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Update on alpha-linolenic acid
Aliza H Stark, Michael A Crawford, and Ram Reifen
Consumption of omega 3 fatty acids is known to have health benefits. For many
years, the importance of the only member of the omega 3 family considered to be
essential, alpha-linolenic acid (ALA), has been overlooked. Current research indicates
that ALA, along with its longer chain metabolites, may play an important role in
many physiological functions. Potential benefits of ALA include cardioprotective
effects, modulation of the inflammatory response, and a positive impact on both
central nervous system function and behavior. Recommended levels for ALA intake
have been set, yet the possible advantages of its consumption are just being
revealed.
© 2008 International Life Sciences Institute
INTRODUCTION
It is now recognized that in humans a-linolenic acid
(ALA 18:3w) an omega-3 (n-3) fatty acid, is an essential
fatty acid (EFA) that cannot be synthesized by the body
and therefore must be supplied by dietary sources.
However, this recognition has been long in gestation. In
1976, Cuthbertson
1
reviewed the requirements for infant
formula and claimed that alpha-linolenic acid was not
essential and only linoleic acid was required to replace
breast milk. His claim was challenged by Crawford et al.
2
who argued that the evidence available at that time not
only favored the essentiality of alpha-linolenic acid but
also the independent need for arachidonic acid and
docosahexaenoic acid for infants. The position of the
essentiality of ALA was confirmed by the 1978 World
Health Organization/Food and Agriculture Organization
Expert Consultation on the Role of Dietary Fats and Oils
in Human Nutrition.
3
For many years there was little
interest in ALA and issues were raised concerning the
danger of consuming highly unsaturated fatty acids that
were susceptible to peroxidation. Holman
4
argued
that omega-3 fatty acids, although susceptible to peroxi-
dation, were in practice, themselves protective. His
conclusions have recently been confirmed by the discov-
ery of neuroprotectins, powerful antioxidants derived
from docosahexaenoic acid.
5
ALA is abundant in certain plant foods including
walnuts, rapeseed (canola), several legumes, flaxseed, and
green leafy vegetables.
6
ALA is the precursor of three
important longer-chain n-3 fatty acids, eicosapentaenoic
acid (EPA 20:5w3), docosapentaenoic acid (DPAw3
22:5w3), and docosahexaenoic acid (DHA 22:6w3), which
have vital roles in brain development and function,
cardiovascular health, and inflammatory response.
7–10
Omega-3 fatty acids are incorporated into the membrane
lipid bilayer in virtually all body cells and affect mem-
brane composition, eicosanoid biosynthesis, cell signaling
cascades, and gene expression.
11
EPA, DPAw3, and DHA are found in large quantities
in fish oils. Most attention has been given to EPA and
DHAwithDPAw3 being ignored. However, DPAw3isthe
main ALA metabolite in the cell membranes of most large
land mammals with the exception of Homo sapiens.
12,13
EPA and DHA derived from fish oil have been investi-
gated widely, but there have been significantly fewer
studies on ALA from plants. In a recent evidence-based
systematic review of the health effects of omega-3 fatty
Affiliations: AH Stark and R Reifen, The Hebrew University of Jerusalem, Faculty of Agricultural, Food and Environmental Quality Sciences,
Institute of Biochemistry, Food Science and Nutrition, School of Nutritional Sciences, Rehovot, Israel. MA Crawford, London Metropolitan
University, Institute of Brain Chemistry and Human Nutrition, London, United Kingdom.
Correspondence: AH Stark, The Hebrew University of Jerusalem, Faculty of Agricultural, Food and Environmental Quality Sciences, Institute
of Biochemistry, Food Science and Nutrition, School of Nutritional Sciences, P.O. Box 12, Rehovot, 76100 Israel. E-mail: stark@agri.huji.ac.il,
Phone: +972-8-9489612, Fax: +972-8-9363206.
Key words: alpha linolenic acid, essential fatty acids, omega 3 fatty acids.
Special Article
doi:10.1111/j.1753-4887.2008.00040.x
Nutrition Reviews® Vol. 66(6):326–332326
acids,
14
166 studies were included that examined the
impact on cardiovascular risk factors and clinical out-
comes. Only 18 of the studies looked at the effects of ALA.
Furthermore, for approximately one-third of the clinical
conditions examined, no studies at all evaluated ALA.
This is unfortunate as ALA may have independent, thera-
peutic properties similar to those of other n-3 fatty acids.
It may also be of particular importance for neural devel-
opment.
15
Land-based sources of ALA are varied and
plentiful while the seafood stocks are under threat. Thus,
it seemed worthwhile to review the literature on the value
of ALA in the human food chain.
ABSORPTION AND METABOLISM OF ALA
As a rule,dietary fats are absorbed very efficiently from the
digestive tract, and ALA is no exception. Burdge
16
recently
reported that absorption levels of ALA are 96% or more.
There are several possible metabolic fates for ALA that
enters the bloodstream. The body can store the fatty acids
in adipose tissue, use them for acetyl-CoA or energy
production through b-oxidation, synthesize other
non-essential saturated or monounsaturated fatty acids
(MUFA),or convert them to longer-chain n-3 polyunsatu-
rated fatty acids (PUFA) in the liver. The activity of the
desaturation/elongation pathway is of unique importance
as it is responsible for the synthesis of EPA and DHA.What
is presently understood concerning conversion of ALA to
EPA,DPAw3,and DHA is that the first step in the pathway,
addition of a fourth double bond by D 6 desaturase, is
considered to be the rate-limiting step. This is followed by
elongation (addition of two carbon atoms) and an addi-
tional desaturation by the enzyme D 5 desaturase, with the
product being 20:5n-3 or EPA (Figure 1).Several possibili-
ties have been suggested for the precise pathway for pro-
duction of DHA (22:6n-3) from EPA in humans. It was
assumed that the conversion of DPAw3 to DHA would be
carried out by a D4 desaturase but, thus far, little or no D4
desaturase has been found. Sprecher
17
has provided evi-
dence for elongation of 22:5w3 to 24:5w3,whichisthen
desaturated by the rate limiting D 6 desaturase to yield a
24:6w3. Two carbons are then cleaved in the peroxisomes
to yield DHA that is then exported to the reticulo-
endothelial system. Thus, the insertion of the last double
bond in DHA production in human metabolism may be
rather indirect and somewhat inefficient. There are some
doubts as to the validity of the Sprecher shunt, but it
provides a possible explanation for the small proportions
of DHA produced and the build up of DPAw3, despite
ample ALA available in foods and in body tissues.
Alpha-linolenic acid is partially converted to EPA in
humans (8–20%), while conversion rates of ALA to DHA
are estimated at 0.5–9%.
18,19
The sex difference in metabo-
lism is well known. Studies in women of reproductive age
showed a substantially greater (2.5-fold) rate of conver-
sion of ALA to EPA than that measured in healthy men.
Thus, the ability to produce long-chain metabolites is
gender dependent. It appears that women have a lower
partitioning of ALA to b-oxidation, leaving more of it
available for conversion to EPA.
16,20
Other possible expla-
nations include a direct effect of estrogen on conversion
Figure 1 Metabolism and dietary sources of omega-6 and omega-3 fatty acids.
Nutrition Reviews® Vol. 66(6):326–332 327
rates.
16,21
Gender differences have also been observed in
the conversion rates of ALA to DHA. In males it is esti-
mated that only 0.5–4% of ALA is converted to DHA
while in females the rates are thought to be as high as
9%.
18,19
It is hypothesized that demands for DHA by the
fetus during pregnancy may stimulate female physiology
to more readily synthesize this fatty acid.
n-6:n-3 Ratio
Dietary patterns have changed over time and in many
developed nations consumption of n-6 fatty acids has
risen dramatically. According to Simopoulos,
22
in the
past, the ratio of n-6 to n-3 essential fatty acids was 1 : 1.
However, in the modern Western diet the ratio is approxi-
mately 15 : 1. The n-6 and n-3 fatty acids are metabolized
by the same set of enzymes to their respective long chain
metabolites. The competition for these enzymes between
the omega 6 and omega 3 as well as between EFAs and
other fatty acids has been known since 1966.
23
It has been
shown that when ALA intake in the diet is increased, an
increased proportion of both ALA and EPA is found con-
sistently in both plasma and cell lipids.
7,20,23
In addition, it
is thought that a higher relative intake of n-6 fatty acids
increases production of arachidonic acid (20:4n-6), which
in turn is used to produce pro-thrombotic and pro-
inflammatory n-6 metabolites.
24,25
Metabolites of n-3
origin are anti-inflammatory and anti-arrhythmic.
8,19,24,25
A high n-6 : n-3 ratio is thought to promote the patho-
genesis of many diseases, including cardiovascular
disease, cancer, osteoporosis, and inflammatory and
autoimmune diseases.
26–28
CARDIOPROTECTIVE EFFECTS OF ALA
In systematic reviews of the literature, numerous well-
designed studies have been carried out to support the
protective role of long-chain n-3 polyunsaturated fatty
acids derived from fish in coronary heart disease
(CHD).
10,14
However, the data concerning the role of ALA
is less definitive. On one hand, Balk et al.
14
described the
evidence for a cardioprotective role of ALA as “inconclu-
sive” and Wang et al.
10
stated there is no high-quality
evidence to support the role of ALA in preventing myo-
cardial infarction and cardiac death. In contrast, a meta-
analysis that looked at five prospective cohort studies
reported that high ALA intake was associated with
reduced risk of fatal heart disease (combined relative risk
0.79, 95% CI 0.60–1.04).
29
There are several studies that
strongly indicate that ALA may indeed be important in
maintaining heart health. Because ALA is a precursor of
the longer chained n-3 fatty acids, it contributes to the
body pools of EPA and DHA that have been associated
with improved vascular tone, heart rate, serum lipid
levels, platelet function, inflammatory responses, arrhyth-
mia, growth rates of atherosclerotic plaques, and blood
pressure.
26,27,30
Most of these beneficial effects have also
been associated specifically with ALA consumption.
Interest in the role of ALA in the prevention of heart
disease arose following the publication of the Lyon Heart
Study carried out in France.
31
This study provided the
first evidence that ALA consumption may play an impor-
tant role in reduction of heart disease. In this secondary
prevention trial, which included 608 patients with known
CHD, participants in the intervention group were
encouraged to follow the Mediterranean diet and were
supplied with canola margarine rich in ALA. At
27-month follow-up, a 73% reduction in cardiovascular
events was observed along with a 70% decrease in overall
mortality.ALA intake in the intervention group was three
times higher than that in the control subjects. Although it
appears that ALA may have contributed to the improve-
ment in cardiac health, there were numerous indepen-
dent variables in the study, which makes it impossible to
isolate the effects of ALA per se.
Mozaffarian
26
reviewed the findings from observa-
tional studies that evaluated the relationship between
ALA intake and risk of CHD or mortality. In most
studies, particularly those in the American population, an
inverse association between ALA consumption and inci-
dence of CHD was found. For example, the Cardiovascu-
lar Health Study,
32
which included a prospective cohort of
close to 6000 older men and women, found that each one
standard deviation increase in plasma phospholipid ALA
levels (a biomarker of ALA intake) was associated with a
52% decrease in risk for fatal CHD. In The Nurses Health
Study,
33,34
over 75,000 women were followed using dietary
intake records to assess ALA consumption. ALA intake
was associated with a significantly lower risk of sudden
cardiac death. These results help support the hypothesis
that ALA may have antiarrhythmic properties. Data from
another large cohort, the National Heart, Lung, and Blood
Institute Family Heart Study,
35
included measurement of
calcified atherosclerotic plaque in the coronary arteries
(CAC) of 2004 participants aged 32 to 93 years.When the
lowest to highest quintiles of ALA consumption were
correlated to CAC, it was found that consumption of ALA
was associated with a lower prevalence of CAC in a dose-
response fashion.
It is interesting to note that in the Health Profession-
als Study
25
(a prospective cohort of 45,722 men), the
strongest evidence linking ALA intake and CHD risk
was observed in participants that consumed very little
seafood. In men who consumed <100 mg of long-chain
n-3 (EPA + DHA), each 1 g/day ALA intake was associ-
ated with a 58% lower risk of nonfatal heart attack and a
47% lower risk of CHD. These data strongly support the
direct role of ALA consumption in decreasing CHD risk
Nutrition Reviews® Vol. 66(6):326–332328
and indicate that intake of plant sources of n-3 fatty acids
may be of particular importance in sectors of the popu-
lations that do not eat fatty fish.
Despite the highly convincing results reported in
these studies, it must be noted that when biomarkers of
intake are not measured, ALA consumption is deter-
mined entirely by self-report. The wide use of food fre-
quency questionnaires to estimate dietary intake is
problematic as they provide only semiquantitative data
and are characterized by large random error.
36
The inac-
curacies of assessing dietary exposures are well-known,
and cannot be ignored when evaluating the reliability of
nutritional studies.
Experimental studies have also been carried out to
assess the specific effects of diets enriched with ALA on
blood clotting. Freese et al.
37
showed that in healthy men,
supplementing the diet with canola oil for 6 weeks (2.3%
energy from ALA) reduced in vitro platelet aggregation.
An additional study determined platelet composition and
function in individuals that consumed either 40 g/d flax-
seed oil (a rich source of ALA) or equal amounts of sun-
flower seed oil.
38
Consumption of flaxseed doubled EPA
levels in platelets and significantly decreased aggregation
response. These results are thought to be beneficial in
preventing thrombosis connected to CHD.
It is difficult to determine the exact impact of ALA on
heart health because very few studies have focused exclu-
sively on ALA. However, available data consistently
support a beneficial effect. Why, then, is there a seeming
lack of consensus between the individual studies
described here and the results of large systematic
evidence-based reviews? This is most likely due to the fact
that there are only a small number of well-designed, high-
quality studies investigating the role of ALA in cardiovas-
cular disease. Large-scale randomized trials with ALA
have not been carried out and observational studies alone
cannot decisively establish causal relationships. Recom-
mendations from a workshop sponsored by the National
Heart, Lung, and Blood Institute and Office of Dietary
Supplements on omega-3 fatty acids and cardiac arrhyth-
mogenesis
39
specified the need for more epidemiological
data on ALA intake and sudden cardiac death and cardiac
arrest, including large-scale, double-blind randomized
trials with well-defined end points. Although the data
available today is not conclusive, the continual appear-
ance of new studies and professional opinions of scien-
tists from around the world
19,26,30,35,40-42
support the ever-
growing body of research that ALA has cardioprotective
effects in its own right.
THE ROLE OF ALA IN INFLAMMATION
Inflammation is a central pathological component in
CHD disease. Other diseases such as rheumatoid arthritis,
psoriasis, Crohn’s disease, chronic obstructive pulmonary
disease, and irritable bowel syndrome (IBD) are also char-
acterized by high levels of inflammation.
28
It is thought
that n-3 fatty acids are able to reduce disease-promoting
inflammatory responses.
43
Eicosanoids,a class of bioactive
molecules, act as inflammatory mediators and play a role
in platelet aggregation. They are produced from both the
n-3 and n-6 fatty acid families and include leukotrienes,
prostaglandins and thromboxanes.
8
Eicosanoids derived
from n-3 fatty acids are considered to be antithrombotic,
anti-inflammatory, and vasodilating.
19,42,44
These physi-
ological effects are attributed, for the most part, to the
longer chained n-3 fatty acids (EPA and DHA), but some
research has looked specifically at the effects of ALA.
In order to assess the role of ALA on systemic inflam-
mation,men suffering from dyslipidemia were provided a
high-ALA diet (canola oil 15 ml/day) or a safflower oil
control diet.
45
After 3 months, in individuals consuming
the canola oil diet, systemic inflammation was signifi-
cantly reduced, as measured by C-reactive protein, serum
amyloid A, and interleukin-6. Additional evidence that
ALA consumption affects biomarkers of inflammation
and endothelial activation was observed in a cross-
sectional study of 727 women in the Nurses Health
Study.
46
An inverse relationship was observed between
ALA intake and plasma concentrations of C-reactive
protein,IL-6, E-selectin, soluble intercellular cell adhesion
molecule 1, and soluble vascular cell adhesion molecule 1.
This provides support for the role of ALA as an anti-
inflammatory agent that also lowers endothelial activa-
tion. Ferrucci et al.
47
also reported that higher levels of
ALA in the blood were associated with lower levels of
inflammatory biomarkers. His study included 1123 indi-
viduals, aged 20–98 years. Fatty acids in fasting plasma
were analyzed in relation to numerous inflammatory
markers.Decreased levels of C-reactive protein and IL-1ra
levels were associated with higher plasma ALA levels.
Zhao et al.
48
have shown that dietary ALA elicits anti-
inflammatory effects by inhibiting IL-6, IL-1beta, and
TNF-alpha production in peripheral blood mononuclear
cells cultured from hypercholesterolemic individuals
exposed to a diet high in ALA (6.5% of energy). Thus,
consumption of ALA may provide protection against
heart disease and other inflammatory diseases by reduc-
tion of inflammatory cytokines.
OTHER POTENTIAL BENEFITS OF ALA CONSUMPTION
ON THE CENTRAL NERVOUS SYSTEM, BEHAVIOR, AND
IN AUTOIMMUNE DISEASES
New studies continually appear suggesting that ALA may
be of physiological importance in a wide range of disor-
ders and diseases.
Nutrition Reviews® Vol. 66(6):326–332 329
Attention deficit hyperactivity disorder: Arecent
study by Joshi et al.
49
reported a significant improvement
in the symptoms of ADHD in children that had received
flax oil and Vitamin C supplements.
Neuroprotection: Lauritzen et al.
50
reported that ALA
prevented neuronal death in an animal model of transient
global ischemia; it also protected animals treated with
kainate against seizures and hippocampal lesions. An
additional study reported neuroprotective effects in rat
models of spinal cord ischemia leading to paraplegia.
51
ALA treatment preserved neurological function and
animals sustained only mild-to-moderate injury. This
suggests that ALA can induce protection against ischemia
in spinal injury, preventing necrosis and apoptosis of
motor neurons.
Autoimmune diseases: Work in animal models by
Reiffen et al.
52
suggests that the addition of flaxseed oil
(70% ALA) to the diet may attenuate the severity of sys-
temic lupus erythematosus (SLE or lupus). SLE mice fed
flaxseed exhibited lower titers of antibodies to DNA and
to cardiolipin and less severe kidney damage than mice
fed other diets, including fish oil.
POSSIBLE ADVERSE EFFECTS OF ALA CONSUMPTION
In the many publications dealing with omega-3 fatty
acid consumption, very few adverse effects have been
reported aside from mild gastrointestinal symp-
toms.
10,14,19
However, concern has arisen from one meta-
analysis that reported an increased incidence of prostate
cancer risk in men with high intake or high blood levels of
ALA (combined relative risk 1.70; 95% CI 1.12–2.58).
29
The data presented was heterogeneous and the authors
themselves stated it is uncertain if the results present a
genuine effect. The results have yet to be confirmed. More
recent reviews did not confirm or refute this possible
deleterious effect of ALA.
14
RECOMMENDATIONS FOR ALA INTAKE
The present dietary reference intakes state that to achieve
nutritional adequacy, ALA should provide 0.6–1.2% of
energy with up to 10% provided by longer chained fatty
acids.
53
In contrast to ALA, which is essential, EPA and
DHA have no minimum requirements, and in large doses,
longer chain n-3 fatty acids present a health risk. Recom-
mendations by the European Commission for fatty acid
composition in infant formulas require that ALA be
included at levels of at least 50–100 mg/100 kcal. Here
too, there is no set minimum for EPA or DHA, but
maximum safe levels have been established.
54
The fact
that several major scientific and medical associations
have published nutritional guidelines including recom-
mendations specifically for ALA emphasizes its perceived
importance in health promotion and disease prevention.
CONCLUSION
Based on the studies and research presented here, it can
be concluded that increasing ALA in the daily diet is a
safe, viable option for meeting dietary requirements and
maintaining the suggested n-6 : n-3 ratio. Foods naturally
rich in ALA should be included in the diet and manufac-
turers can help the public meet dietary recommendations
by increasing ALA content in processed foods. Results of
many of the studies described here indicate that ALA is
not only essential, it also has therapeutic properties. Nev-
ertheless, it is clear that additional well-designed research
is needed in order to establish unequivocal support for
health claims.
Although there is some interconversion of the
omega 3 fatty acids, each fatty acid has it own place in
biology. It is important to remember that of the omega-3
fatty acids, ALA is the parent molecule, and greater atten-
tion should be paid to its independent physiological
function.
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