Rumen Lipid Metabolism
Recent discoveries in the area of functional foods indicate that specific fatty acids produced in the rumen may have beneficial effects on human health. These developments have brought about a keen interest in the possibility of designing natural food products with enhanced levels of these fatty acids. However, the dynamic nature of the rumen environment results in the production of a complex pattern of fatty acids from a limited number of dietary fatty acids. Therefore, it is difficult to predict the output of fatty acids from the rumen based on fatty acid intake. While this page will only cover some very basic concepts pertaining to lipid metabolism in the rumen, a more complete explanation of this process can be found in a 2003 CNC paper written by our group.
Dietary Supply
Fat is an important energy component in the diet of ruminants. Forages and concentrates are the primary sources of lipid in the ruminant diet, and over the last decade fat supplementation has become a common practice to increase the energy density of the diet for high producing dairy cows. Forages typically contain 4 to 6% of the dry weight of the leaf as lipid, of which the major lipid class is glycolipids. The lipid content of concentrates is usually higher than that of forages, and the majority is present in the form of triglycerides. Fat supplements that are by-products of rendering and vegetable oil refining industries contain the lipid predominantly as triglycerides whereas rumen-protected supplements of Ca-salts are comprised of free fatty acids. Along with lipid class, the fatty acid profile of dietary components also varies. Generally forage sources contain a higher concentration of linolenic acid (18:3) whereas linoleic acid (18:2) is the predominant fatty acid in cereal grains and seeds. Below is a table showing the representative fatty acid composition of some typical feedstuffs and fat supplements.
Hydrolysis
When dietary lipid enters the rumen, the initial step in lipid metabolism is the hydrolysis of the ester linkages found in triglycerides, phospholipids, and glycolipids. Hydrolysis of dietary lipids is predominantly due to rumen bacteria with little contribution by rumen protozoa and fungi or salivary and plant lipases. Hydrolysis of lipids is extracellular, and the glycerol and sugars that are liberated are readily metabolized by the rumen bacteria. Although the extent of hydrolysis is generally high (>85%), a number of factors that affect the rate and extent of hydrolysis have been identified. For example, the extent of hydrolysis is reduced as the dietary level of fat is increased, or when factors such as low rumen pH and ionophores inhibit the activity and growth of bacteria.
Biohydrogenation of unsaturated fatty acids is the second major transformation that dietary lipids can undergo in the rumen. The process of biohydrogenation requires a free fatty acid to proceed and as a consequence rates are always less than those of hydrolysis, and factors that affect hydrolysis also impact biohydrogenation. The initial step in rumen biohydrogenation typically involves an isomerization of the cis-12 double bond to a trans-11 configuration resulting in a conjugated di- or trienoic fatty acid.
The next step is a hydrogenation reaction, which results in the conversion of an unsaturated double bond to a saturated single bond. In the case of linoleic and linolenic acids this is a reduction of the cis-9 double bond resulting in a trans-11 fatty acid. The final step is a further hydrogenation of the trans-11 double bond producing stearic acid (linoleic and linolenic acid pathways) or trans-15 18:1 (linolenic acid pathway). It is also possible to differentially affect steps in the biohydrogenation process. This can be caused by factors such as dietary addition of fish oil or reduced rumen pH.
The major biohydrogenation substrates are linoleic and linolenic acids. The rate of rumen biohydrogenation is typically more rapid with increasing unsaturation. For most diets linoleic acid and linolenic acid are hydrogenated to the extent of 70-95% and 85-100% respectively. Based on their metabolic pathways, rumen bacteria involved in biohydrogenation have been classified into two groups (A and B). In order to obtain complete biohydrogenation of polyunsaturated fatty acids, bacteria from both groups are generally required.
Diet and changes in the rumen environment can shift the pathways of biohydrogenation resulting in dramatic changes in the fatty acid intermediates produced. Through improved analytical techniques we have gained an appreciation for the complexity of the biohydrogenation processes occurring in the rumen. As shown in the figure above, the biohydrogenation intermediates formed from the classical pathways of biohydrogenation are trans-11 18:1 and cis-9, trans-11 CLA. However, the table below demonstrates the remarkable range of trans 18:1 and CLA isomers flowing from the rumen, indicating that in addition to the major pathways described above there must be many additional pathways of biohydrogenation involving a variety of bacterial isomerases and hydrolyases.
Absorption
Lipid entering the small intestine is virtually identical to that leaving the rumen. The total amount of lipid entering the duodenum may exceed lipid intake due to the contribution of microbial lipid synthesis. Approximately 80-90% of the lipid entering the small intestine is as free fatty acids attached to feed particles. The remaining lipid components are microbial phospholipids plus small amounts of triglycerides and glycolipids from residual feed material. The fatty acids are highly saturated and mainly in the form of palmitic (16:0) and stearic acids (18:0). Micelle formation is critical for efficient fatty acid absorption. In ruminants, lysolecithins act together with bile salts to desorb fatty acids from feed particles and bacteria allowing for the formation of micelles. Formed micelles are taken up by the epithelial cells of the jejunum where the fatty acids are re-esterified into triglycerides and then packaged into chylomicrons for transport.