| Consumption of animal fat has been associated with an increased incidence of chronic diseases, as cardiovascular disease, obesity and diabetes. Ruminant products, milk and meat, have high saturated fatty acids (FA) content as a result of biohydrogenation by microbial metabolic activity in the rumen. Biohydrogenation also originates FA with health promoting or disease preventing properties, including conjugated linoleic acids (CLA) and vaccenic acid (18:1 trans-11). Improvement of FA composition of milk and meat might promote human health but research is needed on ruminal lipid metabolism, which plays a major role in FA profile. The overall aim of this thesis was to investigate on specific microorganism and factors affecting the rumen lipid metabolism. Different methodological approaches were used, ranging from in vitro studies, with pure and mixed cultures, to in vivo studies. Polyunsaturated FA metabolism by Butyrivibrio fibrisolvens, a major hydrogenating rumen bacterium, was evaluated (Chapter 2). Toxicity of non-esterified unsaturated FA to B. fibrisolvens increased with FA length, number of unsaturations, and cis geometrical configuration. Biohydrogenation was shown to be a detoxifying mechanism, although mechanisms by which toxicity occur were not identified. Results suggest a protective role of 18:1 trans-11 to FA toxicity. A semi-continuous in vitro fermentation system was used to determine the effects of pH and starch inclusion level, in diets supplemented with oil, on the biohydrogenation pattern, namely in identifying the factor responsible for the trans-10 shift, associated with milk fat depression (Chapter 3). The trans-10 shift was observed to be associated with high starch level, independently of pH incubation level (6 or 7). Modification of the biohydrogenation pathway was suggested to be associated with changes in microbial population. Dietary nitrogen source and soybean oil supplementation effects on rumen contents and bacterial fractions FA composition were evaluated in vivo (Chapter 4). Biohydrogenation pattern was not affected by nitrogen source, yet CLA was promoted in true protein diets with soybean oil. The increase in cis-vaccenic acid (18:1 cis-11) in non-protein nitrogen diets may suggest a regulatory mechanism of bacterial membrane fluidity in the presence of lower proportions of odd- and branched-chain FA. |
