Fat Digestion and Absorption in Ruminants
Processes in the rumen
There are major differences in the process of fat digestion and absorption between ruminants and non-ruminants animals. Dairy cows mainly consume a diet that contains “polyunsaturated” fatty acids (PUFA) as part of plant triglycerides and glycolipids. The bacteria in the rumen will split off the fatty acids (and sugars) from the glycerol backbone. The glycerol and the sugars released from glycolipids are fermented to the volatile fatty acids (VFA). The breakdown of dietary lipids by the rumen bacterial generally occurs very rapidly as the lipids are exposed during rumination and bacterial digestion of feed particles. Moreover, this process is completed so that no monoglycerides or diglycerides are passed to the lower digestive tract. There is an exception however, when a highly saturated (or hydrogenated ) triglyceride is fed. This is due to the high melting point of such fats, resulting in low solubility, causing bacterial enzymes to not able to gain access to the bonds linking fatty acids to glycerol, so these would pass to the lower digestive tract. Unfortunately, the same limitations of solubility and melting point result in poor access of the animal’s digestive enzymes in the small intestine, and very poor digestibility in that site as well.
The fatty acids released in the rumen are not immediately absorbed in the rumen, but rather will pass to the abomasum and then the small intestine, which is the primary site for absorption of the fatty acids. However, the profile of fatty acids that reaches the intestine will be very different from what the animal has consumed. This is because of the extensive biohydrogenation that occurs in the rumen as a result of bacterial activity.
Unsaturated fatty acids are toxic to many of the species of rumen bacteria, especially to those that are involved in the fiber digestion. There will also be an excess of hydrogen that the microbial population is continually interested in getting rid of due to the anaerobic environment of the rumen. As unsaturated fatty acids are released from glycerol backbone, they are quickly hydrogenated to saturated fatty acids. In cows that are fed with typical diets, over 90% of the unsaturated fatty acids will be biohydrogenated to produce saturated fatty acids that flow to the small intestine.
Biohydrogenation is considered a favourable process from the standpoint of rumen carbohydrate fermentation because it reduces potential negative effects of unsaturated fatty acids on rumen fermentation of fiber. The negative effects on microbial fiber digestion is the main reason why large amounts of free vegetable oils cannot be fed to dairy cows. The breakdown of dietary lipids to free fatty acids occurs more rapidly compared to the biohydrogenation process. Hence, large amounts of unsaturated oils can "overwhelm" the biohydrogenation process and result in undesirable effects on the rumen microbial population.
During the biohydrogenation process, intermediate compounds with trans-double bonds are produced. Some of these trans-intermediates escape from the rumen and are incorporated into body fat and milk fat of ruminants. This accounts for the relatively high content of trans-fatty acids in ruminant products. Under low rumen pH conditions that may result from excessive grain or insufficient effective fiber in the diet, a different set of trans-intermediates may be produced. Some of these alternate products, particularly those with a trans-double bond between the 10th and 11th carbons, have very powerful inhibitory effects on milk fat synthesis and so milk
fat depression (or low milk fat tests) may result.
Processes in the small intestine
The lipids that leave the rumen are predominantly free fatty acids (85-90%) and phospholipids (10-15%) found as part of microbial cell membranes. Most of the free fatty acids found in the rumen are potassium, sodium, or calcium salts of fatty acids because of the near-neutral pH in the rumen contents (6.0 - 6.8). After passing through the acid conditions (pH ~2.0) of the abomasum, the fatty acid salts will dissociate and the free fatty acids will be found absorbed (or "stuck") to the surface of small feed particles that pass as part of the digestive contents. The fatty acids making up the free fatty acid portion will be predominantly saturated (approximately 80-90%), usually around two thirds being stearic acid while one third being palmitic acid. For non ruminants animal, they will have a hard time trying to absorb such high-melting point, insoluble fatty acids, but ruminants have developed processes that result in saturated fatty acids being absorbed nearly as well as unsaturated fatty acids, with much greater efficiency compared to non-ruminants animals.
Regardless of ruminants or non-ruminants, the key to absorption of fatty acids is the formation in the intestine of complexes called micelles, which are bi-layer disks consisting of bile salts (secreted in bile from the liver into the intestine by way of the gall bladder), phospholipids, and the insoluble lipids in the middle. The main function of micelles is to move the fatty acids to the surface of the intestinal cells where they can be absorbed into the cells. In non-ruminants, monoglycerides that result from digestion of triglycerides in the small intestine are needed for fat absorption. Bile salts and monoglyceride have portions of their molecular structure that can interact with aqueous systems (like the fluid in the intestinal lumen) as well as portions that can interact with lipids, so they form an “interface” between fat and water. In the absence of monoglycerides, nonruminants are not able to absorb many fatty acids.
In ruminants, a compound called lysolecithin takes the place of monoglyceride. Lysolecithin is produced from lecithin by action of an enzyme called phospholipase that is secreted from the pancreas of the cow into the upper small intestine. Phospholipase converts
lecithin into lycolecithin, which is an extremely efficient emulsifier particularly for saturated free fatty acids.
Intestinal processing and delivery of dietary fat
After the absorption of fatty acids into intestinal cells, the fatty acids are reconverted to triglycerides by combining with the glycerol produced from metabolism of blood glucose. The triglycerides are packaged into lipoprotein particles (chylomicrons or very low density
lipoproteins, VLDL) in combination with cholesterol, phospholipids, and specific proteins. These proteins (called apoproteins) serve to direct the trafficking and use of the lipoprotein triglycerides. The lipoproteins are secreted into the lymph, due to its size being too large to pass directly into the venous blood stream draining the intestinal cells. They are then delivered back into the blood stream near the heart. The lipoprotein particles are delivered to various organs of the body such as the mammary gland, muscle, and heart that can use the triglycerides after that blood is oxygenated through the lungs. Triglycerides in chylomicrons or VLDL are broken down to free fatty acids by an enzyme called lipoprotein lipase that is found in the capillaries of these tissues. The free fatty acids then enter the cells where they can be formed back into triglycerides (such as milk fat) or burned to release energy that can fuel cell functions (such as contraction in skeletal or heart muscle).
It should be noted from the scheme for lymphatic absorption described here that dietary fats do not reach the liver directly, in contrast to other absorbed nutrients like amino acids or propionate. Consequently, dietary fats do not contribute appreciably to fat accumulation in the liver (fatty liver) that is often observed around calving. Fatty liver results from the accumulation of triglyceride driven by extensive mobilization of NEFA from adipose tissue during negative energy balance, and the conversion of NEFA back to triglyceride in the liver.
In ruminant animals, oxidative or fuel use of long-chain fatty acids is more limited than in most nonruminant animals. This may be due, at least in part, to the abundance of acetic acid from rumen fermentation. Acetate which is the salt form of acetic acid present at normal body pH is the most abundant oxidative fuel in ruminants such as dairy cows, and seems to be more actively taken up and prepared for fuel use than long-chain fatty acids. Consequently, the main use for fatty acids from dietary fats and oils will be for triglyceride synthesis. For cows in negative energy balance during early lactation, which are mobilising body fat, the only substantial use for dietary fatty acids will be for milk fat synthesis. For cows in positive energy balance that are also depositing body fat, some dietary fat can be stored in adipose tissue as well as used for milk fat synthesis.
The small amounts of PUFA that escape through the rumen without being hydrogenated are very important for proper structure of membranes. The PUFA cannot be made in the cow’s body and so must be absorbed from the intestine. Intestinal cells primarily attach absorbed PUFA to phospholipids and cholesterol esters rather than triglycerides. In this way, the PUFA are protected from being burned for energy and instead are incorporated into cell membrane phospholipids. Here, they maintain normal structure and function of cell membranes. They also can be released and converted into important signaling molecules such as prostaglandins and leukotrienes. The unique structure and metabolism of PUFA result in their incorporation into cell membranes of tissues all over the body, including immune cells and the reproductive tract. Proposed benefits of partially rumen-protected PUFA supplements likely arise in this way. The challenge is to prevent their biohydrogenation in the rumen.