No abstract

Ruminal disappearance of linoleic and linolenic acid has been studied extensively. Less is known of the metabolism of docosahexaenoic acid (22:6n-3). The aim of this study was to identify factors which affect the disappearance of 22:6n-3 during in vitro batch incubations using rumen fluid from sheep. In experiment 1, the effect of the rumen fluid/buffer ratio (0.2 or 0.4), substrate (cellulose or cellulose/glucose), time of 22:6n-3 addition (0.08 mg/mL after 0 or 6 h of incubation) and incubation time (24 or 48 h) was evaluated. A mixture design was used in experiment 2 to evaluate the effect of carbohydrate type (cellulose, glucose, cellobiose and starch) on 22:6n-3 disappearance (0.08 mg/mL). In experiment 3, several concentrations of 22:6n-3 (0.05-0.30 mg/mL) were evaluated with different substrate mixtures (combinations of cellobiose, starch and cellulose). In a final experiment, the effect of the rumen fluid/buffer ratio (0.20, 0.35 and 0.50) and substrate (glucose, cellobiose and starch) was evaluated. In this experiment, 22:6n-3 was added as a proportion of rumen fluid ranging from 0.1 to 0.4 mg/mL rumen fluid, contrary to former experiments where concentrations were relative to culture medium. Low levels of 22:6n-3 (0.05 mg/mL) allowed extensive metabolism whereas increasing amounts of 22:6n-3 hampered its disappearance. A greater proportion of rumen fluid resulted in increased disappearance of 22:6n-3. The effect of carbohydrate type was small compared with the former two factors. These results suggest that in vitro metabolism of 22:6n-3 is mostly dictated by the conditions at the start of the incubation, i.e., inoculum, probably reflecting the density of bacteria able to metabolize 22:6n-3.