LDL is called “bad cholesterol”; HDL “good.” But LDL and HDL are not good or bad, and they are not cholesterol. LDL (low density lipoprotein) and HDL (high density lipoprotein) are lipoproteins – vehicles that travel in the blood and deliver cholesterol to and from tissues in the body.
Lipoprotein blood transport came early in evolution – Birds and mosquitoes have LDL, they just don’t fret about it. There are two distinct lipoprotein systems: (A) gut-based system using a gut-assembled chylomicron to deliver dietary fat and cholesterol (exogenous lipids); and (B) liver-based system using VLDL to deliver liver-made fat (endogenous lipids) and cholesterol.
Liver-made VLDL – Very Low Density Lipoprotein – is identified by its protein marker apolipoprotein B-100 (apoB-100). The apoB-100 protein serves as the chassis of the lipoprotein and identifies VLDL as liver-assembled fat and cholesterol to cell surface receptors throughout the body.
Excess dietary carbohydrates are converted in the liver into fat. These liver-made-fats or triglycerides are combined with cholesterol and assembled into VLDL. VLDL production in the liver is very complex and can go on for several hours after a carbohydrate-rich meal. More than anything else, the triglyceride number (TG) in your lipid panel (blood work) reflects how much carbohydrate you are eating.
VLDL and LDL are a continuum
The more carbs you eat, the more triglycerides your liver makes, the more VLDL is required to transport the fat out to the body. As VLDL circulates in the bloodstream shedding its fatty bounty, it morphs into LDL – also identified as apoB-100 – now delivering mostly cholesterol.
LDL is the metabolic offspring of fat-rich VLDL. A key point: Carbohydrate intake – not dietary fat – determines how much VLDL and triglycerides your body makes – and elevated triglycerides (TG) are potent risk factors for heart disease.
Regardless of how much VLDL is assembled in the liver, LDL levels are independently regulated. As an example, you can have elevated VLDL and low LDL. But – and this is crucial – the longer it takes VLDL to circulate and unload triglycerides, the more likely its LDL offspring will morph into a small, dense particle.
There are several subclasses of LDL – different size particles. The small, dense particles are associated with elevated triglycerides and increased risk of heart disease. The greater the VLDL and triglyceride number, the more likely that LDL – whether low, medium or high – will be the small, dense Pattern B variety.
Your LDL could be below 130 mg/dl – what your doctor calls a safe level – but if your triglycerides are elevated, your LDL will be the small, dense particle associated with risk of heart disease. The take-away lesson: Protect yourself from heart disease – elevated VLDL, elevated triglycerides, and small dense LDL – by restricting your carbohydrates to a level appropriate for your metabolism – usually less than 60 grams per day.