VII. Lipids, Digestion

Monogastric Animals
The digestion process involves the breakdown of lipid molecules into smaller ones that are eventually absorbed into the blood. Lipids are not soluble in water, which is the aqueous medium of the digestive tract (lipids are hydrophobic). Therefore, the initial step in lipid digestion is to make them dissolve in water. How? Through a process called emulsification, or the dispersion of lipids in small droplets.

Dietary lipids (mostly triglycerides), upon their entry into the small intestine, are emulsified by bile salt (also called bile acid) released from the gall bladder. Bile salt functions as a detergent (due to their OH and COOH groups), and large lipid molecules form smaller lipid droplets surrounded by a layer of bile. Emulsified lipids are acted upon by enzyme pancreatic lipase and converted into fatty acids, monoglycerides and glycerol.

The lipid digestion products are assembled into micelles. These are temporary combinations of bile salt, fatty acids, monoglycerides, and other fat-soluble substances such as vitamins and cholesterol. The micelles are water soluble and enable the lipid digestion products to be transported to the small intestinal surface for absorption. At the site of absorption, the micelle breaks down and the bile salt returns to the intestine for continuing emulsification processes (bile salt recycling). The components are absorbed into the small intestine by passive diffusion. In a nutshell, the ability to form micelles and the presence of bile salt are very important for lipid digestion, and the lack of it can affect digestibility. For example, saturated fatty acids are less efficient than unsaturated fatty acids in forming micelles. So a blend of saturated and unsaturated fatty acids is used in animal rations.

Once inside the intestinal cell (or enterocyte), the monoglycerides and fatty acids are reesterified, and together with free and esterified cholesterol, lipoproteins and phospholipids are assembled into chylomicrons. The chylomicrons are secreted into the lymphatic system.

Ruminant Animals

In ruminant animals, the lipid content of the diet is low (under 5%) and comes from different sources such as grass, leaves, oil seeds, or cereal grains. Leaf or grass lipids are mainly galactolipids, phospholipids, waxes, pigments, and essential oils, and oil seed or grain lipids are mainly triglycerides.
In the rumen, there is no emulsifying agent or pancreatic lipase enzyme. Instead, there are rumen microbes producing microbial lipases. When dietary lipids enter the rumen, the initial step is the hydrolysis of the ester linkages in triglycerides, phospholipids, and glycolipids. Hydrolysis of dietary lipids is done by microbial lipases, which releases glycerol and fatty acids (free fatty acids) from the lipid backbone. Glycerol is readily metabolized by the rumen bacteria to form propionic acid. Feeding of supplemental fat increases the proportion of propionic acid (one of the volatile fatty acids, or VFAs) and the propionate:acetate ratio in ruminants. Hydrolysis is a prerequisite for the next step.

Biohydrogenation of unsaturated fatty acids is the second major transformation that dietary lipids can undergo in the rumen. Fatty acids with double bonds are altered by microbes to form more stable fatty acids. Fatty acids such as linoleic acid are converted “conjugated” fatty acids (e.g., conjugated linoleic acid, or CLA) in which the double bonds are not separated by methylene (CH2) groups. The position of double bonds is altered, and the fatty acids are converted to more stable “trans” fats. Some odd-numbered (e.g., C19:0) and branched-chain fatty acids are also created during this process. For example, linoleic acid (C18:2 n-6), where the double bonds are in the cis position (cis9-cis12), is converted to several isomers of CLAs during this conversion step.

Lipid digestion in the ruminant small intestine is very similar to lipid digestion in monogastric animals. The two key secretions enabling this process are bile and pancreatic juices. These secretions enable the lipids to form micelles for absorption. Bile supplies bile salts and pancreatic juice and enzymes. These compounds desorb the fatty acids from feed particles and bacteria, allowing the formation of micelles. Once micelles are formed, they facilitate the transfer of water-insoluble lipids across the intestinal epithelial cells of the jejunum, where the fatty acids are absorbed. Within the intestinal epithelial cells, the fatty acids are reesterified into triglycerides and then packaged into chylomicrons for transport in lymph to the blood.