Arachidonic acid (AA) is a fatty acid of the omega-6 class, and is the main fatty acid of interest when referring to an omega-3:6 ratio (relative to fish oil fatty acids). It is proinflammatory and immunosupportive.
Sources and Structure
Arachidonic acid (AA) is the most biologically relevant omega-6 fatty acid, and in the lipid membrane of a cell serves as the fatty acid that is compared against the two fish oil fatty acids (EPA and DHA) in constituating an omega-3:6 ratio. Recent data suggests a daily intake of 50-250mg of arachidonic acid with some sources estimated levels of up to 500mg daily; arachidonic acid intake appears to be lower in vegetarians
Dietary sources of arachidonic acid include:
- Lean beef (460mg/kg)
- Raw beef ribeye (460mg/kg raw, 200mg/kg cooked; 1.1% total fatty acids)
- Whole eggs (2,390mg/kg raw, 1,490mg/kg cooked; 1.9% total fatty acids)
- Chicken breast (640mg/kg raw, 400mg/kg cooked; 4.9% total fatty acids)
- Chicken thigh (1060mg/kg raw; 2.9% total fatty acids)
- Turkey breast (590mg/kg raw, 300mg/kg cooked; 3.1% total fatty acids)
- Pork loin (530mg/kg raw, 300mg/kg cooked; 2.2% total fatty acids)
- White tuna (packed in water and 330mg/kg;)
- Duck (990mg/kg)
- Kangaroo (620+/-120mg/kg)
- Emu (1,300+/-300mg/kg)
- Kidney (Lamb; 1,530+/-110mg/kg)
- Liver (Oxen; 2,940+/-640mg/kg)
- Barramundi (260+/-60mg/kg)
- Salmon (1,000+/-920mg/kg)
Arachidonic acid is present in the visible fat ('grisle') of meat products at similar levels as the meat and despite the above numbers it is not clear what occurs to arachidonic acid during the cooking process. Some studies note an increase in total fatty acids on a weight basis during the cooking process while others note no significant differences (relative to other fatty acids).
Arachidonic acid is found naturally occurring in the diet, mostly through animal products. If dietary arachidonic acid is not ingested, linoleic acid (the parent omega-6 fatty acid, also found in animal products) can be used to create arachidonic acid in the body
Concentrations of AA in the body follow a non-linear dose-dependent relationship with dietary linoleic acid (the parent omega-6 fatty acid) where human diets consisting of less than 2% linoleic acid experience increases in plasma arachidonic acid when supplementing extra linoleic acid but those at 6% or more ('standard' western diet) do not. Conversely, dietary arachidonica acid itself dose-dependently increases plasma arachidonic acid.
Dietary linoleic acid (the parent omega-6 fatty acid) can increase plasma arachidonic acid levels, which is how omega-6 fatty acids mediate their effects. There appears to be a limit cap in place, and supplementing arachidonic acid circumvents it and dose-dependently increases plasma arachidonic acid concentrations
Reducing arachidonic acid in the diet slightly (244% instead of 217%) increases the amount of EPA stored in the membranes of erythrocytes (from supplementation of fish oil) with no influence on DHA.
Arachidonic acid is the reason for linoleic acid (dietary omega-6 fatty acid) having the status of an essential fatty acid, as the latter is required in the diet for synthesis into the former.
Additionally, arachidonic acid may be produced as a catablite of anandamide (one of the main endogenous cannabinoids acting upon the cannabinoid system, also known as arachidonoylethanolamide) via the FAAH enzyme and may share some similar properties as anandamide such as acting upon TRPV4 receptors. The endocannabinoid 2-arachidonoylglycerol can also be hydrolyzed into arachidonic acid via monoacylglycerol lipase or similar esterases.
Arachidonic acid may also be produced in the body via the breaking down of endocannabinoids
In aging rats and humans, there appears to be less bodily and neuronal stores of arachidonic acid (in plasma membranes) associated with less activity of the biosynthetic enzymes that convert linoleic acid into arachidonic acid.
Arachidonic acid appears to be reduced in older subjects relative to younger subjects due to less conversion of dietary linoleic acid into arachidonic acid
Bioactivation of Eicosanoids
Eicosanoids are fatty acid metabolites that are derived from either arachidonic acid (if belonging to the omega-6 class) or from both eicosapentaenoic acid and docosahexaenoic acid (EPA and DHA, the two fish oil fatty acids, if belonging to the omega-3 class).
DHA, EPA, and AA all tend to be in the middle of a triglyceride backbone (at the sn-2 binding position) and are thus liberated form the membrane when the enzyme known as phospholipase A2 is activated; when this enzyme is activated (seizures, ischemia, NMDA-receptor stimulation, as well and various inflammatory cytokines such as IL-1β, TNF-α, and PMA; basically cell stressors) and due to the indiscriminatory nature of the phospholipase A2 enzyme (releasing DHA/EPA and AA with similar efficacy) the amount of eicosanoids produced are dependent on the ratio of omega3:6 fatty acids in the cell membrane.
Eicosanoids are signalling molecules derived from long chain fatty acids, and the eicosanoids from arachidonic acid are liberated by the same enzyme that liberates the fish oil fatty acids. This is the stage that determines which eicosanoids will be used in cell signalling, and is the mechanism underlying the importance of a dietary ratio of omega3:6 fatty acids (as the eicosanoids released into a cell reflect the membrane ratio)
Similar to the fish oil fatty acids, arachidonic acid can follow one of three pathways after it is liberated from the membrane. It can go into:
- A COX-dependent pathway to produce PGH2 (the parent prostaglandin, and all prostaglandins are derived from this pathway); prostaglandins are signalling molecules with a pentacyclic structure (pentagon) in their fatty acid side-chain
- A LOX-dependent pathway to produce lipoxins and leukotrienes
- A P450 pathway, which can be further subject to either the epoxygenase enzyme (to produce epoxyeicosatrienoic acids or EETs) or the hydroxylase enzyme (to produce hydroxysaeicosatrienoic acids or HETEs)
Arachidonic acid can take one of three pathways after being liberated; the COX pathway (for prostaglandins), the LOX pathway (lipoxins and leukotrienes), or one of the two routes of the P450 pathway to make EETs or HETEs. All these classes of signalling molecules are known as omega-6 eicosanoids
After being liberated from the cell membrane by phospholipase A2, arachidonic acid is converted to Prostaglandin H2 (PGH2) via the Prostaglandin Endoperoxide H Synthases 1 and 2 (alternate names for the cyclooxygenase enzymes COX1 and COX2) and this process uses O2 to convert arachidonic acid to the unstable peroxide intermediate of PGG2 which then passively reconfigures into PGH2; PGH2 serves as a parent intermediate for all other AA-derived prostaglandins (a subset of eicosanoid). This first stage of eicosanoid synthesis is one of the reason for the antiinflammatory and antiplatelet effects of COX inhibitors (such as aspirin) which prevent AA eicosanoids from being made by reducing PGH2 production.
In regards to the enzymes that mediate this conversion, COX2 is the inducable form that can be activated in response to inflammatory stressors in as little as 2-6 hours in a variety of cells although it may be expressed at basal conditions in some cells (brain, testes, and the kidney cells known as macula densa) while COX1 is just generally expressed in all cells; it is due to this variation that COX2 is referred to as the inducible variant and COX1 the constitutive variant.
Arachidonic acid (AA) is liberated from the cell membrane by phospholipase A2, and then is converted into PGH2 (a prostaglandin) via one of the two COX enzymes. Inhibiting this stage inhibits production of all AA-derived eicosanoids, and after PGH2 is synthesized then it can branch out to other eicosanoids
PGH2 can be converted to Prostaglandin D2 via the enzyme prostaglandin D synthase (in the presence of sulfhydryl compounds) and PDG2 is most well known to signal via the DP2 receptor (initially discovered on T cells and named CRTh2 and also referred to as GRP44 and coupled to a Gi or G12 protein). In this sense and via signalling through its receptor, PGD2 is biologically active.
PGD2 can be converted to PGF2α which can bind to its own receptor (PGF2α receptor) as well as the DP2 receptor, albeit 3.5-fold weaker than PGD2. An isomer of PGF2α known as 9α,11β-PGF2 can also be produced from PGD2 and is equivalent in potency on the DP2 receptor.
PGH2 can be converted to the prostaglandin D2, which is one of the few metabolic 'branches' of prostaglandins. After conversion into PGD2, further metabolism into 9α,11β-PGF2 and PGF2α can occur and these three molecules all possess similar effects
PGH2 (parent prostaglandin) can also be converted into prostaglandin E2 (PGE2) via the enzyme PGE synthase (of which there are a few, the membrane bound ones mPGES-1 and mPGES-2 and the cytosolic one cPGES), and further metabolism of PGE2 results in PGF2. Interestingly, selective inhibition of the inducible enzyme (mPGES-1) appears to attenuate production of PGE2 without affecting concentrations of other prostaglandins downstream of PGH2 while indiscriminately suppressing COX enzymes suppresses all prostaglandins, and inhibiting PGE2 production causes a slight recompensation and increase in PGI2 levels (via COX2).
There are four receptors for prostaglandin E2 named EP1-4, which are all G-protein coupled receptors. EP1 is coupled to the Gq/11 protein and its activation can increase activity of phospholipase C (producing IP3 and diacylglycerol and thus activating PKC). The EP2 and EP4 receptors are both coupled to the Gs protein and activate adenyl cyclase (creatine cAMP and activating PKA). EP3 receptors appear to be a tad more complex (being spliced into alpha, beta, and gamma variants; EP3α, EP3β, and EP3γ) but are all coupled to Gi which suppresses the activity of adenyl cyclase (and thus opposes EP2 and EP4) except EP3γ is coupled to both Gi and Gs proteins (inhibiting and activating adenyl cylase).
The group of enzymes known as PGE synthases, but particularly mPGES-1, converts the parent prostaglandin into PGE2 which serves a role in promoting inflammation and pain perception. PGE2 activates the prostaglandin E receptors (EP1-4)
PGH2 (parent prostaglandin) may be subject to the enzyme prostacyclin synthase and be converted into the metabolite known as prostacyclin or PGI2, which can be further converted into 6-keto-PGF1α (and then converted to the urinary metabolite known as 2,3-dinor-6-keto Prostaglandin F1α). PGI2 is known to activate the I prostanoid receptor (PI) which is expressed in the endothelium, kidneys, platelets, and brain.
Prostacyclin production attenuates the pro-platelet function of thromboxanes (next section).
PGH2 can be converted to PGI2, which is also called prostacyclin, and then this prostaglandin signals via the PI receptor
Somewhat unrelated to the prostaglandin class but still derived from the parent prostaglandin, when PGH2 is subject to the enzyme known as thromboxane synthase it is converted into Thromboxane A2. Thromboxane A2 (TxA2) signals through the the T prostanoid receptors (TP) which is a G-protein coupled receptor with two splice variants (TPα and TPβ) coupled to Gq, G12/13
Thromboxane A2 is best known for being produced in activated platelets from when the platelet is stimulated and arachidonic acid is released and its suppression by COX inhibitors (namely aspirin) underlies the antiplatelet effects of COX inhibition.
Thromboxane A2 is a metabolite of the parent prostaglandin (PGH2) that acts upon the T prostanoid receptors, it is most well known to being very pro-platelet formation and exacerbating blood clotting (and inhibition of Thromboxane A2 underlies the antiplatelet benefits of aspirin)
Epoxyeicosatrienoic acids (EETs) are eicosanoid metabolites that are produced when arachidonic acid is subject to the P450 pathway and then immediately subject to the epoxygenase enzyme; hydroxyeicosatrienoic acids (HETEs) are also metabolites of the P450 pathway, but subject to a hydroxylase enzyme instead of the epoxygenase enzyme.
The HETEs include mostly 19-HETE and 20-HETE.
The EETs include 5,6-EET (converted to 5,6-DHET via the soluble epoxide hydroxylase enzyme), 8,9-EET (also converted, but to 8.9-DHET), 11,12-EET (into 11,12-DHET), and 14,15-EET (14,15-DHET).
The P450 pathway mediates both EET and HETE synthesis
In the LOX pathway (to confirm, prostaglandins are due to the COX pathway and both EETs and HETEs due to the P450 pathway) the major eicosanoid metabolites are the leukotrienes. Arachidonic acid is directly converted by the LOX enzyme into the novel metabolite 5-hydroperoxyeicosatrienoic acid (5-HPETE) which is then converted into Leukotriene A4.
Leukotriene A4 can take one of two routes, either conversion into Leukotriene B4 (LTB4) via the addition of a water group, or conversion into Leukotriene C4 via glutathione S-transferase. If it is converted into the C4 metabolite, it can be further converted into Leukotriene D4 and then Leukotriene E4.
Leukotrienes may be made near the nucleus.
The LOX pathway tends to mediate leukotriene synthesis
240-720mg arachidonic acid to elderly persons for 4 weeks was able to increase plasma membrane arachidonic acid concentration (at 2 weeks with no further effect at 4 weeks) but urinary metabolites and serum PGE2 and lipoxin A4 were not significantly influenced.
Arachidonic acid supplementation at rest does not necessarily increase plasma level of eicosanoid metabolites despite an increase in arachidonic acid stores
Autism spectrum disorders is a neurological condition associated with impaired social functioning and communication. Arachidonic acid has been investigated as, as well as DHA from fish oil, AA is critical for neuronal development in neonates and disturbances in polyunsaturated fatty acid metabolism have been associated with autism disorders (somewhat unreliably).
Supplementation of 240mg AA and 240mg DHA (with 0.96mg astaxanthin as antioxidant) over 16 weeks in 13 patients with autism (half the dose if aged 6-10yrs) failed to find any reduction in scores on the SRS and ABC rating scales for autism although there was improvement in the subscales of social withdrawal (ABC) and communication (SRS) although the percentage of patients experiencing a 50% reduction in symptoms was not significantly different than placebo.
Very limited evidence to support the role of arachidonic acid and fish oil DHA in the attenuation of autism symptoms, but there is a small bit of promise in aiding social symptoms that requires larger future trials
Memory and Learning
Phospholipase A2 activation has been noted to promote neurite outgrowth following neuronal injury and axon elongation. The aforementioned implications eicosanoids (of both arachidonic acid and fish oil origin, mostly DHA) and arachidonic acid has been noted to promote neurite outgrowth via the 5-LOX pathway with maximal efficacy around 100µM although at higher concentrations (10mM) this same pathway is neurotoxic via excessive oxidation (prevented with Vitamin E). The neurite outgrowth may be related to acting upon calcium channels.
In the body, arachidonic acid has a role in promoting neuronal development and elongation although unnaturally high concentrations of arachidonic acid appear cytotoxic
It has been noted that, in rats, the activity of the enzymes that convert linoleic acid into arachidonic acid are reduced with aging and in aged rats supplementation of arachidonic acid to the diet appears to promote cognition which has been replicated in otherwise healthy elderly men with 240mg AA (via 600mg triglycerides) as assessed by P300 amplitude and latency.
Due to less production of arachidonic acid during the aging process, supplemental arachidonic acid may have a cognitive enhancing role in older persons (not clear if this would also extend to youth, seems unlikely)
Activation of phospholipase A2 has been implicated in being a link between immune cells and demyelination of neurons possibly via COX dependent mechanisms, as celecoxib (COX2 inhibitor) has been noted to improve neuronal healing rates. This implicates eicosanoids of both omega-3 and omega-6 origin.
Arachidonic acid (4.28% of the rat diet) appears to fully reverse the age-related increase in vasoconstriction induced by phenylephrine in rats by an endothelial-dependent means and is able to slightly augment an acetylcholine-induced vasorelaxation effect; there was no apparent benefit to young rats. When tested in aged (65yrs average) humans, 240mg arachidonic acid paired with 240mg DHA (one of the fish oil fatty acids) for three months was able to improve coronary blood flow in periods of hyperemia but not at rest.
Arachidonic acid supplementation in older ages may be cardioprotective by promoting blood flow, although human evidence at this point is time is fairly weak
Skeletal Muscle and Performance
Arachidonic acid is thought to be important for skeletal muscle metabolism as the phospholipids in the membrane of the sarcoplasm are thought to be reflective of the diet, training itself appears to alter phospholipid content of the muscle (independent of muscle fiber composition and associated with a lower omega6:3 ratio) and eicosanoids from arachidonic acid interact with muscle protein synthesis via their receptors.
Arachidonic acid signals for muscle protein sythesis via a COX-2 dependent pathway (suggesting prostaglandins are involved) which is associated with increases in both prostaglandin E2 (PGE2) and PGF(2α), although incubation with isolated PGE2 or PGF(2α) doesn't appear to fully replicate the hypertrophic effects of arachidonic acid. PGE2 and PGF(2α) are also induced by exercise (specifically, a stretching of muscle cells in vitro) and has been noted both in serum and intramuscularly (four-fold, from 0.95+/-0.26ng/mL to 3.97+/-0.75ng/mL) of exercising subjects, which are normalized within an hour after working out. The ability of the stretch reflex to increase concentrations of PGE2 and PGF(2α) may merely be due to stretching increasing the activity of COX2.
It should be noted that supplementation of 1,500mg arachidonic acid (relative to a control diet containing 200mg) for 49 days has been noted to increase PGE2 secretion from stimulated immune cells (by 50-100%) in otherwise healthy young men, but the relevance to skeletal muscle of this study is not known. This study also noted that, without stimulation, there were no significant differences between groups. Elsewhere, a trend to increase serum PGE2 concentrations has been noted at rest in trained men given 1,000mg arachidonic acid for 50 days.
Arachidonic acid, via the eicosanoids known as PGF(2α) and PGE2, stimulate muscle protein synthesis. They are produced from arachidonic acid, but normally do not form their respective muscle building eicosanoids until the cell is stimulated by a stressor (such as the stretch reflex on a muscle cell) which then induce their production
The receptor for PGF(2α) (FP receptor) appears to be upregulated by COX1 inhibitors (acetaminophen used in this study) and enhanced signalling of PGF(2α) is thought to underlie the improvement in muscle protein synthesis seen in older individuals with antiinflammatory drugs. Supplementation of arachidonic acid does not appear to affect the amount of FP receptors in youth; while exercise itself can increase EP3 receptor content neither COX1 inhibitors nor arachidonic acid appear to further influence it.
However, usage of COX2 inhibitors (in youth) have been found to abolish the exercise-induced increase of PGF(2α) (Ibuprofen and Acetaminophen) as well as PGE2, which is thought to be due to conversion from PGH2 into these metabolites being reliant on COX2 activity.
Due to the production of these eicosanoids being dependent on the COX2 enzyme, inhibiting this enzyme is thought to reduce the anabolic effects of exercise when taken prior to
Arachidonic acid (as well as EPA from fish oil) have not been noted to impair glucose uptake into isolated muscle cells and 10μM of fatty acid is able to attenuate saturated fat-induced insulin resistance; a phenomena seen with saturated fats of 18 carbon chains or longer that doesn't appear to apply to polyunsaturated fatty acids of equal chain length and is likely related to increasing intracellular ceramides that impair Akt signalling and reduce GLUT4-mediated glucose uptake from insulin.
Arachidonic acid and omega-3 polyunsaturated fats are both associated with improved insulin sensitivity of muscle cells, which may be secondary to reducing the levels of saturated fats in the lipid membrane and thus reducing intracellular ceramide concentrations. It is possible that this is not related to eicosanoids or the omega-3:6 ratio
Exercise is known to release vasoactive metabolites which induce blood vessel relaxation of which alongside some common vasodilatory agents (nitric oxide, adenosine, hydrogen ions) prostanoids are also released. Arachidonic acid levels in serum are acutely suppressed by exercise (normalized within a few minutes) and there were increases in a few eicosanoids of arachidonic acid including 11,12-DHET, 14,15-DHET, 8,9-DHET, and 14,15-EET when cycling at 80% VO2 max acutely and higher 2,3-dinor-6-keto Prostaglandin F1α urinary concentrations (indicative of higher PGI2 and 6-keto-PGF1α concentrations) have been noted at rest after 4 weeks of training in previously untrained youth.
In 31 trained males subject to a standardized weight lifting program and diet (500kcal excess with 2g/kg protein) given either 1g arachidonic acid daily or placebo, supplementation over 50 days appeared to increase peak power (7.1%) and average power (3.6%) on a Wingate test but failed to positively influence muscle mass or weight lifting measures of power (bench press and leg press).
Bone Metabolism and the Skeleton
Inflammation and Immunology
In patients of rheumatoid arthritis, a reduction of dietary arachidonic acid (from 171mg to 49mg; eicosapentaenoic acid nonsignificantly increased) and linoleic acid (from 12.7g to 7.9g) is able to reduce pain symptoms of rheumatoid arthritis (14%) and was able to improve the efficacy of fish oil supplementation from 17% to 31-37%.
Limiting dietary intake of arachidonic acid appears to aid in symptoms of rheumatoid arthritis and appears to augment the efficacy of fish oil supplementation
Interactions with Hormones
Both total and free testosterone appear to be unaffected following 50 days supplementation of arachidonic acid in resistance trained men.
Interactions with the Lungs
It is thought that signalling through the DP-1 and DP-2 receptors mediate the pro-asthmatic effects of these prostaglandins as those are the receptors best known to respond and abolishing this receptor genetically is associated with reduced airway inflammation.
Arachidonic acid eicosanoids appear to be pro-asthmatic
Interactions with Aesthetics
Prostaglandin D2 (of arachidonic acid origin) and the enzyme that produces it (prostaglandin D2 synthase) are 10.8-fold higher in the scalp of men with androgenic alpoceria (receding hair line) relative to the parts of their heads with hair and appears to suppress hair growth via signalling through the DP2 receptor (also known as GPR44 or CRTh2), with the PGD2 receptor 1 not being associated with hair growth suppression and the prostaglandin 15-ΔPGJ2 also having suppressive effects. Overexpression of the enzyme is able to mimic androgenic alpoceria, suggesting that the enzyme is a therapeutic target and this enzyme is known to be highly responsive to androgenic signalling.
Prostaglandin D2 and its metabolites (produced from Prostaglandin H2 via the prostaglandin D2 synthase enzyme) are increased in the balding portion of androgenic alopecia relative to the haired portion, and the enzyme itself is increased in activity by androgens. Signalling via the DP2 receptor (named after prostaglandin D2) appears to suppress hair growth
There appears to be more prostaglandin E2 (PGE2) in the sections of the head in balding men with hair relative to bald areas (2.06-fold). Increasing PGE2 is thought to be one of the possible mechanisms of minoxidil in promoting hair growth.
Other prostaglandins derived from arachidonic acid
Safety and Toxicology
Arachidonic acid is known to be increase in breast milk following oral consumption (either via foods or supplementation, in a relatively dose-dependent manner) although supplemental DHA (fish oil) in isolation may reduce the concentration of arachidonic acid in breast milk. The increase has been quantified at 14-23% over 2-12 weeks (220mg arachidonic acid supplementation) while one week of 300mg arachidonic acid being insufficient to increase concentrations significantly. This apparent time delay may be due to most fatty acids coming from material stores rather than directly from the diet of the mother.
Arachidonic acid concentrations in breast milk tend to correlate with the diet, with low concentrations noted in some studies with lower dietary arachidonic acid intake in general and higher breast milk concentrations noted in studies where the study population tends to have higher arachidonic acid intake.
Arachidonic acid is known to accumulate in the breast milk of mothers, and its concentrations in breast milk appear to correlate with dietary intake.