Nutrition Myths
Digestion, absorption and metabolism of triglycerides

SUMMARY

  • All triglycerides are digested similarly, regardless of the length of the fatty acids
  • Medium chain fatty acids passively diffuse through the cell membranes and get rapidly absorbed to the liver via the portal vein.
  • Long chain fatty acids need to be detached from the glycerol package to be able to pass through the intestinal wall. After absorption, they are sent to the adipose tissue via the lymphatic and blood systems.
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Digestion, absorption and metabolism of triglycerides

This article addresses the different fate of long and medium chain fatty acids in the body.

It is a reference point to articles discussing the benefits of medium chain fatty acids such as MCT Oil, coconut oil, palm kernel oil and lauric acid.

Please note that:

Long chain fatty acids (LCFAs): >12 carbon length

Medium chain fatty acids (MCFAs): 6-12 carbons

Digestion of triglycerides

The digestion rate of triglycerides in the mouth and stomach appears to be the same, regardless of their type.

The digestion of fatty acids starts in the mouth, and then continues in the stomach with the help of enzymes called lipases.

Only when these molecules reach the intestine, does the digestion of MCTs and LCTs starts to vary.

  • Long chain fatty acids

    The presence of LCFAs in the intestine stimulates the release of bile and pancreatic lipase which help to break it down even further to be ready for absorption by the intestine cells. (1)

    However, before LCTs are absorbed they are broken down, separating fatty acids from the glycerol backbone.

    The detached fatty acids form micelles (transport mechanisms) that approach the microvilli in the intestine.

    At the surface of the intestinal wall, monoglycerides and fatty acids get absorbed.

  • Medium chain fatty acids

    MCFAs don’t stimulate the release of bile and pancreatic lipase and can passively cross the intestinal wall as quick as glucose does. (2, 3)

MAIN POINTS

  • The rate of digestion of triglycerides is similar, regardless of the length of the fatty acids
  • The absorption process starts to differentiate, once the triglycerides get close to the intestinal microvilli.

General behavior of triglycerides after digestion

A smaller number of carbon atoms in a fatty acid generally means that: (4)

  • Its molecule is shorter;
  • Its molecular mass is smaller;
  • It has a higher solubility in an aqueous environment and, therefore, it is easier to be transported in blood or in and out of cells;
  • It is easier to ionize at neutral pH and the more ionized MCFAs are, the easier they dissolve in blood (5);
  • It is easier to pass through the cell membranes;
  • Its transport and metabolic processing is quick;
  • It has a higher priority to travel via the portal vein.

With an increasing number of atoms, the absorption, transport and metabolism of the fatty acids slows down and involves more processes and binding proteins.

Diet also influences the distribution of medium chain fatty acids to either the portal vein or the lymphatic system. When a high dose of medium chain triglycerides is applied, their proportion sent via the lymphatic system increases.

MAIN POINTS

  • Less carbons in fatty acid chain means quicker absorption and transport and less processing
  • The absorption and metabolism of fatty acids also depends on the diet

Absorption and metabolism of long chain triglycerides

After the absorption through the intestinal wall, most long chain fatty acids get attached to glycerides to form triglycerides and get packed to chylomicrons (transport vehicles).

They then travel through the lymphatic system to the blood stream.

Once in the blood flow, most long chain triglycerides are ready to be absorbed by fatty cells and stored as body fat.

The metabolism of long chain triglycerides is diet dependent. (6)

If the diet is rich in carbohydrates, more triglycerides get absorbed in the fatty cells while glucose becomes the preferred energy source for the body cells. (7, 8)

If the diet is very low in carbohydrates (as in the ketogenic diet), the metabolism of long chain triglycerides is altered, since there is insufficient energy coming from glucose. In this case, the body starts producing ketone bodies as the preferred energy source. The fatty cells release more triglycerides for energy usage.

This explains why this type of metabolism is the most efficient way to lose weight.

MAIN POINTS

  • Most long chain fatty acids are packaged into chylomicrons, sent through the lymphatic system and deposited in the fatty tissue.
  • The metabolism rate of long chain fatty acids depends primarily on whether the diet is high in fat or high in carbohydrates.

Absorption and metabolism of C6-C10 medium chain fatty acids

Medium chain triglycerides don’t form micelles and don’t need bile acid emulsification or proteins for transport.

  • Distribution channels of medium chain fatty acids to the portal vein or the lymphatic system

    This distribution depends on the length and chemical properties of fatty acid chains and on the diet and composition of fats in each meal.

    Generally, most medium chain fatty acids of 10 carbons or less are rapidly transferred from the intestine, by passive diffusion, and to the liver via the portal vein, with a small percentage transferred to the fat cells.  

  • Energy production

    In the liver, medium chain fatty acids get rapidly converted into ketone bodies which are used by the brain, heart muscles and other organs as a source of energy.

    The shorter the carbon chains, the more efficiently the ketone bodies are produced. For instance, one study found that the number of ketones detected after one hour of administration of 8C is more than double than in the case of 10C.

MAIN POINTS

  • Most MCTs are immediately absorbed from the intestine and transported to the liver where they are rapidly converted into energy.
  • Shorter medium chain fatty acids are more efficient in producing ketone bodies than longer ones.

Absorption and metabolism of lauric acid (C12)

Although lauric acid belongs to the medium chain fatty acid group, it is sometimes referred to as long chain. This is the reason why I included it under a separate subtitle.

The confusion comes from a small number of studies which show that under certain conditions, a large part of the lauric acid can be sent through the lymphatic system just like long chain fatty acids.

It is important to note that what primarily differentiates lauric acid from long chain fatty acids, is its ability to pass through the portal vein.

However, when diet and the intake of medium chain triglycerides are altered, the distribution of medium chain fatty acids changes. This explains why lauric acid (the longest of the MCFAs) sometimes is redirected to the lymphatic system. (4)

MAIN POINTS

  • Under normal conditions, most lauric acid is sent via the portal vein to the liver, just like other medium chain fatty acids.
  • When unusually large amounts of medium chain fatty acids are used, a large proportion of lauric acid is sent via the lymphatic system, just like long chain fatty acids.

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