How Lipids are Digested

Lipid breakdown starts in the mouth with the action of lingual lipases which is secreted by tongue cells. The majority of lipids will reach the stomach intact. In the stomach, lipids are mixed and dispersed into smaller droplets and degraded further by gastric lipases. Final digestion takes place in the small intestine with pancreatic lipases breaking down triglycerides, phospholipids and cholesterol esters to smaller components. For example, triglycerides are converted to monoglycerides and free fatty acids.

Bile salts from the gall bladder aid in distribution of lipids for greater lipase-lipid interaction and digestion. These bile salts consists of phospholipids which are amphiphilic, meaning that they are both hydrophilic (water-loving) and hydrophobic (water-fearing). This characteristic allows them to form a micelle around the lipids. In this form they can be transported to the surface of the intestinal absorptive cells, called enterocytes, where they can be absorbed into the cells.

Within the enterocytes, lipids take one of two pathways. If they are short-chained (<14 carbons long), they will be absorbed directly into blood vessels which takes lipids to tissue cells via the liver. This is possible due to the ability of short chain fatty acids (SCFA) to bind with albumin and phospholipids. Alternately, if the fatty acids are long (>14 carbons) they enter the endoplasmic reticulum of the enterocytes where they are modified to triglycerides and packaged as chylomicrons. Chylomicrons are lipoproteins. Lipoproteins are spherical compounds packed with lipids on the hydrophobic interior, while at the surface are the hydrophilic phosphate groups and protein. The chylomicrons are then absorbed into the lacteals (part of the lymphatic system) which transport lipids into the blood via the left subclavian vein. Once in the circulatory system they may be taken up by cells to make energy or to make lipid-containing compounds. Alternately, lipids may be stored in adipose cells for future use.

Function of Lipids

Lipids provide us with energy; in fact more energy than any other macronutrient. They make up our cell walls and the majority of our nervous tissue. They absorb fat-soluble vitamins, absorb flavor to make our food taste better and modifies food texture to produce improved mouthfeel. In addition, lipids carry out many metabolic functions in the form of essential fatty acids. These include blood clotting, reducing inflammation, preventing DNA damage and enabling normal growth of fetus. 

How Much Lipids do We Need?

Based on the DRI, the acceptable macronutrient distribution range (AMDR) for lipids is 20-35%. That means, of all the calories that you consume for the day, 20-35% of it should be in the form of lipids. Keep in mind that not all lipids are healthy. Therefore, you should reduce your saturated fat intake and eliminate trans fats from your diet. Remember that these contribute to heart disease. Also, reduce cholesterol since your body makes as much as you need. High cholesterol may also contribute to heart disease. Also, for good overall health, make sure to eat adequate amounts of essential fatty acids. 

A Note on Lipoproteins 

Note that there are three other types of lipoproteins other than chylomicrons. They are classified based on their density. Since lipids have a low density, the higher the percentage of lipids in the lipoprotein, the lower its density. the three other lipoproteins are:

1. VLDL (Very low density lipoprotein): Used to transport lipids that are made in the body (endogenous lipids) to various body tissues.
2. LDL (Low density lipoprotein): Used to transport cholesterol to the cells of the body. They are also called bad cholesterol.
3. HDL (High density lipoproteins): Used to transport cholesterol from tissues back to the liver. They are also called good cholesterol.

Health targets for blood lipids are:

1. Total cholesterol: <200 mg/dl
2. LDL: <130 mg/dl
3. HDL: >40 mg/dl
4. Triglycerides: < 150 mg/dl

Reference: Thompson, J. L., Manore, M. M.  & Vaughan, L. A. (2017). The Science of Nutrition (4th ed.). New York City, NY: Pearson Education.