Digestion and Absorption
of Fat and Cholesterol
How are triglycerides and cholesterol digested?
Digestion is a watery affair and likewise, digestive enzymes are water-soluble. The digestion of lipids presents a special problem as these substances will tend to clump together and form large droplets in the watery environment. Bile serves as an emulsifier or detergent in lipid digestion. Components of bile will coat smaller droplets of lipid rendering them water-soluble. This keeps larger lipid droplets from forming. So instead of having a few very large droplets of lipid we have a lot of tiny droplets. When lipids are present as tiny droplets, digestive enzymes have no problem attacking them. ats and oils, and therefore triglycerides, are present in both animals and plants. Oil is a natural component of many plants, seeds, and animals. Oils can be extracted from seeds or plants by a process called pressing which squeezes out the oil. Common oils include sunflower, safflower, corn, olive, coconut, and palm oil. Contrarily, butter is made from the fat in milk while lard is hog fat, and tallow is the fat of cattle or sheep. Cholesterol is not a necessary substance for plants, therefore they do not have the need to make it. Contrarily, mammals will make cholesterol to help meet their body needs. As a result, diet cholesterol intake is only attributed to consuming animal foods or foods that use animal products in their recipe.
Although, a triglyceride digesting enzyme called lingual lipase is present in our saliva, the job of digesting triglyceride is handled mostly by an enzyme called pancreatic lipase. Pancreatic lipase detaches 2 fatty acids from glycerol which results in a monoglyceride and two fatty acids. Some of the monoglycerides can also have the remaining fatty acid detached by another enzyme which results in glycerol and a fatty acid. Thus, the products of triglyceride digestion are fatty acids, monoglycerides, and glycerol. These substances are now small enough to move into the cells lining our small intestine, along with cholesterol. Some of the cholesterol in our diet is linked to other molecules. Another digestive enzyme will liberate cholesterol so that it can be absorbed.
How efficient is fat digestion?
Normally the efficiency of digestion and absorption of dietary lipid is >90%. In the absence of bile, much of the dietary lipid would escape digestion and absorption. Bile is made in our liver and stored in our gall bladder during non-digestive times. When fat-containing food particles arrive in our small intestine, bile is squeezed out of the gall bladder and travels to our small intestine through a duct. Some people may have had their gall bladder removed for medical reasons. Since bile is made in the liver, and the gall bladder merely functions as a temporary storage depot for that bile, this is not a large concern. In most cases the liver sends adequate amounts of bile directly to our small intestine during digestion to compensate for the absence of the gall bladder. Typically, a lower fat diet is recommended by the individual's physician.
How are triglycerides and cholesterol absorbed?
The process of absorbing lipids into our blood requires special consideration. Since our blood is mostly water, how can we introduce these water-insoluble substances into this watery medium? Cells lining the wall of our small intestine reassemble triglycerides and package them up along with cholesterol into protein-containing shells called chylomicrons. Chylomicrons are ejected from those cells and enter our circulation indirectly through lymphatic vessels. Chylomicrons are unable to directly enter our blood like monosaccharides because they are simply too large to squeeze into capillaries that supply the portal vein. As chylomicrons move through our lymphatic vessels, they eventually dump into our blood and circulate.
What does triglyceride do in our body?
Fat (triglyceride) is an energy source for most cells in our body and is our primary means of storing the excessive energy from the foods we eat. Storing excessive energy as fat rather than as protein or carbohydrate has great advantages. First, we are able to store more than twice the amount of energy in one gram of fat as we can in one gram of carbohydrate or protein. Secondly, stored fat, by virtue of its water-insolubility, will have less water associated with it then would be stored carbohydrate and protein. The net effect of storing excessive diet energy as fat versus carbohydrate or protein is that our body weight and volume is minimized.
What is Brown Adipose Tissue?
Sometimes magazine articles or blogs mention brown adipose tissue (BAT). Fat stores around our internal organs provides some cushioning while fat underlying our skin provides a layer of insulation that helps maintain our body temperature. A special type of fat called Brown Adipose Tissue (BAT) is specifically designed to generate heat. BAT is especially important for infants trying to regulate their body temperature. Adults generally have small amounts of BAT relative to infants. Because the level of BAT in adults is inversely related to obesity and that having more BAT would potentially help dispose of blood glucose and burn more calories daily, interest continues to see if we can leverage this specialized fat tissue.
What are Omega-3 fats?
All too often we think of fat as stored energy and curse it when we accumulate it beyond a desired level. However, some fats seem to be important in regulating key operations in various parts of the body and in doing so they tend to first become incorporated into the membranes of cells at the point of action. Among these specialized fatty acids are omega-3s. The plasma membrane of our cells contain molecules somewhat similar to triglycerides, called phospholipids. Phospholipids provide the water-insoluble barrier component to the membrane and may contain special fatty acids which can be detached and modified. Modified versions of these special fatty acids can be released from a cell and have an effect on neighboring cells. These special fatty acid-derived substances are an extraordinary class of molecules called the eicosanoids.
