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A General Clinical And Medical Overview Of Hyperlipidemias As Metabolic Disorders
Food Sources Of Lipids
Endogenous And Exogenous Lipids
The Exogenous Lipids
A liberal diet supplied about 100 g of triglycerides and 1 g of cholesterol daily. After absorption, they circulate in blood as chylomicrons from which the fatty acids pass to muscle cells and fat cells where they are utilized for energy or re-esterified into triglycerides. The remaining portion of the chylomicron which is rich in cholesteryl esters is taken up by the liver for the formation of bile acids and the rest passes unaltered in bile. Free fatty acids liberated from fat depots are also metabolized in the liver to form trigylcerides, phospholipids, and cholesterol which circulate in blood as lipoproteins.
The endogenous lipids
The liver synthesizes triglycerides from surplus carbohydrates obtained from diet. The triglycerides are esterified and released into circulation as very low density lipoproteins (VLDL). The triglycerides is split off and it enters the adipocytes. The VLDL remnant which contains mainly cholesterol esters gets physically transformed into low density lipoprotein (LDL). Around 75% of circulating cholesterol is present in the form of LDL, and this is the form in which cholesterol is supplied to extrahepatic tissues like the adrenal cortex, muscle cells, renal cells and lymphocytes for the formation of cell membranes, hormones etc. Unesterified cholesterol which is liberated into plasma from disintegrating hepatic parenchymal and phagocytic cells constitutes high density lipoproteins (HDL). The levels of cholesterol and triglycerides in plasma show considerable variation in health. Cholesterol levels above 240 mg/dl and triglyceride levels above 200 mg/dl indicate hyperlipidemia in adults. In individuals below the age of 20 years, upper limits are 200 mg/dl and 150 mg/dl respectively. The cholesterol of atheromatous lesion is derived principally from plasma.
The blood levels of cholesterol and triglycerides give valuable information for the assessment of errors of lipid metabolism. LDL and HDL together account for 90% of the cholesterol in the plasma. Lipids are present in circulation as lipoproteins and free fatty acids (non-esterified fatty acids) bound to albumin. The lipoproteins are formed by the combination of lipids, triglycerides, phospholipid and cholesterol with specific apoproteins. These have been classified as very low density lipoproteins (VLDL, prebeta) low density lipoproteins (LDL, beta lipoproteins), or high density lipoproteins (HDL, alpha lipoproteins) based on ultracentrifugal studies and electrophoretic patterns. The LDL and HDL particles are heterogenous. The terms “pre-beta”, “beta”, and “alpha” refer to the bands seen on electrophoresis.
The role of HDL is not still clear. Probably its role is to transport cholesterol and remove lipid from the arterial walls. The HDL reduces uptake of LDL by cells. Increase in levels of LDL and VLDL and decrease of HDL are associated with higher indicence of atheroma and its complications. Specific subclasses are selectively related to coronary heart disease. The HDL has a protective role against atherogenesis. In coronary heart disease, the concentration of the small HDL 2 fraction is particularly low. Raised concentration of LDL is readily decreased by controlled diet and appropriate drugs. High levels of LDL (above 500 to 700 mg/dl, 6.7 mmol/liter) lead to acute pancreatitis.
© 2014 Funom Theophilus Makama