Nutrition and Metabolism of Lipids 113Monounsaturated fatty acids
Little is known about the nutritional or health impli-
cations of palmitoleate (16:1n-7), but there is a bur-
geoning interest in the main dietary monounsatu-
rated fatty acid, oleate, and the health implications of
olive oil. In the context of the same total fat intake,
the main benefi t of higher oleate intake seems to be
that this reduces intake of palmitate and stearate and
that this helps to lower serum cholesterol.
Partially hydrogenated fatty acids
Partially hydrogenated fatty acids contain a large pro-
portion of trans fatty acids that are not naturally
occurring but arise directly from food processing.
Hence, unlike saturates and cis-unsaturated fatty
acids, they are not a necessary component of the diet
except in the small amounts found in cow’s milk.
Their physical characteristics make them economi-
cally suitable for inclusion in a wide variety of baked,
fried, and oil-based foods, from which they can easily
contribute up to 10% of dietary fat depending on
food selection. Epidemiological evidence and some
experimental studies show that common dietary trans
fatty acids raise LDL cholesterol and lower HDLs in
healthy adults, so the main nutritional concern is that
they may contribute to an increased risk of cardiovas-
cular disease (see Section 6.11).
Trans fatty acids have also been experimentally
shown to compete with and impair the metabolism
of other dietary long-chain fatty acids, but the rele-
vance of these observations in humans is unclear.
Trans fatty acids can be present in baby foods at rela-
tively high concentrations but, so far, there is no evi-
dence of deleterious effects on growth or development.
Some information on the metabolism of trans fatty
acids in humans has been gained from tracer studies,
but fundamental information, such as the rate at
which they are oxidized, is still unknown.
Polyunsaturated fatty acids
Unlike saturates and monounsaturates, a dietary
source of n-6 and n-3 polyunsaturates is a necessity
for normal growth and development. As with other
essential nutrients, this has given rise to assessment of
the dietary requirements for polyunsaturates and the
implications of inadequate dietary intake of them.
It has been accepted for over 50 years that n-6
polyunsaturates, particularly linoleate, are required in
the diet of all mammals, including humans. Offi cial
dietary guidelines generally recommend a dietary
source of linoleate at 1–2% of energy intake. It has
taken much longer to demonstrate that n-3 PUFAs
are required by humans. Although this now seems
widely accepted among nutrition researchers, some
countries, including the USA, still do not yet offi cially
recognize that, as a minimum, α-linolenate is a
required nutrient. As with other nutrients, the require-
ment for polyunsaturates varies according to the stage
of the life cycle, with pregnancy, lactation, and infancy
being the most vulnerable. Symptoms of linoleate
defi ciency are virtually impossible to induce in healthy
adult humans, so the concept of “conditional indis-
pensability or dispensability” of PUFAs has recently
emerged to replace the older but ambiguous term
“essential fatty acid.” Linoleate appears to be condi-
tionally dispensable in healthy nonpregnant adults,
but is not in pregnancy, lactation, or infancy.
Because of the competition between the two fami-
lies of PUFAs, defi ciency of n-3 PUFA is commonly
induced by an excess of dietary linoleate. Hence, dis-
cussion of the requirements for linoleate and α-lino-
lenate has focused on their ratio in the diet. The ratio
of n-6 to n-3 polyunsaturates in human milk (5:1 to
10:1) has been widely viewed as a suitable reference
for this ratio in the general diet. In most affl uent
countries, this ratio remains much higher, at about
20:1, and has been implicated in subclinical defi ciency
of n-3 polyunsaturates. There is recent evidence to
suggest that it is the absolute amounts of long-chain
n-3 and n-6 fatty acids that are important in predict-
ing health outcomes, and not the dietary ratio of these
PUFAs.Essential fatty acid defi ciency
The fi rst experimental model of defi ciency of polyun-
saturates was total fat defi ciency. The elimination of
dietary fat had to be extreme because the traces of fat
found in starch and dietary proteins were suffi cient
to prevent reproducible symptoms of fat defi ciency.
The defi ciency symptoms are now well known and
involve dry, scaly skin, growth retardation, and repro-
ductive failure. Most of these gross symptoms are
relieved by linoleate and arachidonate. Although α-
linolenate cannot be synthesized de novo, it has little
effect on these gross symptoms. However, careful
studies using a diet that is extremely defi cient in n-3
polyunsaturates and contains an excess of n-6 poly-