90 Introduction to Human Nutrition
Hydrogenated and conjugated fatty
acid isomers
The introduction of unsaturation with one double
bond creates the possibility of both positional and
geometric isomers in fatty acids. Among long-chain
unsaturated fatty acids, positional isomers exist
because the double bond can be introduced into
several different locations, i.e., 18:1n-7, 18:1n-9,
18:1n-11, etc. Geometric isomers exist because the
two remaining hydrogens at each double bond can be
opposite each other (trans) or on the same side of the
molecule (cis; Figure 6.2). Thus, there is cis-18:1n-9
(oleate) and trans-18:1n-9 (elaidate), and so on for all
unsaturated fatty acids, with the combinations
mounting exponentially as the number of double
bonds increases.
Trans isomers of monounsaturated or polyunsatu-
rated fatty acids arise primarily from partial hydroge-
nation during food processing of oils, but some also
occur naturally in ruminants. The number of trans
isomers increases with the number of double bonds,
so there is only one trans isomer of oleate but there
are three trans isomers of linoleate and seven of α-
linolenate. Virtually all naturally occurring polyun-
saturated fatty acids have double bonds that are
methylene interrupted, i.e., have a CH 2 group between
the two double bonds. However, methylene interrup-
tion between double bonds can be lost, again, through
food processing, and the bonds moved one carbon
closer together, becoming conjugated. Thus, the
double bonds in linoleate are at the 9–10 and 11–12
carbons, but in conjugated linoleate they are at the
9–10 carbons and the 11–12 carbons. Some degree of
further desaturation and chain elongation can occur
in conjugated fatty acids, but much less than with
methylene-interrupted polyunsaturates. Thus, conju-
gated linoleate is the main conjugated fatty acid that
has attracted considerable attention with respect to its
potential role in nutritional health.
Fats and oils
Fats are esters of fatty acids with glycerol (Table 6.1).
They usually occur as triesters or triacylglycerols
(TAGs), although monoacylglycerols and diacylglyc-
erols occur during fat digestion and are used in food
processing. Most common dietary fats contain a
mixture of 16- to 18-carbon saturated and unsatu-
rated fatty acids. By convention, fats that are liquid at
room temperature are called oils, a feature arising
from their lower proportion of saturated (straight-
chain) and higher proportion of unsaturated (bent-
chain) fatty acids. Unsaturated fatty acids usually
have a lower melting point; this facilitates liquefaction
of the fats of which they are a component. TAGs of
animal origin are commonly fats, whereas those of
fi sh or plant origin are usually oils. Animal fats and
fi sh oils frequently contain cholesterol, whereas plant
oils do not contain cholesterol but usually contain
other “phyto” sterols.
TAGs are primarily used as fuels, so dietary fats
(mostly TAGs) are commonly associated with energy
metabolism rather than with structural lipids
found in membranes. However, membrane lipids as
well as TAGs are extracted with lipid solvents used to
determine the fat content of foods, tissues, or plant
material. Hence, because organs such as brain are
rich in membrane phospholipids, when the total
lipids are extracted to determine the organ’s chemical
composition, these organs are said to have a certain
fat content. On a chemical basis this is true, but this
description often misconstrues the nature of the lipid
because the brain in particular contains virtually no
TAG.Phospholipids
Phospholipids contain two nonpolar, hydrophobic
acyl tail groups and a single functional head group
that is polar and hydrophilic. Hence, they are rela-
tively balanced amphiphilic lipids and, in this capac-
ity, are crucial components of biological membranes.
The head groups contain phosphorus and amino
acids (choline, serine, ethanolamine), sugars (inosi-
tol), or an alcohol (glycerol). Phosphatidylcholine
(lecithin) is the most abundant phospholipid in
animal tissues but phosphatidylglycerols (glycosides)
predominate in plant lipids. Phospholipids contain-
ing a fatty acid amide are sphingolipids. Various phos-
pholipases can hydrolyze the acyl groups or head
group during digestion or metabolism.
One of the outstanding characteristics that make
phospholipids suitable as major constituents of bio-
logical membranes is that, in water, they naturally
aggregate into spherical or rod-like liposomes or ves-
icles, with the hydrophilic portion facing outwards
and the hydrophobic portion facing inwards (Figure
6.3). Changing the constituent acyl groups from satu-
rated to polyunsaturated changes the fl uidity of these
aggregates because of the greater amount of space