Introduction to Human Nutrition

88 Introduction to Human Nutrition

Lengthening of the chain and the introduction of
additional double bonds beyond the fi rst one occur
from the carboxyl-terminal. The presence of one or
more double bonds in a fatty acid defi nes it as “unsat-
urated,” compared with a saturated fatty acid which
contains no double bonds. A saturated fatty acid gen-
erally occupies less space than an equivalent chain
length unsaturated fatty acid (Figure 6.1). Double
bonds allow for isomerization or different orientation
(cis or trans) of the adjoining carbons across the
double bond (Figure 6.2). In longer chain fatty acids,
double bonds can also be at different positions in the
molecule. Hence, unsaturation introduces a large
amount of structural variety in fatty acids and
the resulting lipids. Further details about the features
of the different families of fatty acids are given in
Sections 6.6 and 6.8.

Short- and medium-chain fatty acids

Short-chain fatty acids (less than eight carbons)
are water soluble. Except in milk lipids, they are
not commonly esterifi ed into body lipids. Short-
chain fatty acids are found primarily in dietary prod-

ucts containing ruminant milk fat. Hence, although

they are produced in relatively large quantities

from the fermentation of undigested carbohydrate in

the colon, as such, they do not become part of the

body lipid pools. Medium-chain fatty acids (8–14

carbons) arise as intermediates in the synthesis of

long-chain fatty acids or by the consumption of

coconut oil or medium-chain TAG derived from it.

Like short-chain fatty acids, medium-chain fatty acids

are present in milk but they are also rarely esterifi ed

into body lipids, except when consumed in large

amounts in clinical situations requiring alternative

energy sources. Medium-chain fatty acids (8–14

carbons) are rare in the diet except for coconut and

milk fat.

Table 6.2 Nomenclature of common fatty acids

Saturated Monounsaturated Polyunsaturated

Formic (1:0) Lauroleic (12:1n-3) Linoleic (18:2n-6)
Acetic (2:0) Myristoleic
(14:1n-5)

γ-Linolenic (18:3n-6)

Propionic (3:0) Palmitoleic
(16:1n-7)

Dihomo-γ-linolenic
(20:3n-6)
Butyric (4:0) Oleic (18:1n-9) Arachidonic (20:4n-6)
Valeric (5:0) Elaidic
(trans-18:1n-9)

Adrenic (22:4n-6)

Caproic (6:0) Vaccenic (18:1n-7) n-6 Docosapentaenoic
(22:5n1-6)
Caprylic (8:0) Petroselinic
(18:1n-12)

α-Linolenic (18:3n-3)

Capric (10:0) Gadoleic
(20:1n-11)

Stearidonic (18:4n-3)

Lauric (12:0) Gondoic (20:1n-9) Eicosapentaenoic
(20:5n-3)
Myristic (14:0) Euricic (22:1n-9) n-3 Docosapentaenoic
(22:5n-3)
Palmitic (16:0) Nervonic (24:1n-9) Docosahexaenoic
(22:6n-3)
Margeric (17:0)
Stearic (18:0)
Arachidic (20:0)
Behenic (22:0)
Lignoceric (24:0)

Saturates

cis-Monounsaturates

cis-Polyunsaturates

Figure 6.1 Stick models illustrating the basic structural differences

between saturated, cis-monounsaturated, and cis-polyunsaturated

fatty acids. As shown in two dimensions, the increasing curvature

caused by inserting one or more double bonds increases the area

occupied by the fatty acid. The physical area occupied by unsaturated

fatty acids is further accentuated in three dimensions because esteri-

fi ed fatty acids rotate around the anchored terminal.