A Tale of Two Fats – the difference between structural fats and fat stores, and why we need both

By Professor Michael A. Crawford

structural fats in white fish Photo credit: avlxyz via Visualhunt / CC BY-SA
Oily fish and white fish – we need both. Photo credit: avlxyz via Visualhunt / CC BY-SA

When reading about the benefits of omega-3 fatty acid, you will invariably be told that fish oils are the best source. While this is true to a certain extent, it’s important to understand that fish oil is made up of triglyceride fats. The basis of the problem, and why white fish is usually pushed out of the equation, is that there are two types of fat in the human body – triglyceride and phosphoglyceride. They have different uses, and both are essential.

  • Triglycerides are for storage. This is the stuff that sits on our waistlines and can clog our arteries if we ingest too much of the wrong type. It is basically a storage system for energy, although it also stores fat soluble nutrients such as vitamin A and omega-3 DHA[1] and EPA[2]. Fish oil is a triglyceride.
  • Cod liver oil (triglyceride) contains 12% EPA and 8% DHA.
  • Phosphoglyceride is a structural fat.This is the building material for cells, forming their solid structures. Its fatty acid composition is relevant to cell function. White fish flesh is a phosphoglyceride.
  • Cod flesh phosphoglycerides contain 47% DHA and very little EPA – six times the amount of omega-3 DHA than a corresponding amount of oil from cod liver.

Unfortunately, there is a great deal of misunderstanding on this issue. People mainly think of the membranes as sheets of cellophane or oily barriers to water, which is an incomplete picture.

The role of structural fats

Phosphoglyceride, or structural fat, is the polyunsaturated part of the phospholipids that make up the cell membrane. It performs the following:

  • Packets the cell with a plasma membrane
  • Packets the DNA in the nucleus by surrounding it with a membrane
  • Does the same with the cell’s mitochondria, with the additional formation of its internal structures which organise its function as the furnace of the cell, providing its energy
  • Lines the blood vessels
  • Makes the myelin sheaths for nerve fibres
  • Provides for the uniquely sensitive signalling systems of the retina and brain.

Moreover, the composition at every point alters the function of membrane-bound proteins, in regard to transporting, signalling and many other things. In this way, the fatty acid composition of the membrane alters cell function, with a direct impact on health and disease. Its composition can be altered by changes in diet.

Structural vs Storage fats – a homely analogy

If you think of these fats as a house, the phospholipids are the external and internal structures – not just the walls, but the doors, windows, stairs, etc, that allow the building to function. The triglycerides are the coal or oil bunker, or gas supply, providing the energy to heat the house.

As with biology, the integrity of the house will depend on the quality of the materials used. For example, the right mixture of sand and cement is vital for the longevity of the walls, just as the right balance of omega-6 and -3 is needed for the wellbeing of the human body. The wall’s protective coat of paint is like the antioxidants (such as vitamin E) that protect the intimate structures of the cell – especially for things like DHA, which is highly susceptible to peroxidation.

What the Brain Needs

Lack of understanding, or even knowledge, is the basis of many misconceptions and badly designed trials on oils of one form or another. Triglyceride and phosphoglyceride are two quite different types of fat.

The brain is largely made of the latter – structural fat. Our arteries are lined with a plasma membrane which in an adult weighs around 1kg, with a total surface area about the size of a football pitch. The endothelial phosphoglycerides are rich in arachidonic acid arachidonic acid which is vital in providing prostacyclin to keep our arteries open and blood flowing. It is also responsible for various derivatives which respond to injury, recruiting immune cells and producing inflammation, and even resolving the damage after the immune cells have done their job. The brain’s neurones, synapses and sensors are especially rich in DHA.

If you live on a diet rich in omega-6 linoleic acid and poor in omega-3 you can expect lack of integrity of the endothelial membranes and arterial damage. You can then expect a persistent arachidonic response to injury which can impede circulation and pose a danger to the heart. EPA competes with arachidonic acid for the response to injury, dampening it down. That is one of the rationales for using fish oil or recommending a fish-based diet.

Given the central role played by omega-3 DHA in healthy brains, and given the high DHA content of white fish, it becomes apparent that both white and oily fish should play a key part in our diets.

[1] Docosahexaenoic acid – primary structural component of the human brain, cerebral cortex, skin, and retina.

[2] Eicosapentaenoic acid (aka icosapentaenoic acid) – polyunsaturated fatty acid (PUFA) that acts as a precursor for DHA, certain lipids and signalling molecules.