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Blood lipids (orblood fats) arelipids in theblood, either free or bound to othermolecules. They are mostly transported in aphospholipid capsule, and the type of protein embedded in this outer shell determines the fate of the particle and its influence onmetabolism. Examples of these lipids includecholesterol andtriglycerides. The concentration of blood lipids depends on intake andexcretion from theintestine, and uptake andsecretion fromcells.Hyperlipidemia is the presence of elevated or abnormal levels oflipids and/orlipoproteins in theblood, and is a major risk factor forcardiovascular disease.
Short- andmedium chain fatty acids are absorbed directly into the blood viaintestine capillaries and travel through theportal vein.Long-chain fatty acids, on the other hand, are too large to be directly released into the tiny intestine capillaries. Instead they are coated with a membrane composed ofphospholipids andproteins, forming a large transporter particle calledchylomicron. The chylomicron enters alymphatic capillary, then it is transported into the bloodstream at the leftsubclavian vein (having bypassed the liver).
In any case, the concentration of blood fatty acids increase temporarily after a meal.
After a meal, when the blood concentration of fatty acids rises, there is an increase in uptake of fatty acids in different cells of the body, mainlyliver cells,adipocytes andmuscle cells. This uptake is stimulated byinsulin from thepancreas. As a result, the blood concentration of fatty acid stabilizes again after a meal.
After a meal, some of the fatty acids taken up by the liver is converted intovery low density lipoproteins (VLDL) and again secreted into the blood.[1]
In addition, when a long time has passed since the last meal, the concentration of fatty acids in the blood decreases, which triggersadipocytes to release stored fatty acids into the blood asfree fatty acids, in order to supply e.g. muscle cells with energy.
In any case, also the fatty acids secreted from cells are anew taken up by other cells in the body, until enteringfatty acid metabolism[clarification needed].
The fate of cholesterol in the blood is highly determined by its constitution oflipoproteins, where some types favour transport towards body tissues and others towards the liver for excretion into the intestines.
The 1987 report ofNational Cholesterol Education Program, Adult Treatment Panels suggest the totalblood cholesterol level should be: <200 mg/dl normal blood cholesterol, 200–239 mg/dl borderline-high, >240 mg/dl high cholesterol.[2]
The average amount ofblood cholesterol varies with age, typically rising gradually until one is about 60 years old. There appear to be seasonal variations in cholesterol levels in humans, more, on average, in winter.[3] These seasonal variations seem to be inversely linked tovitamin C intake.[4][5]
Inlipid digestion, cholesterol is packed intochylomicrons in thesmall intestine, which are delivered to theportal vein andlymph. The chylomicrons are ultimately taken up by liverhepatocytes via interaction betweenapolipoprotein E and theLDL receptor orlipoprotein receptor-related proteins.
Cholesterol is minimally soluble inwater; it cannot dissolve and travel in the water-based bloodstream. Instead, it is transported in the bloodstream bylipoproteins that are water-soluble and carry cholesterol andtriglycerides internally. Theapolipoproteins forming the surface of the given lipoprotein particle determine from what cells cholesterol will be removed and to where it will be supplied.
The largest lipoproteins, which primarily transport fats from theintestinalmucosa to theliver, are calledchylomicrons. They carry mostly fats in the form of triglycerides. In the liver, chylomicron particles release triglycerides and some cholesterol. The liver converts unburned food metabolites intovery low density lipoproteins (VLDL) and secretes them into plasma where they are converted to intermediate-density lipoproteins(IDL), which thereafter are converted tolow-density lipoprotein (LDL) particles and non-esterified fatty acids, which can affect other body cells. In healthy individuals, most of the LDL particles arelarge and buoyant (less dense, also known as lb-LDL) and they are cardiovascularly neutral: they have no negative and no positive effect on cardiovascular health. In contrast, large numbers ofsmall and dense LDL (sd-LDL) particles are strongly associated with the presence ofatheromatous disease within the arteries. For this reason, total LDL is referred to as "bad cholesterol," although only a fraction of it is actually bad.
Standard chemistry panels typically include total triglyceride, LDL and HDL levels in the blood. Measuring the concentration of sd-LDL is expensive. However, since it is produced from VLDL, it can be inferred indirectly by estimating VLDL levels in the blood. That estimate is typically obtained by measuring triglyceride levels after at least eight hours of fasting, when chylomicrons have been totally removed from the blood by the liver. In the absence of chylomicrons, triglyceride levels have a much larger correlation with risk ofcardiovascular diseases than total LDL levels.
After being transported to the liver by HDL, cholesterol is delivered to the intestines via bile production. However, 92-97% is reabsorbed in the intestines and recycled viaenterohepatic circulation.
Cholesterol circulates in the blood inlow-density lipoproteins and these are taken into the cell byLDL receptor-mediatedendocytosis inclathrin-coated pits, and then hydrolysed in lysosomes.
In response to low blood cholesterol, different cells of the body, mainly in theliver andintestines, start to synthesize cholesterol fromacetyl-CoA by the enzymeHMG-CoA reductase. This is then released into the blood.
Hyperlipidemia is the presence of elevated or abnormal levels oflipids and/orlipoproteins in theblood.
Lipid andlipoprotein abnormalities are extremely common in the general population, and are regarded as a highly modifiable risk factor forcardiovascular disease. In addition, some forms may predispose toacute pancreatitis. One of the most clinically relevant lipid substances ischolesterol, especially onatherosclerosis andcardiovascular disease. The presence of high levels of cholesterol in the blood is calledhypercholesterolemia.[6]
Hyperlipoproteinemia is elevated levels oflipoproteins.
Hypercholesterolemia is the presence of high levels ofcholesterol in the blood.[6] It is not adisease but ametabolic derangement that can be secondary to many diseases and can contribute to many forms of disease, most notablycardiovascular disease.Familial hypercholesterolemia is a raregenetic disorder that can occur in families, where sufferers cannot properly metabolise cholesterol.
Abnormally low levels of cholesterol are calledhypocholesterolemia.
