Introduction to Human Nutrition

118 Introduction to Human Nutrition

day for 3 years) reduces the incidence of death from
CHD in healthy, free-living subjects. This longer term
benefi t may be linked to the effects of eicosapentae-
noic acid/docosahexaenoic acid on a host of other
cardiovascular risk factors, including plasma TAGs
and lipoproteins.
Long-chain n-3 PUFAs exert multiple effects on
lipid metabolism, the most notable of which is the
capacity to decrease postabsorptive plasma TAG levels
by 20–30%. Fish oil-enriched diets have also been
shown to attenuate the magnitude and duration of
postprandial lipemia following the ingestion of a fat-
containing meal. These effects are frequently accom-
panied by benefi cial changes in circulating LDLs and
HDLs, and the correction of an ALP.
Widespread knowledge of the favorable effects of
eicosapentaenoic acid and docosahexaenoic acid has
raised awareness of the need to increase intakes of
these fatty acids, and to reduce the amount of n-6

PUFA at the same time. However, in practice, this will

be diffi cult to achieve, not least because of a mass

resistance to the increased consumption of oily fi sh

and diminishing fi sh stocks. An obvious alternative

would be to increase the intake of the shorter chain

precursor of eicosapentaenoic acid/docosahexaenoic

acid, α-linolenic acid (C18:3n-3). The latter is derived

from plant seeds such as fl ax and rapeseed, and is

desaturated and elongated to its longer chain relatives

in the body. Unfortunately, the rate of conversion to

eicosapentaenoic acid and especially docosahexaenoic

acid is slow, and the effi ciency of conversion is reduced

by high levels of linoleic acid, which competes more

effectively than α-linolenic acid for desaturation.

There is, as yet, no evidence to suggest that the rate

of conversion of dietary α-linolenic acid to eicosa-

pentaenoic acid and especially docosahexaenoic acid

is suffi cient to achieve fi sh oil-like effects on blood

lipids (Table 6.6).

Table 6.6 Effects on plasma lipids of substituting dietary saturated fats with polyunsaturated fatty acids, monounsaturated fatty

acids, and carbohydrates

Carbohydrate

Sucrose/fructose?

b

a

LDL-C HDL-C TAG TC:HDL

SFA

(C12–C16)

n-6 PUFA

Linoleic acid

(sunflower/corn)

n-3 PUFA

LC = EPA/DHA (fish oil)

SC = linolenic acid

(flax/soyabean/rape)

n-9 MUFA

Oleic acid (olive oil/rape)

a n-6 PUFA in excess of 10% energy.

b Increase in response to redistribution of LDL subsclasses.

PUFA: polyunsatruated fatty acids; MUFA: monounsaturated fatty acids.

SFA: saturated fatty acids; LC: long chain; EPA: eicosapentaenoic acid; DHA: docosahexaenoic acid; short chain;

LDL-C: low-density lipoprotein cholesterol; HDL-C: high-density lipoprotein cholesterol; TAG: triacylglycerols; TC: total

cholesterol.