Cholesterol & the lipoprotein cycle

So a quick (and simplified) biology lesson for todays post....

Hopefully your on-board with the fact that cholesterol isn't actually the killer substance that certain people/organisations/Dr's/etc would have you believe, but did you know how important it actually is?

Cholesterol is required to build cell walls, it is essential for building many of our hormones , it is a key part of our immune system, and essential for tissue repair. Cholesterol literally holds your body together and it is so important that the majority of our cells have the ability to manufacture their own cholesterol. Whilst talking about cholesterol it is important to also understand what triglycerides and lipoproteins are as well.

Triglycerides are a fat based fuel source made of three fat molecules grouped into one glycerol (or sugar-like) molecule. Triglycerides can be consumed through fat containing food and they can be created in your liver.

Cholesterol and triglycerides cannot travel around the bloodstream by themselves, and this is where lipoproteins come in. These act like 'boats' to carry cholesterol and triglycerides around the body. Lipoproteins are hollow spheres within which cholesterol and triglycerides are packed for delivery to the brain and body tissues. Lipoproteins move freely within the blood and there will be millions moving around your body at this very moment on delivery and collection duties. The lipoproteins all have a protein signalling molecule in their outer shell (called a apolipoprotein) to identify themselves to the various 'docks' in the body. There are two main types of lipoproteins in your body, LDL and HDL, HDL is unique and there are a number of classes of LDL.

VLDL (Very Low Density Lipoprotein) is where LDL starts and it is the largest lipoprotein. It is created in the liver and ferries cholesterol & triglycerides around the body. Its main function is to deliver triglycerides as a fuel to skeletal and heart muscle.

LDL (Low Density Lipoprotein) is formed when the VLDL drops off its cargo and shrinks in size, some cholesterol & triglyceride remains inside LDL and VLDL is used for both delivery and return trips in the cholesterol delivery system.

sdLDL (Small dense LDL) is created when LDL becomes distorted in an inflammatory environment. Your immune system will identify sdLDL as un-unwelcome guest and mop it up (but when the system is broken/overwhelmed this doesn't always work). Also unlike LDL, sdLDL is less likely to return to the liver, so will remain in the blood stream for longer which increases the chance of it becoming damaged further.

Now I've simplifying things somewhat but essentially VLDL transports triglycerides to muscles for energy, and then after delivering the energy it becomes LDL which continues to transport triglycerides and also distributes cholesterol to tissues. LDL then returns to the liver where remaining cholesterol is recycled and re-used and the cycle repeats.

HDL (High Density Lipoprotein) is a bit different and is created in the gut, it has many important functions and acts as the manager of the cholesterol/triglyceride transport system. It manages the cholesterol and triglyceride content of LDL by swapping cholesterol & triglyceride molecules with LDL as required, protecting LDL from oxidation damage. It also delivers cholesterol to the few tissues that LDL can't reach including the gonads and adrenal glands AND removes cholesterol from where it shouldn't be (like arterial walls).

So cholesterol is essential for our bodies to grow/function/repair and we have evolved a complex system to manage the distribution of this important substance so why the bad rap? Well like any system, when abused it can become damaged and dysfunctional, and this is where problems can occur. You've probably heard the terms 'good' and 'bad' cholesterol, HDL is this 'good' guy and LDL the 'bad' guy. Now this isn't a fair description as both LDL and HDL are essential in the cycle but low levels of measured HDL and high levels of measured LDL are symptomatic of underlying problems.  When LDL becomes damaged they no longer get recycled properly by the liver and instead hang around in the blood a lot longer than they are supposed too. If not cleaned up by the immune system they can end up inside inflamed arterial walls and this can lead to arterial plaque, blockages, and eventually heart attacks. HDL will try and clean this up but if the system is damaged then it can only do so much.  I'll try and cover how this system gets broken in more detail in a future post (spoiler alert its not from eating animals!) but its important to note that blocked arteries and heart disease are symptoms of a dysfunctional system and not a result of eating too much cholesterol, furthermore cutting dietary cholesterol will likely only cause other problems.