Basic Metabolism of Fats

by Catherine Haug, December 17, 2010

NOTE: This overview is another post in the Cat’s Fat’s series.


Metabolism is the process used by the body to keep it functioning, and fats are an essential part of this function.

In addition to being a part of our diet, many fats can be made from carbohydrates or proteins in the liver. However, the essential fats (Omega-3 and Omega-6) cannot be made in our liver and must be obtained in our diet.

The process of digestion, absorption & metabolism of fats is not the same for all types of fatty acids. And while all fats can be used for energy production (just as carbs and proteins) they are also used for other purposes.

Excess fats are stored primarily in adipose tissue (fat cells), but may also be stored in the liver, for future use. Furthermore, excess dietary carbohydrates are converted to fats.

This article addresses:

  • Short-chain (SCFA) and medium-chain (MCFA) fatty acids
  • Long-chain fatty acids (LFCA)
  • Fats and energy production
  • Conversion of carbs to body fat

See also The Medical Biochemistry Page on Carbohydrate Metabolism (5) and Fat Metabolism(6) for much more detail.

Short- & medium-chain fatty acids

Short- & medium-chain fatty acids (SCFAs & MCFAs) are all saturated, and are slightly polar (slightly charged) so can form a suspension in watery fluids. An abundance of SCFAs in the bowel has been associated with reduced risk of (4):

  • irritable bowel syndrome;
  • inflammatory bowel disease,
  • cardiovascular disease, and
  • cancer.

NOTE: SCFTs are also produced in the bowel by probiotic digestion of dietary fiber. These fatty acids are then metabolized in the same manner as dietary SCFTs.

Contrary to popular belief concerning all saturated fats, recent research has shown that MCFAs affect control over adipose (fat) tissue to reduce fat mass, body weight and particularly body fat. (1)

Digestion & absorption of SCFAs & MCFAs

The first step in digesting fats is to strip (hydrolyze) the individual fatty acids from the glycerol backbone, in an enzyme-driven process called ‘lipolysis.’ For short- and medium-chain fatty acids, the presence of bile is not required.

Once lipolysis is complete, these relatively small and slightly polar molecules are readily absorbed through the intestinal wall into the hepatic portal vein for transport to the liver, where they are a preferred source of energy. This is a passive process, meaning that they do not require the assistance of proteins and cholesterol for transport across the intestinal wall.

Metabolism of SCFAs & MCFAs

If SCFAs and MCFAs are not being used for energy production in the liver, they can be actively transported from the liver to muscle and other cells for energy production within the mitochondria.(3)  This transport involves bundling with longer chain fatty acids, cholesterol and protein as triglyceride particles (TGs), LDL or HDL cholesterol for transport in the blood.

Long chain fatty acids

Long-chain fatty acids (LCFAs) can be saturated, mono-unsaturated or  poly-unsaturated, and comprised of 14 or more carbons in the chain. They can also be branched-chain.

Digestion & absorption of LCFAs

All long chain fatty acids require bile salts from the liver for lipolysis, which is the first step of their digestion.

Once freed from the glycerol backbone, they are too large to move through the intestinal wall on their own. Instead, they must first be coupled with special proteins and cholesterol as chylomicrons (a type of cholesterol particle) for active transport through the intestinal wall, then carried to all the cells of the body for metabolic use.

Metabolism of LCFAs

While LCFTs can be used for energy production in cells, they are also an integral part of cell and mitochondrial membranes, where there is a high demand for them. Fatty acids in the membrane are in constant state of flux, so that a steady supply of replacements is always needed. Saturated LCFAs provide stability for the membrane; unsaturated LCFAs provide flexibility. And all types are involved in communication between the inside of the cell and the cell’s surroundings.

Poly-unsaturated LCFAs in cell membranes are used to produce prostaglandins, which are localized hormones that affect the immediate surroundings of the cell or the cell itself, in response to stress triggers such as: trauma, virus, bacteria, heavy metals, free-radical or glycation products and other toxins. In this role, LCFAs are an important part of our immune system.

Generally, prostaglandins made from omega-3 fatty acids fight inflammation, while those made from omega-6 fatty acids promote inflammation; but there are exceptions to this generalization. Both inflammatory and anti-inflammatory processes are important for healing.

It is important to note that man-made poly-unsaturated fats, such as those made by partial hydrogenation of vegetable oils to make margarine or shortening, cannot form prostaglandins and are always pro-inflammatory.

Fats and energy production

As mentioned above, fats can be used for energy production in the cells.

  • SCFAs and MCFAs produce energy in the liver, to support it’s detox function.
  • LCFAs are transported in the blood to cells in other parts of the body (primarily muscles) for energy production via the TCA cycle (Tricarboxylic Acid cycle). This is an enzyme driven process that produces energy in the form of ATP (Adenosine Triphosphate), and used by the cell to keep it alive and doing what it is supposed to do.

For some people, fats are a preferred source of muscle energy; for others, carbs are the preferred source. This preference depends upon whether the parasympathetic (7) or Sympathetic (7) nervous system is dominant.

Fats can also be used by cells for cell and mitochondrial membranes, and production of prostaglandins (a type of hormone). Or for insulation of nerve tissue.

Conversion of Carbs to Body Fat

Body fat

Contrary to general belief, dietary fats are not the major contributor to body fat. Rather it is excess dietary carbohydrates that are the primary cause of belly fat accumulation. Excess dietary fats are far more likely to be used for other purposes, such as incorporation into cell membranes, or to form insulation for nervous tissue.

Review of carbohydrates

  • Simple Sugars: The primary carb metabolized in the body to produce energy, such as for muscle or brain activity, is glucose, a simple sugar (mono-saccharide). Fructose is another simple sugar, but is metabolized only in the liver. Other simple sugars can also be metabolized, but are not discussed here.
  • Di-saccharides must first be broken down into their constituent simple sugars before they can be metabolized; for example, sucrose (table sugar) is broken down into one molecule of glucose and fructose.
  • Starches are broken down by digestive enzymes into individual molecules of glucose.
  • Fibers: We don’t have the enzymes to break down fibers so they do not produce simple sugars. Instead they are either excreted, or converted to short chain fatty acids (acetic, propionic, lactic, malic, and glucuronic)  in the gut by the action of beneficial bacteria (probiotics), then carried to the liver where they are the primary source of energy for detox functions.

See Medical Biochemistry Page on Carbohydrate Metabolism and Non-glucose Sugar Metabolism, for more. (5)

Metabolism of glucose

Glucose is readily absorbed through the intestinal wall into the blood, and then carried to the cells of the body, primarily muscle and brain cells, where it is burned for energy.

But before it can be burned, it must be taken up by the cell, a process that requires insulin, insulin receptors, and active-transport molecules, because the cell membrane is not friendly to glucose on its own. Once in the cell, glucose enters the  glycolytic pathway and TCA cycle (Tricarboxylic Acid cycle), enzyme processes that produce energy in the form of ATP (Adenosine Triphosphate).

If the cells already have enough ATP, glucose is converted into glycogen (a type of starch) for temporary storage in the cell. The glycogen can be converted back to glucose, when more energy is needed.

However, when the cells have enough glucose/glycogen, the insulin receptors on the cell membrane are turned off, so that no more glucose can be taken into the cell. This results in a buildup of glucose in the blood, which must be dealt with to avoid catastrophic problems such as coma.

Storage of excess blood sugar

Excess glucose in the blood is taken to the liver where it may be converted to glycogen for temporary storage. However, if the liver has enough glycogen, the glucose is converted into saturated fatty acids which are incorporated into triglycerides (fat). These are then transported by cholesterol particles (LDL and TGs) to the fat cells of the body, primarily those in the belly.

Metabolism of Fructose

Fructose metabolism is distinctly different from that of glucose. It does not invoke the insulin response but rather is taken directly to the liver after absorption from the gut. In the liver it is converted into glucose. (5)

While the glucose can be burned in the liver for energy, the liver prefers to burn short chain fatty acids. Instead, it is either:

  • Converted into glycogen for storage in the liver cells; or
  • Into saturated fatty acids, which are incorporated into triglycerides (fat), then transported via cholesterol particles (LDL and TGs) to fat cells, primarily in the belly. (5)

The latter happens when the liver has enough stored glycogen from excess glucose as well as fructose.

Modern diets are much higher in fructose than traditional diets because of the use of high fructose corn syrup and agave nectar in our foods. These are a leading cause of belly fat and obesity.


  1. Medium-Chain Triglycerides; a Review:
  2. What is Life: Fatty Acids, an overview:
  3. PubMed, Medium chain fatty acid metabolism and energy expenditure: obesity treatment implications:
  4. Topical Review: Short Chain Fatty Acids and Colonic Health, by E. Hijova & A. Chmelarova:
  5. The Medical Biochemistry Page: on Carbohydrate Metabolism: and on Non-Glucose Sugar Metabolism:
  6. The Medical Biochemistry Page: on Fat & Lipid Metabolism:
  7. Wikipedia on Sympathetic & Parasympathetic nervous systems:

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