Just like other types of fats, triglycerides are carried in the bloodstream by lipoproteins. Chylomicron and VLDL (very low-density lipoprotein) er examples of triglyceride-rich lipoproteins.
Chylomicron and VLDL remnants contribute to the building of plaques in our arteries (atherosclerosis) and are therefore considered atherogenic.
The fats we consume in our diet are mainly composed of triglycerides. Therefore, following a fatty meal, blood levels of triglycerides will rise.
It is important to understand that raised blood triglycerides following a meal (postprandial hypertriglyceridemia) are caused by chylomicrons while elevated fasting levels are due to high VLDL, often caused by excessive carbohydrate intake. Both these types of hypertriglyceridemia may contribute to atherosclerosis.
Chylomicrons
Chylomicrons are formed in the intestine after a meal. They contain triglycerides and small amounts of cholesterol. They are initially exported to intestinal lymph nodes and after that reach the circulation where they may contribute to postprandial hypertriglyceridemia.
An enzyme called lipoprotein lipase subsequently breaks down chylomicrons into free fatty acids that are used as fuel by the heart and skeletal muscles or stored in adipose (fat) tissue.
Chylomicron remnants are removed from the circulation, partly through the LDL-receptor.
VLDL
VLDL is synthesized by the liver. It contains both triglycerides and cholesterol.
Once in circulation, VLDL comes in contact with lipoprotein lipase in the capillary beds. Lipoprotein lipase catalyzes the breakdown of VLDL, releasing triglycerides for energy production or storage in adipose tissue.
After the removal of triglycerides from VLDL, the composition of the lipoprotein changes and it becomes intermediate-density lipoprotein (IDL). Later, when the amount of cholesterol increases, IDL becomes low-density lipoprotein (LDL). These LDL particles are typically small. Small LDL particles are associated with increased risk of cardiovascular events (3).
Inflammation
Both chylomicrons and VLDL promote inflammation.
High levels of chylomicrons increase the risk of acute pancreatitis, an inflammation of the pancreas. Chylomicron and VLDL remnants increase inflammation of the endothelium (the innermost layer of the artery).
Triglyceride rich lipoproteins increase atherosclerosis, inflammation and the lipid content of arterial plaques (4).
VLDL Cholesterol and Atherosclerosis
Non-HDL cholesterol is a measure of the cholesterol content transported in all atherogenic lipoproteins. It is calculated by subtracting HDL cholesterol from the total cholesterol. Non-HDL cholesterol is a greater predictor of cardiovascular risk than LDL cholesterol.
High triglycerides are usually accompanied by elevated levels of non-HDL cholesterol, small LDL particle size, and high LDL particle concentration.
VLDL cholesterol can be calculated by subtracting directly measured LDL cholesterol from non-HDL cholesterol. Recent data suggests that VLDL cholesterol or remnant cholesterol is a stronger promoter of atherosclerosis than LDL cholesterol (5).
Chylomicron remnants and VLDL remnants have been shown to rapidly penetrate the arterial wall and promote atherosclerosis (6). Triglycerides and Coronary Heart Disease
Triglycerides and Coronary Heart Disease
Despite evidence that chylomicron and VLDL remnants promote atherosclerosis, the importance of lowering triglycerides has not been considered a priority for patients with coronary heart disease.
Recent genetic studies have addressed the relationship between triglyceride-rich lipoproteins and the risk of cardiovascular events. The results of one of these studies imply that the elevated cholesterol content of triglyceride-rich lipoprotein particles may cause coronary heart disease (7). Another study found that a genetic mutation associated with low blood levels of triglycerides was associated with less risk of cardiovascular disease (8).
These studies strongly support the hypothesis that high blood levels of triglycerides may increase the risk of cardiovascular disease and that low levels may be protective.
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