What Happens to the Fat You Eat?
As with all nutrients, the digestion of fat begins in your mouth. Chewing mechanically breaks down the food-in this case, a slice of pizza.
The enzyme lingual lipase in the mouth plays a minor role in breaking down some fat. Once food is swallowed, the stomach begins to breaks down fat further. Let’s follow the fat in the cheese pizza through the rest of the GI tract.
You Digest Most Fat in Your Stomach and Small Intestine
In the stomach, fat mixes with gastric lipase, an enzyme that breaks down some of it into a fatty acid and a diglyceride (the remnant of fat digestion when only two fatty acids are left joined to the glycerol backbone). The majority of fat digestion occurs in your small intestine, where an enzyme released from your pancreas, pancreatic lipase, continues to break down the fat into two fatty acids and a monoglyceride (the remnant of fat digestion when only one fatty acid is left joined to the glycerol backbone).
Just as oil and water don’t mix, fat can’t mix with the watery fluids in your digestive tract. The fat globules tend to cluster together rather than disperse throughout the fluids. Mixing fats with watery fluids requires the addition of bile, which is made in your liver and stored in your gallbladder. When the fat from the pizza arrives in your intestines, your gallbladder releases bile. Bile contains bile acids that help to emulsify the fat into smaller globules within the watery digestive solution. This keeps the smaller fat globules dispersed throughout the fluids, and provides more surface area so that the pancreatic lipase can more easily break down the fat.
Monoglycerides and fatty acids are next packaged with lecithin, which is in the bile, and other substances to create micelles (small transport carriers). Once close to the mucosa of your small intestine, micelles travel through your intestinal cells. The length of the fatty acid chain determines what happens next. Short-chain fatty acids will enter your bloodstream and go directly to your liver. The long-chain fatty acids can’t enter your bloodstream directly. They enter your lymph and need transport carriers.
Lipoproteins Transport Fat through the Lymph and Blood
Long-chain fatty acids are reformulated into a fat within the wall of your intestines as they are absorbed. These reformulated fats (as well as other lipids, such as cholesterol) are not soluble in your watery blood. They need to be packaged inside protein containing carriers called lipoproteins. These capsule-shaped fat “carriers” have an outer shell high in protein and phospholipids and an inner compartment that carries the insoluble fat, as well as cholesterol, through your lymph and bloodstream. One example of a lipoprotein carrier that transports these lipids is a chylomicron.
Chylomicrons are too large to be absorbed directly into your bloodstream, so they travel through your lymph system first and then enter your blood. Once in the blood, the fat is broken down into fatty acids and glycerol with the help of the enzyme lipoprotein lipase, which is located in the walls of the capillaries. After the fat is removed from the chylomicrons, the remnants of these lipoproteins go to your liver to be dismantled.
The liver produces other lipoproteins with different roles in your body:
- Very low-density lipoprotein (VLDL)
- Low-density lipoprotein (LDL)
- High-density lipoprotein (HDL)
Although all lipoproteins contain fat, phospholipids, cholesterol, and protein, the proportion of the protein in these substances differs in the various types. Protein is denser than fat, so the proportion of protein in the lipoproteins determines their overall density.
For example, VLDLs are composed mostly of triglycerides and have very little protein, so they are considered to be of very low density. The LDLs, which are mostly made of cholesterol, have more protein than the VLDLs but less than HDLs, which have the highest density. The protein in the lipoproteins helps them to perform their functions in your body. For example, the high protein content in HDLs not only helps remove cholesterol from your cells, but also enables the carrier to expand and contract, depending on the amount of fat and cholesterol it is carrying.
Why is the proportion of protein in a lipoprotein carrier important? Each lipoprotein has a different role. The main role of the VLDLs is to deliver fat that is made in the liver to your tissues. Once the fat is delivered, the VLDL remnants are converted into LDLs. The LDLs deliver cholesterol to your cells and are often referred to as the “bad” cholesterol carriers because they deposit cholesterol in the walls of your arteries, which can lead to heart disease. To help you remember this, you may want to think of the “L” in LDL as being of “Little” health benefit.
The HDLs, as mentioned earlier, are mostly protein. They remove cholesterol from your cells and deliver it to your liver to be used to make bile or to be excreted from your body. For this reason, the HDLs are often referred to as the “good” cholesterol carriers, as they help remove cholesterol from your arteries. An easy way to remember this is to think of the “H” in HDL as referring to “Healthy”.
Message:
The digestion of fat begins in your mouth, and with the help of enzymes and emulsifying bile acids, most of it is digested and then absorbed in your small intestine. Fat is generally packaged as part of a chylomicron lipoprotein carrier, and travels in the lymph before entering your bloodstream. The other lipoproteins are VLDLs, LDLs, and HDLs. The VLDLs are converted to the “bad” LDL cholesterol carriers, which can deposit cholesterol in the walls of your arteries. The “good” HDL cholesterol carriers remove cholesterol from your arteries and deliver it to your liver to be excreted from your body.
Digesting and Absorbing Fat
1. Mouth – Chewing or mechanically breaking down fatty foods begins in your mouth.
2. Stomach – Fat digestion continues in the stomach with the aid of the enzyme gastric lipase.
3. Small intestine – Bile acids in bile secreted from the gallbladder help emulsify the fat into smaller globules, enabling pancreatic lipase to break it down more easily.
4. Absorption – By-products of fat digestion travel through your intestinal cells. Short-chain fatty acids enter your bloodstream directly. Long-chain fatty acids are reformulated into fats and need transport carriers, called chylomicrons, which travel in your lymph before entering your blood.
Terms:
Diglyceride – A glycerol with only two attached fatty acids.
Monoglyceride – A glycerol with only one attached fatty acid.
Bile – A secretion that’s squirted into the small intestine to emulsify fat into smaller globules, which allows enzymes to break the fat down. Bile is made in the liver and stored in the gallbladder
Micelles – Small transport carriers in the intestine that enable fatty acids and other compounds to be absorbed.
Lipoproteins – Capsule-shaped transport carriers that enable fat and cholesterol to travel through the lymph and blood.
Chylomicron – A type of lipoprotein that carries digested fat and other lipids through the lymph system into the blood
Very low-density lipoprotein (VLDL) – A lipoprotein that delivers fat made in the liver to the tissues. VLDL remnants are converted into LDLs.
Low-density lipoprotein (LDL) – A lipoprotein that deposits cholesterol in the walls of the arteries. Because this can lead to heart disease, LDL is referred to as the bad cholesterol carrier.
High-density lipoprotein (HDL) – A lipoprotein that removes cholesterol from the tissues and delivers it to the liver to be used as part of bile and/or to be excreted from the body. Because of this, it is known as the good cholesterol carrier.
