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meat, and dairy products, and so is commonly found in low levels in human membranes [35] .
The fact that elaidic acid has been in the human diet for so long cannot account for the large
increase in heart disease observed over the past century. It is doubtful that elaidic acid causes
these health problems. Instead, the culprit is likely to be partially hydrogenated plant oils.
The first successful hydrogenation of plant oils was reported in 1897 [35] . An industrial
process was developed to harden fluid plant oils and to decrease their susceptibility to oxida-
tion, opening their application to food processing. Since then there has been a steady increase
in the amount of TFAs appearing in the human diet, and with it, a concomitant increase in
heart disease. Partial hydrogenation of plant lipids produces a bewildering array of TFAs,
the result of cis double bond reduction and migration up and down the chain and partial
conversion of normal cis to deleterious trans double bonds. The process creates a wide range
of geometric and positional fatty acid isomers. In recent years, the increase in heart disease
has resulted in banning trans fats from many parts of the world.
The question, however, remains as to why cis fatty acids are essential for human health,
while trans fatty acids are so harmful. One possible answer, related to the theme of this
topic, involves incorporation of unnatural trans fatty acids into membrane phospholipids,
where they replace the natural cis lipids, thus altering the structure and function of the
membranes. All fatty acids, including TFAs, can be incorporated into phospholipids and
thereby affect the hydrophobic interior of membranes [36,37] .
A first approach to investigating how cis and trans fatty acids differ in their effect on
membrane physical properties can be extracted from the main phase transition temperatures
(T m s) reported in Chapter 4, Tables 4.3 and 4.7. For the 18-carbon series:
Fatty acid
Designation
T m
D T m
Stearic
Saturated (18:0)
69.6 C
0 C
cis (18:1 D 9c )
16.2 C
53.4 C
Oleic
trans (18:1 D 9t )
43.7 C
25.9 C
Elaidic
Both unsaturated fatty acids (oleic and elaidic) exhibit T m s that are lower than that of
saturated stearic acid. However, the change in T m s(
T m ) is much larger for the cis than
for the trans fatty acid. Therefore, at least by T m analysis, although both elaidic and oleic
acid have 18-carbons and a single
D
D
9 double bond, elaidic acid is more similar to the
saturated stearic acid than it is to oleic acid. From the T m s it can be concluded that cis double
bonds have a larger effect on lipid packing than do trans double bonds that exhibit consider-
able saturated fatty acid-like properties.
Roach et al. [38] extended the T m measurements on oleic versus elaidic acids to several PC
model membranes (monolayers and bilayers). They compared the effect of oleic versus elai-
dic and linoleic versus linelaidic on homo-chain and hetero-chain PCs using molecular
dynamics, lateral lipid packing, thermotropic phase behavior, 'fluidity', lateral mobility,
and permeability. In all cases the cis unsaturated chains induced much larger membrane
perturbations than did the trans unsaturated analogs. Once again, the trans double bond
acyl chains behaved more like a saturated chain than a cis unsaturated chain. Corroborating
this conclusion is the molecular dynamics simulations of Pasenkiewicz-Gierula and
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