*Geek Box: Lipid Metabolism
The term ‘lipid’ is the technical term for fats in the body, which are taken in through diet or are produced in the body. Because fats don’t dissolve well in water, they need transport to be carried around the body through blood. The terminology ‘blood lipids’ is often misinterpreted to mean ‘blood cholesterol’, but in fact blood lipids include triglycerides, cholesterol, free fatty acids, and phospholipids [which provide structure to cells and are stored in cell membranes]. Triglycerides [also known as ‘triacylglycerol’ or TAG], are, formed of three fatty acids [‘tri’] bound to a glycerol [a sugar compound] backbone. TG’s are the main form of fat consumed through the diet, and the main form of storage fat in the body. Chylomicrons [CMs] are also important, as these are the vehicle to transport TGs absorbed from diet from intestinal cells.
‘Blood cholesterol’ may also be misleading terminology, as it in fact is referring to lipoproteins, which are complexes of proteins and lipids that transport both TG’s and cholesterol around the body. They are defined by density, which is a characteristic of the ratio of protein to lipid. The main lipoproteins are very- low-density lipoprotein [VLDL], low-density lipoprotein [LDL], high-density lipoprotein [HDL]. The higher the protein volume, the less capacity there is for TG transport. Thus, VLDL is larger and less dense and therefore has more capacity to transport TGs. Conversely, HDL has much greater protein density, and therefore not much capacity to carry TGs. VLDL operates to transport TGs newly produced in the liver, as opposed to CM transport of TGs from diet. The general role of LDL is to transport cholesterol from the liver to body tissues; HDL operates reverse transport of cholesterol from tissues to the liver. The relationships between the size and density of lipoproteins is a factor in the promotion of, or protection against, cardiovascular disease.
Lipid metabolism is a complex interaction of multiple factors, including the composition and quantity of fats in the diet, the composition and quantity of carbohydrates, the level of fat in a meal and in preceding/subsequent meals, and the site of stored fat in the body [visceral fat, and ectopic fat around the liver, being particularly problematic].
Generally, cardiovascular risk has been viewed through the lens of high total cholesterol and high LDL in particular. This is a well-established risk factor. The European Atherosclerosis Society Consensus Panel in 2017 confirmed that high LDL is a causative factor in atherosclerosis. However, we also understand CVD risk is more nuanced, and the relationships are not as simple as often portrayed.