108 Introduction to Human Nutrition
block peroxidation in living tissues. Humans and
animals readily detect peroxidized fats in foods by
their disagreeable odor and avoid them. However,
modeling the effects of peroxides produced in vivo
and in vitro is particularly challenging because lipid
peroxidation undoubtedly is an important part of
several necessary biological processes such as activa-
tion of the immune response.
Desaturation, chain elongation,
and chain shortening
One important characteristic of long-chain fatty acid
metabolism in both plants and animals is the capacity
to convert one to another via the processes of desatu-
ration, chain elongation, and chain shortening.
Plants and animals use desaturases to insert a
double bond into long-chain fatty acids. There are
several desaturases, depending on the position in the
acyl chain into which the double bond is inserted.
Although myristate (14:0) and palmitate can be
converted to their monounsaturated derivatives,
myristoleate (14:1n-5) and palmitoleate (16:1n-7)
respectively, commonly it is only the fatty acids of 18
or more carbons that undergo desaturation. The Δ^9
desaturases in all organisms, except for anaerobic bac-
teria, use oxygen and NADPH to introduce a cis
double bond at carbons 9 and 10 of stearate. This is
accomplished by an enzyme complex consisting of a
series of two cytochromes and the terminal desatu-
rase itself. The acyl-CoA form of fatty acids is the
usual substrate for the desaturases, but fatty acids
esterifi ed to phospholipids can also be desaturated
in situ.
All mammals that have been studied can convert
stearate to oleate via Δ^9 desaturase. However, in the
absence of dietary oleate, young rats may have insuf-
fi cient capacity to sustain normal tissue oleate levels.
Normal values depend on the reference, which can
vary widely depending on the source and amount of
oleate in the diet. Nevertheless, it is important to dis-
tinguish between the existence of a given desaturase
and the capacity of that pathway to make suffi cient of
the necessary product fatty acid. Hence, as with the
long-chain polyunsaturates and, indeed, with other
nutrients such as amino acids (see Chapter 4), it is
important to keep in mind that the existence of a
pathway to make a particular fatty acid or amino acid
does not guarantee suffi cient capacity of that pathway
to make that product. This is the origin of the concept
of “conditional essentiality” or “indispensability.”
Both plants and animals are capable of desaturating
at the 9–10 carbon (Δ^9 desaturase) of stearate, result-
ing in oleate. However, only plants are capable of
desaturating oleate to linoleate and then to α-lino-
lenate. Once linoleate and α-linolenate are consumed
by animals, their conversion to the longer chain
PUFAs of their respective families proceeds primarily
by an alternating series of desaturation (Δ^6 and Δ^5
desaturases) and chain-elongation steps (Figure 6.13).
Sequential desaturations or chain elongations are also
a possibility, resulting in a large variety, though low
abundance, of other PUFAs.
During dietary defi ciency of linoleate or α-lino-
lenate, oleate can also be desaturated and chain elon-
gated to the PUFA eicosatrienoate (20:3n-9). Hence,
most but not all PUFAs are derived from linoleate or
α-linolenate.HH
X•XHHHHOOO OH OOHHHOO 2HydroperoxideEndoperoxide+ R•MalondialdehydeOFigure 6.12 Principal steps in peroxidation of a polyunsaturated fatty
acid.