Introduction to Human Nutrition

Nutrition and Metabolism of Lipids 89

Long-chain saturated and
monounsaturated fatty acids

Long-chain fatty acids (>14 carbons) are the main
constituents of dietary fat. The most common satu-
rated fatty acids in the body are palmitate and stea-
rate. They originate from three sources: directly from
the diet, by complete synthesis from acetyl-coenzyme
A (CoA), or by lengthening (chain elongation) of a
pre-existing shorter-chain fatty acid. Hence, dietary
or newly synthesized palmitate can be elongated
within the body to form stearate and on to arachidate
(20:0), behenate (22:0), and lignocerate (24:0). In
practice, little stearate present in the human body
appears to be derived by chain elongation of pre-
existing palmitate. In humans, saturates longer than
24 carbons do exist but usually arise only during
genetic defects in fatty acid oxidation, as will be dis-
cussed later.
Palmitate and stearate are important membrane
constituents, being found in most tissue phospholip-
ids at 20–40% of the total fatty acid profi le. Brain
membranes contain 20- to 24-carbon saturates that,
like palmitate and stearate, are synthesized within the
brain and have little or no access to the brain from
the circulation. The normal membrane content of

long-chain saturates can probably be sustained

without a dietary source of these fatty acids. Com-

pared with all other classes of dietary fatty acid, espe-

cially monounsaturated or polyunsaturated fatty

acids, excess intake or synthesis of long-chain satu-

rates is associated with an increased risk of cardiovas-

cular disease.

The most common long-chain cis-monounsatu-

rated fatty acids in diet and in the body are oleate

(18:1n-9) and palmitoleate (16:1n-7), with the former

predominating by far in both the body’s storage and

membrane lipids. As with stearate, most oleate in the

human body appears to be of dietary origin. Hence,

although humans have the capacity to desaturate

stearate to oleate, dietary oleate is probably the domi-

nant source of oleate in the body. Only plants can

further desaturate oleate to linoleate and again to α-

linolenate. As with saturates of >18 carbons in length,

20-, 22-, and 24-carbon monounsaturates derived

from oleate are present in specialized membranes

such as myelin.

Polyunsaturated fatty acids (PUFAs)

Linoleate and α-linolenate are the primary dietary

cis-polyunsaturated fatty acids in most diets. Neither

can be synthesized de novo (from acetate) in animals

so are ‘essential’ fatty acids. They can be made

by chain elongation from the two respective 16-

carbon precursors, hexadecadienoate (16:2n-6) and

hexadecatrienoate (16:3n-3), which are found in

common edible green plants at up to 13% of total

fatty acids. Hence, signifi cant consumption of green

vegetables will provide 16-carbon polyunsaturates

that contribute to the total available linoleate and

α-linolenate.

Linoleate is the predominant polyunsaturated fatty

acid in the body, commonly accounting for 12–15%

of adipose tissue fatty acids. In the body’s lean tissues

there are at least three polyunsaturates present in

amounts >5% of the fatty acid profi le (linoleate, ara-

chidonate, docosahexaenoate). In addition, at least

two other biologically active polyunsaturates are

present in body lipids [dihomo-γ-linolenate (20:3n-

6) and eicosapentaenoate (20:5n-3)], although usually

in amounts between 1% and 3% of total fatty acids.

Marine fi sh are the richest source of 20- to 22-carbon

polyunsaturates. α-Linolenate and its precursor,

hexadecatrienoate (16:3n-3), are the only n-3 polyun-

saturates in common terrestrial plants.

cis-Monounsaturates

trans-Monounsaturates

Figure 6.2 Stick models comparing a cis- with a trans-unsaturated
fatty acid. A cis-unsaturated double bond creates a U-shaped space
and confers curvature to the molecule because, relative to the longi-
tudinal axis of the fatty acid, the two hydrogens at the double bond
are on the same side of the molecule. A trans-unsaturated double bond
does not confer curvature to the molecule because the hydrogens are
on opposite sides of the double bond. A trans-double bond therefore
tends to give the fatty acid physicochemical properties more like that
of a saturated fatty acid.