Food Lipids: Chemistry, Nutrition, and Biotechnology

70% of the total amount of cholesterol in the plasma. Most of the LDL is now
removed from the circulation through the hepatic LDL receptor pathway, which recognizes
apolipoprotein B-100 (apoB-100). LDL uptake from the blood by the LDL
receptor–mediated pathway is highly controlled and this pathway will be downregulated
if the amount of cholesterol in the cell becomes too high. A smaller part,
however, is removed via the scavenger pathway. Uptake via this pathway is not
saturable and is positively related with the LDL cholesterol concentration. Thus, the
higher the blood LDL cholesterol concentration, the more LDL will be taken up via
the scavenger pathway [1]. When too much LDL is taken up via the scavenger
pathway from macrophages, cells loaded with cholesterol are formed—so-called
foam cells—which are frequently found in atherosclerotic lesions.
Cholesterol can also be transported out of tissues by reverse cholesterol transport.
This system is mediated by HDL, lecithin cholesteryl acyltransferase (LCAT)
and cholesterol ester transfer protein (CETP). HDL binds free cholesterol from tissues,
which is esterified by LCAT, a protein associated with apolipoprotein A-I
(apoA-I). The formed cholesterylesters move to the core of the HDL particle and the
HDL is converted to a larger particle. The acquired cholesterylesters can now be
transferred with the assistance of CETP to apoB-100-containing lipoproteins in
exchange for triacylglycerols. The apoB-100-containing lipoproteins are further metabolized,
as has already been described. The large HDL particles may also be taken
up in the liver by a putative HDL receptor or lose a part of its content by the action
of hepatic lipase and then reenter the circulation again.
V. PLASMA LIPOPROTEINS AND CORONARY HEART DISEASE
LDL and HDL have different effects on the risk for CHD. High concentrations of
LDL are atherogenic, whereas high levels of HDL are negatively associated with the
risk for CHD. As LDL carries most of the plasma cholesterol, the total plasma
cholesterol is also a good index for the risk of CHD. It should be realized, however,
that some people have high total cholesterol concentrations, due to high HDL levels.
Therefore, the total cholesterol to HDL cholesterol might be the most efficient predictor
for the risk for CHD [2]. Also, high levels of triacylglycerols, which are in
the fasting condition mainly found in the VLDLs, are positively related to the risk
for CHD [3].
VI. DIETARY FATS AND PLASMA LIPOPROTEINS
A. Earlier Studies
In the 1950s Keys and coworkers started a series of well-controlled experiments to
examine the effects of dietary fatty acids on plasma total cholesterol concentrations
[4,5]. Groups of physically healthy men were fed diets that differed widely in the
amount of fat and in dietary fatty acid composition. During the studies, individual
allowances were adjusted weekly to keep body weight stable so that changes in
plasma total cholesterol concentrations could be attributed solely to dietary changes.
At the end of the studies, an empirical formula was derived that could be used to
predict for a group of subjects changes in plasma cholesterol concentrations from
changes in dietary fatty acid composition:
Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved.