Linoleic acid conjugation by human intestinal microorganisms is inhibited by glucose and other substrates in vitro and in gnotobiotic rats

The anticarcinogen conjugated linoleic acid (CLA) is a product of bacterial activity that isomerizes linoleic acid (LA) in the rumen of herbivores. Th erefore, fatty dairy products in the human diet are enriched with CLA. Alth ough bacteria capable of in vitro LA conjugation were detected in the human intestinal tract, CLA synthesis from dietary sunflower seed oil was not ob served in gnotobiotic rats associated with these intestinal bacteria. The o bjective of the study was to investigate variables that affect LA conjugati on. In vitro, LA conjugation was strongly inhibited by glucose and other su bstrates. Concentrations of 1.5 mmol glucose/L inhibited LA conjugation by 50%. Methyl-alpha-D-glucoside was a less effective inhibitor than glucose, and 2-deoxy-D-glucose did not inhibit LA conjugation at all. To analyze the concentration of carbohydrates in intestinal contents, the LA-conjugating bacterial mixed culture and human fecal microorganisms were introduced into germfree rats. Samples of feces and cecum and colon contents of both group s exhibited in vitro LA-conjugating activity. Rats associated with human in testinal microorganisms contained 5.7 +/- 1.3 mmol glucose/L in the cecal c ontents and 6.6 +/- 1.0 mmol glucose/L in the colonic contents. Rats associ ated with CLA-producing bacterial culture contained 3.4 +/- 1.3 mmol glucos e/L in the cecal contents and 4.2 +/- 1.0 mmol glucose/L in the colonic con tents. These values are within a range that may explain the observed inhibi tion of LA conjugation in vivo.