104 Introduction to Human Nutrition
potentially harmful cholesterol-rich remnants and
LDLs that contribute to coronary atherosclerosis. The
effects of diet, and in particular dietary fats, in modu-
lating the clearance of TAG-rich lipoproteins in the
postprandial period is of paramount importance in
preventing the accumulation of atherogenic remnants
and development of proatherogenic abnormalities in
LDL and HDL. The actions of insulin coordinate the
metabolism of TAG-rich lipoproteins but can become
defective through energy imbalance, weight gain,
and ultimately obesity. As a consequence, the most
common abnormalities in lipoproteins to increase
risk in populations arise from a primary defect in the
metabolism of TAG, induced through insulin resis-
tance and not cholesterol per se. Equally important is
the fact that these metabolic defects originate, in part,
through nutrient–gene interactions and are thus
highly amenable to dietary modifi cation.
6.6 Body lipid pools
Lipids in the human body exist in two major pools:
structural lipids in membranes and storage lipids in
body fat. The lipid composition and metabolic fate of
these two pools are quite distinct, although many of
the fatty acids occupying both pools are the same. The
main components of both membrane and storage
lipids are the long-chain (16–24 carbons) saturated,
monounsaturated, and polyunsaturated fatty acids.
Although several of the major long-chain fatty acids
in the body are common to both membrane and
storage lipids, namely palmitate, stearate, oleate, and
linoleate, three important distinctions exist between
membrane and storage lipids.
1 Membrane lipids are not usually hydrolyzed to
release free fatty acids for energy metabolism.
2 Membrane lipids contain a much higher propor-
tion of long-chain PUFAs.
3 Membrane lipids are more diverse and rarely
include TAGs, which are the main component of
storage lipids.
Structural lipid pool
Biological membranes surrounding cells and subcel-
lular organelles exist primarily as lipid bilayers (Figure
6.3). The lipids in both the inner and outer surfaces
of membranes are composed mainly of phospholip-
ids and free cholesterol, which interface with a myriad
of proteins functioning as receptors, transporters,
enzymes, ion channels, etc. Some lipids, i.e., PUFAs,
confer the feature of “fl uidity” to membranes, whereas
others, i.e., cholesterol and saturated fatty acids, have
the opposite rigidifying effect. Membranes have
extraordinarily diverse fatty acid profi les and phos-
pholipid composition depending on their tissue and
subcellular location. They are also the body’s reservoir
of both fat-soluble vitamins and eicosanoid precur-
sors such as arachidonate.
Most of the body’s cholesterol is present in the
unesterifi ed form in membranes, where it represents
35–45% of total lipids. Skin, plasma, and adrenal
cortex contain 55–75% of cholesterol in the esterifi ed
form. Bile also contains free cholesterol and bile salts
derived from cholesterol.Storage lipid pool
Triacylglycerols are the main energy storage form of
lipids and they are the principal component of
body fat. TAG-containing fatty acids destined for
oxidation are also present in measurable but much
lower amounts in all tissues that can oxidize long-
chain fatty acids, i.e., muscle and heart. TAG is syn-
thesized by the intestine and liver, where it is subse-
quently incorporated into lipoproteins (see Section
6.4) for the transport of lipids to and from other
tissues.
The main fatty acids in the TAG of adult human
body fat are palmitate (20–30%), stearate (10–20%),
oleate (45–55%), and linoleate (10–15%). The fatty
acid profi le of adult body fat always refl ects the profi le
of dietary fat. Only rarely would this result in other
fatty acids being more prevalent in body fat than the
four listed here. At birth, the fatty acid profi le of body
fat is unusual in having very low linoleate (<3%) and
α-linolenate (<1%) but a higher proportion of long-
chain polyunsaturates than later in life. Body fat occu-
pies several discrete sites that expand and contract as
needed. Body fat is about 82% by weight TAG, making
it by far the main body pool of palmitate, stearate,
oleate, and linoleate.
The main sites of body fat are subcutaneous and
intravisceral, and they have different rates of response
to stimuli for accumulation or release of fatty acids.
Within a given site, growing evidence suggests that
PUFAs are more easily released from adipose tissue
TAG than are saturated fatty acids, especially during
fasting or longer term energy defi cit.