The fate of cholesterol in the blood is highly determined by its constitution of
lipoproteins, where some types favour transport towards body tissues and others towards the liver for excretion into the intestines. The 1987 report of
National Cholesterol Education Program, Adult Treatment Panels suggest the total
blood cholesterol level should be: 240 mg/dl high cholesterol. The average amount of
blood 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. These seasonal variations seem to be inversely linked to
vitamin C intake.
Intestine intake In
lipid digestion, cholesterol is packed into
chylomicrons in the
small intestine, which are delivered to the
portal vein and
lymph. The chylomicrons are ultimately taken up by liver
hepatocytes via interaction between
apolipoprotein E and the
LDL receptor or
lipoprotein receptor-related proteins.
In lipoproteins Cholesterol is minimally soluble in
water; it cannot dissolve and travel in the water-based bloodstream. Instead, it is transported in the bloodstream by
lipoproteins that are water-soluble and carry cholesterol and
triglycerides internally. The
apolipoproteins 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 the
intestinal mucosa to the
liver, are called
chylomicrons. 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 into
very low density lipoproteins (VLDL) and secretes them into plasma where they are converted to intermediate-density lipoproteins(IDL), which thereafter are converted to
low-density lipoprotein (LDL) particles and non-esterified fatty acids, which can affect other body cells. In healthy individuals, most of the LDL particles are
large 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 of
small and dense LDL (sd-LDL) particles are strongly associated with the presence of
atheromatous 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 of
cardiovascular diseases than total LDL levels.
Intestine excretion 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 via
enterohepatic circulation.
Cell uptake Cholesterol circulates in the blood in
low-density lipoproteins and these are taken into the cell by
LDL receptor-mediated
endocytosis in
clathrin-
coated pits, and then hydrolysed in lysosomes.
Cell secretion In response to low blood cholesterol, different cells of the body, mainly in the
liver and
intestines, start to synthesize cholesterol from
acetyl-CoA by the enzyme
HMG-CoA reductase. This is then released into the blood. ==Related medical conditions==