Beth talks blood: part 6

Let’s talk about fat…

Fats that circulate in the blood are called lipids (fat-soluble components of living cells). Cholesterol and triglycerides are both lipids that have essential roles in the body but in excess they can be harmful [1-3]. 

What is cholesterol?

Cholesterol is required to build cell walls and to produce hormones and vitamin D. Some of our cholesterol comes from our diet but most of our cholesterol is made in the liver. When cholesterol is broken down it is used to make bile acids which helps us to digest food [3].

What are triglycerides?

To answer that we need to take a look at fatty acids. A fatty acid is an important component of lipids. They are the building blocks of fat in our bodies and in the food that we eat. During digestion the body breaks down fats into fatty acids which can then be absorbed into the blood [1-3].

A fatty acid consists of a straight chain of an even number of carbon atoms, with hydrogen atoms along the length of the chain and at one end of the chain, with a carboxyl group (-COOH) at the other end. The carboxyl group is what makes it an acid (i.e. carboxylic acid). Fatty acid molecules are usually joined together in groups of three, forming a molecule known as a triglyceride [2].

Triglycerides are the fats found in our food. During digestion any fat in our food is absorbed by our gut. It is then moved around the body in the blood as triglycerides. The liver also makes triglycerides, and they provide the energy that is needed for our muscles and organs, they also help to keep us warm [3].

Lipoproteins: 

Lipids such as cholesterol and triglycerides cannot circulate loosely in the blood, so they need to be packaged in lipoproteins to be able to travel in the blood. Lipoproteins contain a mix of fats and proteins which allow them to circulate in the blood [3]. 

There are 4 main lipoproteins [3-5]:
1. Chylomicrons

2. Very low density lipoproteins (VLDL)

3. Low density lipoproteins (LDL)

4. High density lipoproteins (HDL)


Chylomicrons:

These are the largest lipoprotein and they carry triglycerides from the intestine to the tissues where they are required as a source of energy [3, 5].

VLDLs:

These transport mainly triglycerides made by the liver to where they are used by our muscles or stored for later use [3, 5].  

LDLs vs HDLs:

LDLs are known as “bad cholesterol” whereas HDLs are known as “good cholesterol”. LDL carries most of the cholesterol in our body from the liver to our cells that need it, whereas HDL plays a vital role in removing excess cholesterol away from cells and artery walls, back to the liver for disposal and recycling. When LDL gets into the walls of arteries, this can cause atherosclerosis which forms a blood vessel disease that can lead to heart attacks or stroke, meaning it is a major risk factor in cardiovascular disease [3-5].

Fatty acids & platelets:

Excess fatty acids can exert a profound effect upon blood clotting and can produce hypercoagulability of the blood leading to thrombosis (platelet aggregates formed within the thrombi) [6].

LDL attaches to a protein on the surface of platelets called CD36 which causes platelets to become “stickier”. When this happens, platelets release chemicals that cause inflammation and can damage the walls of the blood vessels, creating hardened areas in the blood vessel called plaques, which can lead to cardiovascular events [7].

Hyperlipidemia:

Hyperlipidemia means your blood has too many lipids e.g. cholesterol and triglycerides. A specific type of hyperlipidemia called hypercholesterolemia means that you have too much LDL (bad) cholesterol and not enough HDL (good) cholesterol in your blood [8].

Hyperlipidemia primes platelets and increases platelet activation in response to various agonists. Plasma cholesterol levels appear to have a critical role in modulating platelet activity. Oxidised LDLs or oxidised phospholipids, which are increased in hyperlipidemia, serve as ligands of platelet CD36 and activate platelets [9]. Oxidised LDL can be a harmful type of cholesterol that is produced in our bodies when normal LDL is damaged by chemical interactions with free radicals. Free radicals are unstable molecules that a produced as result of normal metabolism, a disease or exposure to certain toxins. Free radicals cause oxidation which de-stabilises molecules such as LDL. Along with inflammatory responses, these free radicals can result in the hardening of arteries known as atherosclerosis. The interaction between (ox)LDL and platelets play an important role in the pathogenesis of atherosclerosis, which can lead to cardiovascular events such as heart attack or stroke [10-11]. 

As you can see excessive fatty acids can cause havoc on our cardiovascular system which can lead to cardiovascular disease. It’s most likely to be a multifactorial process with interactions between LDLs, platelets, blood vessel walls, inflammation and clotting factors, that all contribute to the development of cardiovascular disease.

Developing our understanding of the cardiovascular system and what goes wrong in incidences of cardiovascular disease will help us to provide better, preventative measures and/or more targeted treatments. 

Thank you for reading, I hope you enjoyed this post!

See you in the next one.

Beth x

References:

[1] Encyclopedia Britannica. 2020. Fatty Acid | Definition, Structure, Functions, Properties, & Examples. [online] Available at: <https://www.britannica.com/science/fatty-acid&gt;

[2] Kidshealth.org. 2020. Definition: Fatty Acids (For Parents) – Nemours Kidshealth. [online] Available at: <https://kidshealth.org/en/parents/fatty-acids.html&gt;

[3] Heartuk.org.uk. 2020. [online] Available at: <https://www.heartuk.org.uk/downloads/health-professionals/publications/blood-fats-explained.pdf&gt;

[4] Cox RA, García-Palmieri MR. Cholesterol, Triglycerides, and Associated Lipoproteins. In: Walker HK, Hall WD, Hurst JW, editors. Clinical Methods: The History, Physical, and Laboratory Examinations. 3rd edition. Boston: Butterworths; 1990. Chapter 31. Available from: https://www.ncbi.nlm.nih.gov/books/NBK351/ 

[5] Siri-Tarino, P., Sun, Q., Hu, F. and Krauss, R., 2010. Saturated Fatty Acids and Risk of Coronary Heart Disease: Modulation by Replacement Nutrients. Current Atherosclerosis Reports, 12(6), pp.384-390.

[6] HOOK, J., WARNER, E. and CONNOR, W., 1967. Platelets, Fatty Acids and Thrombosis. Circulation Research, 20(1), pp.11-17.

[7] Bhf.org.uk. 2020. How Do Cholesterol And Platelets Team Up To Cause Blood Vessel Damage?. [online] Available at: <https://www.bhf.org.uk/research-projects/characterising-the-thromboinflammatory-roles-of-platelet-cd36&gt;

[8] http://www.heart.org. 2020. Prevention And Treatment Of High Cholesterol (Hyperlipidemia). [online] Available at: <https://www.heart.org/en/health-topics/cholesterol/prevention-and-treatment-of-high-cholesterol-hyperlipidemia&gt;.

[9] Wang, N. and Tall, A., 2016. Cholesterol in platelet biogenesis and activation. Blood, 127(16), pp.1949-1953.

[10] Verywell Health. 2020. Understand How Oxidized LDL Cholesterol Affects The Body. [online] Available at: <https://www.verywellhealth.com/what-is-oxidized-ldl-698079&gt; [Accessed 28 November 2020].

[11] http://www.heart.org. 2020. Atherosclerosis. [online] Available at: <https://www.heart.org/en/health-topics/cholesterol/about-cholesterol/atherosclerosis&gt;

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