Marine
Omega-3 Fatty Acids Are the Preferred Source of EPA and
DHA in Humans
by
Morten Bryhn, MD, Ph D
3/5/2003
Omega-3 fatty acids are essential
nutrients, which have to be provided with the diet. Medium
chain omega-3 fatty acids from seeds are converted to the
long-chain fatty acids, however, based on new data from
clinical studies the conversion rate is low indicating that
marine omega-3 fatty acids are the preferred fatty acids
for prevention and treatment of cardiovascular disease.
Long-chain
omega-3 fatty acids are important structural components
of cell membranes and they are also utilised as raw material
for production of local acting hormones. Docosahexaenoic
acid (DHA) is the longest of the marine omega-3 fatty acids
with 22 carbon atoms and 6 double bonds. DHA is enriched
in brain tissue and in rhodopsin in the retina of the eye.
The reason for the local enrichment of DHA in the brain
and the specific function of this fatty acid is not fully
elucidated. However, newborn babies deprived of DHA does
not obtain the same visual acuity and intellectual functions
as children given mothers milk. Even if the effects of DHA
in the brain is not fully elucidated, it is known that patients
with cognitive defects have less DHA in red blood cell membranes
and patients with Alzheimer’s dementia have significantly
less DHA in the brain tissue compared with normal age-matched
controls. DHA is even important for normal production of
sperms and since DHA is enriched in the myocardium as well
as in lung tissue we will very soon know that this marine
fatty acid has a series of vital functions and that deprivation
may cause disease and malfunction.
Omega-3
fatty acids are essential fatty acids meaning that they
have to be provided by food. However, till now it has been
generally accepted that the medium chain alfa-linolenic
acid (ALA) from linseed or flaxseed can be easily converted
to eicosapentaenoic acid (EPA) and DHA, the marine omega-3
fatty acids, meaning that intake of fish is not absolutely
necessary. The conversion of ALA to EPA and DHA is described
in Sprechers Pathway (Fig. 1)
Fig. 1
The rate limiting step is the delta-6 desaturation and this
enzyme as well as delta-5 desaturase may be subjected to
different capacities in different species. Till now we have
had information mainly from rodents but recently there has
been three publications addressing this issue in humans
and there has also been issued a consensus report from the
UK Food Standard Agency. These reports and the implications
of the conclusions will be discussed.
The
first study of two from the Institution of Human Nutrition
in Southampton examined the capacity for conversion of ALA
to EPA and DHA in young men (1). Emulsified ALA tagged with
the radioactive isotope carbon 13 was given in addition
to a habitual diet. Approximately 33% of administered radio
labelled ALA was recovered as CO2 on breath over the first
24 hours. The time scale of conversion of ALA to EPA indicated
that the liver was the main site of desaturation and elongation,
which has been shown previously in animals. However, there
was no enrichment of DHA in total plasma to be recorded
at any time point up to 21 days. The distribution of total
plasma omega-3 fatty acids over this time period was 84%
ALA, 7,9% EPA and 0% DHA.
The
authors concluded that even if some ALA seems to be converted
into EPA the extent of DHA synthesis is likely to be of
negligible biological significance. A conversion of ALA
to DHA in tissues where the fatty acid was not released
into plasma could, however, not be excluded. The nutritional
demands for DHA in healthy adults are likely to be modest
and the need for DHA may be satisfied by existing pools
of DHA within the body or preferably by eating fatty fish.
Since, however, elderly people with cognition defects have
low levels of DHA in red blood cells membranes, it seems
that these pools will have to be continuously replenished
in order to counteract a negative balance.
The
second study from Southampton was performed in women of
reproductive age with the same methodological set-up as
in the previous study (2). The results were slightly different
compared to what was found in men since the extent of ALA
conversion to DHA was greater in the women. A direct comparison
is difficult due to methodological reasons, but in this
study the fractional excursions of labelled fatty acids
in total plasma lipids was 21% for EPA and 9,2% for DHA.
These data suggest greater synthesis of both EPA and DHA
in women compared with men. It has been shown previously
that estrogens used for anticonception increase the conversion
of ALA to DHA (3). The capacity to regulate ALA conversion
by the action of sex hormones may contribute to the higher
DHA concentration in women. This may be important during
pregnancy and lactation in order to provide the foetus with
sufficient DHA for brain development and maturisation.
However,
a study from Oregon Health and Science University could
not find any increase of DHA in lactating women given flaxseed
oil, which is rich in ALA (4). Seven lactating women were
given this oil providing a daily intake of 10,7g ALA. Breast
milk and plasma ALA and EPA increased significantly while
no increase was observed for DHA. The authors concluded
that flaxseed oil supplementation would not be an adequate
method of increasing the availability of DHA for the developing
infant.
A workshop
initiated by the UK Food Standards Agency with the aim of
reviewing current research investigated whether ALA was
as beneficial to cardiovascular health as the omega-3 fatty
acids from marine sources (5). The conclusions from the
studies presented was that ALA supplementation either as
flaxseed oil or as fortified food such as margarine could
increase the levels of ALA and EPA, but hardly any, if at
all, DHA. The studies presented suggested little, if any,
benefit of ALA on risk factors for cardiovascular disease.
The effects previously observed with supplementation of
marine omega-3 fatty acids were not replicated by ALA supplementation.
The workshop had reservations about the evidence suggesting
beneficial effect of ALA on the secondary prevention of
coronary heart disease, and felt it was still needed to
be established.
The
industry in the US with commercial interest in the commercialisation
of flaxseed crop has during many years been heavily promoting
the use of ALA as a fully substitute for the marine omega-3
fatty acids, claiming that conversion from ALA to EPA and
DHA is efficient and reliable. Data on conversion from ALA
to EPA and DHA has been based on studies in rodents but
the yield of the Sprecher’s pathway from ALA to EPA,
and even more important DHA, is probably very different
between species, not least between men and mice. In the
promotion of food products and food supplements containing
ALA data from studies on EPA and DHA have been extensively
used. Now it seems that this arguments are not valid based
on recent studies in humans. Even if conversion from ALA
to EPA may be effective to a certain extent, DHA has to
be provided either by eating fatty fish or by taking marine
omega-3 dietary supplements. The claimed net effects of
providing ALA on primary prevention and secondary prevention
against cardio vascular disease seems to be non-existing
and further large clinical studies will have to be presented
before health claims of flaxseed and linseed oils can be
warranted.
REFERENCES
1)Burdge
GC, Jones AE and Wooton SA. Eicosapentaenoic and docosapentaenoic
acids are the principal products of alfa-linolenic acid
metabolism in young men. Br J of Nutr 2002;88:355-363
2)Burdge GC and Wooton SA. Conversion of alfa-linolenic
acid to eicosapentaenoic, docosapentaenoic and docosahexaenoic
acids in young women. Br J of Nutr 2002;88:411-420
3)Silfverstolpe G, Johnson P, Samsice G, et al. Effects
induced by two different estrogens on serum individual phospholipid
and serum lecitin fatty acid composition. Hormone and Metabol
Res 1981;13:141-145
4)Francois CA, Connor SL, Bolewicz LC, et al. Supplementing
lactating women with flaxseed oil does not increase docosahexaenoic
acid in their milk. Am J Clin Nutr 2003;77:226-233
5)Sanderson P, Finnegan YE, Williams CM, et al. UK Food
Standards Agency alfa-linolenic acid workshop report. Br
J of Nutr 2002;88:573-579
back
to top
|
-
-
Omega-3 linked to lower colorectal cancer risk
-- read
-
Omega-3
greater anti-inflammatory potential
-- read
- Marine
omega-3 fatty acids are the preferred source of EPA and
DHA in humans
-- read
-
Quality
and Safety of omega-3 products
-- read
-
Omega-3
Fatty Acids
-- read
-
Preventing
skin damage in the sun
-- read
- Preventing
obesity and obesity-related diseases
-- read
- Pilot
studies of ethyl-EPA in the treatment of anorexia nervosa
-- read
- A
vast range of applications
-- read
- Research
in various health fields
-- read
|
