Macronutrients 101: Cholesterol

In my previous posts on lipids, I have mentioned cholesterol. I thought it’d be important to dedicate a whole post to cholesterol because a lot of people don’t have a solid understanding of what it is. I hear people talking about cholesterol and LDL, using the two terms interchangeably, or automatically assuming that cholesterol clogs arteries.  Well let’s dispel some of these myths 😉

Cholesterol is a macronutrient required for metabolism and cell structure in animals but not plants. Hence, only animal food products contain cholesterol. The four ring steroid structure of cholesterol makes it insoluble in water and is the reason it’s considered to be a fat. However, it is structurally and functionally different from triglycerides.

In our society, cholesterol has a horrible reputation as people fail to understand that it is absolutely essential for our bodies. In fact, our cells are capable of making cholesterol out of acetyl-CoA, a component of glucose metabolism. This same building block is involved in fatty acid synthesis but the two pathways involve two different sets of enzymes. Now before anyone gets excited, cholesterol synthesis is a tightly regulated process and glucose doesn’t magically turn into unnecessary cholesterol.

Well then, what does cholesterol do?

As I’ve previously explained, the cell membrane is made up of phospholipids. In order to make phospholipid bilayers more fluid, there are cholesterol molecules embedded within it. This allows for various protein channels to be found in the membrane which can transport other proteins or molecules in or out of the cell.

Not only is cholesterol found in cell membranes but it also acts as the starting material for manufacturing various hormones. These hormones include sex hormones (estrogen, progesterone, testosterone), cortisol, and aldosterone. Note how all of these hormones have the same four ring steroid backbone.

Furthermore, cholesterol is a precursor for vitamin D and is used by the liver to make bile acids. These bile acids are crucial for helping us digest fats.

As with almost anything health related, balance is key. Although cholesterol is vital for our bodies, too much of it in the bloodstream has profound health consequences. Hence, our bodies have devised sophisticated strategies to keep plasma cholesterol levels intact. These mechanisms include:

* Synthesizing new cholesterol

* Reducing cholesterol absorption in the intestines

* Pumping cholesterol into or out of a cell

* Transporting cholesterol to the intestines for excretion

Now you may wondering, what LDL and HDL are? Low-density and high-density lipoproteins are carriers of cholesterol. Remember that cholesterol is insoluble in water and since our blood is water based, cholesterol needs a transportation shuttle. LDL is what transports cholesterol to cells that require it. Alternatively, HDL is where cholesterol is packaged for excretion. In this way you can think of LDL as “bad” since it raises blood cholesterol levels while HDL is “good” because it remove cholesterol from the body.

ldl vs hdl

Does cholesterol cause heart disease? Yes, in the bloodstream LDL oxidation triggers an immune response that leads to plaque formation.

Does dietary cholesterol cause heart disease? I wish I had a concrete answer… The thing is that dietary cholesterol absorption is complex and there’re high interpersonal variations in percentage of dietary cholesterol absorbed. By no means would I recommend someone to eat foods high in cholesterol. Personally, I think more research needs to be done and this topic deserves a follow up in-depth post of its own 😉

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