Technical note: An in vivo method to determine kinetics of unsaturated fatty acid biohydrogenation in the rumen.

Rumen microbial biohydrogenation (BH) of unsaturated fatty acids (UFA) has been extensively studied in vitro; however, in vitro BH pathways, rates, and extents may not parallel those in vivo. The objective was to develop an assay to assess in vivo rates, pathways, and extent of BH of oleic (OA), linoleic (LA), and α-linolenic (ALA) acids. Each UFA was characterized in a separate experiment, each using 4 ruminally cannulated lactating Holstein cows. A single bolus consisting of 200 g of a UFA-oil [experiment 1 (EXP1): 87% OA sunflower, experiment 2 (EXP2): 70% LA safflower, and experiment 3 (EXP3): 54% ALA flaxseed] and 12 g of heptadecanoic acid (C17:0) was mixed into the rumen through the fistula. Rumen digesta was collected at -1, -0.25, 0.1, 0.25, 0.5, 0.75, 1, 1.5, 2, 3, 4, and 6 h relative to the bolus. Overall, the triglyceride boluses increased total fatty acids (FA) in the rumen from 3.9 (standard deviation = ±1.4) to 7.3% (±1.4) of rumen dry matter and enriched C17:0 from 0.4 (±0.1) to 2.5% (±0.5) of FA. The bolus enriched OA from 8.9 (±1.0) to 30.1% (±4.6) of FA in EXP1, LA from 11.1 (±1.8) to 35.9% (±5.0) of FA in EXP2, and ALA from 2.1 (±0.1) to 19.8% (±4.3) of FA in EXP3. The disappearances of C17:0, OA, LA, and ALA were fit to a single exponential decay model. The first-order rate of C17:0 rumen disappearance (turnover) was 9.1, 6.9, and 5.2%/h in EXP1, EXP2, and EXP3, respectively, and was used as a marker of FA passage. The rate of total rumen turnover of OA was 54.1%/h, LA was 60.5%/h, and ALA was 93.0%/h in EXP1, EXP2, and EXP3, respectively. Rumen concentration of all 3 UFA reached prebolus concentrations within 4 h. The calculated extent of lipolysis and initial isomerization was 85.6% for OA, 89.8% for LA, and 94.7% for ALA in EXP1, EXP2, and EXP3, respectively. Assuming that BH equals total disappearance minus passage, the rates of lipolysis and initial isomerization were 45.0, 53.6, and 87.8%/h for OA, LA, and ALA in EXP1, EXP2, and EXP3, respectively. Analysis of the data using compartmental modeling showed that the normal BH pathways proposed in the literature explained 46.0, 37.3, and 49.8% of the BH of OA, LA, and ALA in EXP1, EXP2, and EXP3, respectively. Based on the model, BH of trans C18:1 FA was the rate-limiting step to complete BH. Importantly, oils were provided as triglycerides and the reported rates represent the rate of lipolysis and BH. In conclusion, the rate of ruminal BH of OA, LA, and ALA was higher than that commonly observed in vitro, but the extent of BH was near expected values. The method developed provides a potential in vivo assay of ruminal BH for use in future experiments and modeling efforts.