Since cholesterol is a fat, it cannot be mixed with water (like oil and water don’t mix). Consequently, cholesterol cannot circulate freely in the blood. So, when in production, cholesterol has to be attached to specific proteins that act as ‘carriers’ throughout the bloodstream. After attaching cholesterol to a ‘carrier’ protein, we get lipoproteins.
Two popular lipoproteins are LDL (low-density lipoprotein), called ‘bad cholesterol’, and HDL (high-density lipoprotein), or ‘good cholesterol’. The liver makes a primary type of LDL and sends it off to the body, delivering fatty molecules to all tissues. This is healthy and necessary because we need fat in all cells. However, and this is when LDL becomes the bad guy, LDL tends to stay in the bloodstream. If there’s too much of it, fat starts to accumulate in the blood vessels, building up a plaque known as atherosclerosis.
This is a risk factor for many cardiovascular complications like strokes or heart attacks, so paying attention to LDL levels in the bloodstream is very important. On the other hand, HDL collects cholesterol in the bloodstream and takes it to the liver, where it is properly recycled. Since HDL keeps cholesterol from causing trouble to the cardiovascular system; it makes sense we see it as the good guy.