 2000) 0163-2116/00/0200-0407$18.00/0 © 2000 Plenum Publishing Corporation