What is a healthy ratio of omega-6 to omega-3? (2024)

PUFAs metabolism

Once consumed from the diet and inside human cells, PUFAs are stored in the phospholipids of the cell and organelle membranes or in glycerides and phospholipids of lipid bodies in human cells. When needed, these fatty acids are released from phospholipids by PLA2 (phospholipase A2) and are further converted to signaling molecules. Shorter chain PUFAs, LA and ALA, can be further processed to produce longer carbon chains and more double bonds by the same set of enzymes (enlogases and desaturases). However, omega-3 and omega-6 fatty acids are not inter-convertible in human and animals. All the longer chain products derived from omega-3 fatty acids remain omega-3, and omega-6 derivatives remain omega-6 (Fig.3). For example, AA (omega-6) can be synthesized from LA (omega-6), but not from ALA (omega-3) in the human body. Similarly, DHA (omega-3) and EPA (omega-3) can be synthesized from ALA (omega-3), but not from LA (omega-6). In addition, both types are also precursors of (and share the enzymes to produce) signaling molecules that work through cell surface receptors like the GPCRs (G protein coupled receptors) as well as through nuclear hormone receptor transcription factors directly to regulate processes related to cardiovascular function and inflammation response.

Figure 3. Metabolism of PUFAs in the human body. Dietary essential PUFAs, LA (ω-6) and ALA (ω-3), are further processed to become longer carbon chains with more double bonds by a series of reactions catalyzed by the same set of enzymes desaturase and elongase in the endoplasmic reticulum. The final β-oxidation step occurs in peroxisomes. The long-chain PUFAs, AA (ω-6), EPA (ω-3) and DHA (ω-3), are substrates for the production of signal molecules (colored coded purple to indicate pro-inflammation and blue for anti-inflammation) catalyzed by the enzymes Cox (cyclooxygenase) and Lox (lipoxygenase).

PUFA-derived signaling molecules

Overall, the signal molecules derived from omega-6 are pro-arrhythmic (irregular heart beat or muscle contraction) and pro-inflammatory while the signal molecules derived from omega-3 are anti-arrhythmic and anti-inflammatory (Table 3).

Table 3. Opposite effects of omega−3 and omega−6 derived signaling molecules.

Most of the signal molecules derived from PUFAs are eicosanoids, the 20-carbon atom (‘eicosa’ means 20 in Greek) signal molecules shown in Table 3, except for RVD and NPD1 which are DHA derived 22-carbon atoms signaling molecules. All of these signaling molecules are autocrines or paracrines that act locally on the cells or in the vicinity of the cells where they are manufactured. They activate different pathways either through GPCRs (G protein coupled receptors) on the cell surface or through nuclear hormone receptor transcription factors directly. Some of the signaling molecules are generated in most of the cells while others are cell type specific. The same signaling molecules can bind to different GPCRs to activate totally opposite pathways. For example, prostaglandin PGE2 causes bronchoconstriction (the constriction of airways in the lungs) when it binds to the receptor EP1 whereas the same molecule causes bronchodilation (the relaxation of the airways in the lungs) when it binds to the receptor EP2. Therefore, the effect of one signaling molecule on a whole body depends on its interaction with many other factors. A combination of different signaling molecules, different cell types and different GPCRs dictate the exact physiological effect.

Prostaglandins are a group of fatty acid derivatives that have 20 carbon atoms and include a 5-carbon ring structure. Historically it was believed these molecules were secreted by the prostate gland, hence named prostaglandins. It is now known that many other tissues secrete prostaglandins for various functions. In relevance to the topic discussed here, the omega-6 fatty acid AA derived PGI2 (also known as prostacyclin) and PGE2 have pro-arrhythmic effects, whereas the omega-3 fatty acid EPA-derived prostaglandins PGI3 and PGE3 are anti-arrhythmic (Lin et al, 1997).

Thromboxanes are a group of fatty acid derivatives that have 20 carbon atoms and include a 6-carbon ether-containing ring structure. Named for their roles in thrombosis, the formation of blood clot, thromboxanes are a vasoconstrictor and facilitate platelet aggregation. Thromboxane-2 series TXA2 and TXB2, produced by activated platelets, have prothrombotic properties, stimulating activation of new platelets as well as increasing platelet aggregation. Whereas the omega-3 fatty acid EPA derived TXA3 and TXB3 has opposite effect. The TXB2-mediated platelet aggregation and vasodilation is also inhibited by EPA-derived prostaglandins.

Leukotrienes are a group of fatty acid derivatives that have 20 carbon atoms and include three conjugated double bonds in their structure. The omega-6 fatty acid derived LTB4 is a potent chemotactic agent for leukocytes. It increases vascular permeability, induces release of lysosomal enzymes and accelerates reactive oxygen species production. It also leads to the production of inflammatory cytokines like TNFa, IL-1 and IL-6. The leukotrienes LTC4, LTD4 and LTE4 increase vascular permeability and promote hypersensitivity. These 4-series leukotrienes are believed to be responsible for hypersensitivity reactions that are involved in asthma, psoriasis, allergic rhinitis, gout, rheumatoid arthritis, adult respiratory distress syndrome, neonatal pulmonary hypertension, and inflammatory bowel disease. The omega-3 fatty acid derived LTB5 blocks biosynthesis of the LTB4 thereby exhibiting anti-inflammatory effects.

Lipoxins (LX) are derived from the omega-6 fatty acid AA and manufactured in leukocytes. There are two types of lipoxins, LXA4 and LXB4. Both types inhibit chemotaxis of polymorphonuclear leukocytes and may have roles in inflammation and wound healing. LXA4 appears to oppose some leukocyte responses to leukotrienes. For example, the binding of LXA4 to polymorphonuclear leukocytes inhibits chemotactic responses and degranulation induced by LTB4. By competing for receptor sites, LXA4 inhibits vasoconstriction induced by LTD4. By inhibiting neutrophil and eosinophil migration and adhesion, lipoxins act as anti-inflammatory signaling molecules. The anti-inflammatory effect of LXA4 is unusual in the sense that most of the omega-6 fatty acid derived eicosanoids are pro-inflammatory.

Resolvins are omega-3 fatty acid derived compounds, so named because they were first encountered in resolving inflammatory exudates. Neuroprotectins are resolvins first discovered in brain tissue. The resolvins have strong anti-inflammatory effects in addition to some immunoregulatory activities at picomolar to nanomolar concentrations. They are part of the molecular mechanisms that contribute to removal of inflammatory cells and restoration of tissue integrity once the need for the inflammatory response is over. The neuroprotectins appear to operate in the same way as the resolvins in brain tissue. The anti-inflammatory effects of NPD1 protect retinal epithelial cells from apoptosis induced by oxidative stress. In addition, it has protective effects in animal models of stroke and of Alzheimer’s disease. Amongst its activities in non-neuronal tissues, it promotes apoptosis of T-cells and it has beneficial effects for asthma. It is evident that such compounds and their metabolism have considerable protective effects in acute inflammation and/or chronic inflammatory disease.

Usually an acute inflammation in response to infection or tissue damage appears as heat, redness, swelling and pain. At the cellular level, it is characterized by edema, accumulation of leukocytes, and then by accumulation of monocytes and macrophages. Leukotrienes (especially LTB4) and prostaglandins (PGE2 and PGD2) derived from omega-6 fatty acid AA are actively involved in the early stages of the inflammatory process. As tissues return to health, resolvins and lipoxins promote resolution of inflammation through removal of leukocytes together with cellular debris.

Cox and Lox: key enzymes for the synthesis of signaling molecules from PUFAs

The key enzymes that are responsible for the production of signaling molecules from PUFAs are Cox (cyclo-oxygenases) and Lox (lipoxygenases).

There are two Cox isozymes in the human body: COX-1 and COX-2. COX-1 is considered a constitutive enzyme, being found in most mammalian cells. COX-2 is an inducible enzyme, becoming abundant in activated macrophages and other cells at sites of inflammation. Both Cox-1 and Cox-2 convert the omega-6 fatty acid AA to the prostaglandin-2 series and thromboxane-2 series molecules and the omega-3 fatty acids, EPA and DHA, to the prostaglandin-3 and thromboxane-3 series and resolvins (Fig. 3). Both Cox-1 and Cox-2 are the targets of nonsteroidal anti-inflammatory drugs (NSAIDs). The anti-inflammation medicine aspirin, which irreversibly inhibits Cox-1 more than Cox-2, causes the reduction of inflammation, analgesia (relief of pain), the prevention of clotting, and the reduction of fever through the decreased production of prostaglandins and thromboxanes.

Lox is a family of enzymes that convert the omega-6 fatty acid AA to leukotriene-4-series and lipoxins. These same enzymes also convert the omega-3 fatty acids EPA to leukotriene-5-series and DHA to resolvins (Fig. 3). The two major isozymes of Lox are 5-lipoxygenase and 15-lipoxygenase. The former is responsible for the synthesis of leukotrienes in myeloid cells and the later lipoxins in leukocytes. The asthma treatment drugs, zileuton and montelukast, and the anti-parasite drug, diethylcarbamazine, are lipoxygenases inhibitors.

A unique feature about resolvin synthesis is that both Cox and Lox enzymes are involved. In the presence of aspirin, the enzyme Cox-2 is acetylated. The acetylated Cox-2, which can no longer convert omega-6 fatty acid AA, converts the omega-3 fatty acid EPA or DHA into the substrates for 15-Lox, which further converts the substrates into resolvins. In the absence of aspirin, 15-Lox can still convert EPA and DHA to resolvins through similar pathways for lipoxin biosynthesis. Without aspirin, most of the Cox activity devoted to omega-6 fatty acid AA metabolism results in limited resolvin synthesis. Therefore, it is reasonable to believe that dietary supplementation of omega-3 fatty acids, taken together with aspirin, may ameliorate the clinical symptoms of many inflammatory disorders by regulating the time course of resolution via the production of resolvins.

PUFAs on gene expression: fatty acids as direct ligands for transcription factors

Many omega-3 and omega-6 fatty acids directly bind to nuclear hormone receptor transcription factors like peroxisome proliferator receptors (PPARs), retinoid X receptors (RXRs), liver X receptors (LXRs) and a basic helix–loop–helix leucine zipper (bHLH-LZ) transcription factor sterol regulatory element-binding protein 1c (SREBP-1c) to modulate target gene expression involved in lipid metabolism pathways. Some also directly interact with the master transcription factor NF-κB to influence immune response.

PPARs are master regulators of lipid and energy metabolism. The functions of the three types of PPAR (PPARα, PPARγ and PPARδ) and their interactions with fatty acids are discussed in more details here. In brief, PPARα regulates the expression of target genes involved in liver fatty acid oxidation (fat burning to produce energy) during fasting; PPARγ, regulates the expression of target genes involved in lipid synthesis in adipocytes (energy storage); and PPARδ regulates the expression of target genes involved in fatty acid synthesis in liver and the genes involved in fat burning in muscle. In addition to energy metabolism, all three types of PPARs exhibit anti-inflammation effects via several distinct mechanisms (Fig. 3). For example, ligand-activated PPARα and PPARγ down-regulate the expression of immune responsive genes by binding directly to transcription factors NF-κB, STAT1 and AP-1, preventing them from activating immune responsive genes such as cytokines (IL-1, IL-2, IL-6, IL-12,TNF-α), chemokines (e.g., IL-8, MIP-1α, MCP1), adhesion molecules (e.g., ICAM, VCAM and E-selectin), and inducible effector enzymes (iNOS and COX-2) (Delerive et al 1999; Ricote et al, 1999; Chung et al, 2000). Alternatively, the fatty acid metabolites of PPARs can act as agonists or antagonists of other transcription factors including NF-κB, STAT1 and AP-1. Various omega-3 and omega-6 fatty acids and their metabolites are natural ligands for all types of PPARs with differential potency (Krey et al, 1997). The omega-3 fatty acids EPA and DHA are more potent as in vivo activators of PPARα than omega-6 fatty acids although EPA binds all PPARs with a Kd ranging between 1 and 4 μM. Even stronger activators are the eicosanoids derived from EPA, DHA and AA. Leukotriene B4 and 8-HEPE (hydroxyeicosapentaenoic) are potent and specific ligands for PPARα. Others such as 9-HODE (9-hydroxyoctadenoic acid) and 13-HODE are specific for PPARγ whereas PGJ2 can activate all three types of PPARs (Desvergne & Wahli, 1999).

PUFAs also inhibit NF-κB activity directly. EPA and DHA blocks NF-κB through decreased degradation of the inhibitory subunit of NF-κB (IκB) in cultured pancreatic cells, THP-1 macrophages and human monocytes. AA-derived EETs inhibit NF-κB translocation and activation by blocking the IκB-kinase complex (IKK). Inactive IKK is associated with the active inhibitor IκB, which functionally retains NF-κB in the cytoplasm and renders it inactive (Schmitz & Ecker, 2008).

PPARs function by forming heterodimers with RXRs, which also form hom*odimers or heterodimers with retinoic acid receptors (RARs), and LXRs to regulate target genes involved in multiple cellular processes such as the retinoid signaling pathway and lipid anabolism and catabolism. LXRs activate the expression of SREBP-1c, a dominant lipogenic gene regulator and cholesterol metabolism controller. PUFAs modulate the heterodimer formation between different nuclear receptors through directly binding to individual factors or by ligand competition. It has been shown that DHA is a RXR ligand while AA is a ligand for RXRa. The conjugated LA isomer trans-9, trans-11-CLA activates SREBP-1c. Over-expression of PPARα and PPARγ inhibit LXR induced SREBP-1c promoter activity through a reduction of LXR/RXR complex formation. Additionally, AA, LA and DHA are ligands for farnesoid X receptor (FXR), the nuclear receptor for bile acids metabolism. Stimulation of FXR enhances the expression of a short heterodimer protein, which has a negative feedback effect on LXR activity. A very recent study describes a novel mechanism for fatty acid regulation of hepatocyte nSREBP-1. DHA suppresses hepatocyte nSREBP-1 through 26S-proteasome and ERK-dependent pathways (Schmitz & Ecker, 2008; Russo, 2009).

PUFAs and membrane function

PUFAs contribute to increased cell membrane fluidity. The numbers of hormone receptors on the cell membrane is determined by cell fluidity. A rigid membrane limits while a fluid membrane increases the number of receptors. For example, enhanced cell membrane fluidity via increased PUFA intake was attributed to an increased number of insulin receptors, an increased affinity of insulin to its receptors, and a reduced insulin resistance. Cell membrane fluidity by PUFAs also interferes with T-cell receptor (TCR). TCR stimulation activates Lck and Fyn, two members of the Src family, leading to the activation of the ERK cascade of signal transduction. Treatment of Jurkat T-cells with EPA and AA results in marked incorporation of PUFAs into phosphatidylethanolamine. This event leads to displacement of palmitate-labeled Lck and Fyn from the lipid rafts, a membrane microdomain that organizes receptor trafficking, and down-regulation of ERK signaling. Replacement of the palmitoyl group by AA or EPA results in Fyn loss of function since the kinase reduces its interaction with the lipid rafts (Russo, 2009).

Cell membrane fluidity is significantly impacted by the number of double bonds in PUFAs. More double bonds result in a more flexible carbon chain conformational change. Since omega-3 fatty acids in general have more double bonds than omega-6 fatty acids, they provide more membrane fluidity than omega-6 fatty acids. The omega-3 fatty acid DHA, for example, is required in the nervous system for optimal neuronal and retinal function and influences signaling events that are vital for neuronal survival and differentiation. DHA is incorporated into phosphatidylserine (PS) in neurons. Clustering of PS at the cytosolic side of membrane lipid rafts facilitates translocation and activation of Akt, a serine-threonine protein kinase. This leads to a suppression of caspase-3 activation and cell death. In the case of omega-3 deficiency, DHA is replaced by DPA (docosapentaenoic acid, 22:5, ω−6) in neuronal cells, resulting in less PS clustering and Akt translocation, thus less cell survival (Simons & Vaz, 2004) and consequent memory loss, learning disabilities, and impaired visual acuity.

The omega-6:omega-3 ratio theory

As described above, omega-3 and omega-6 fatty acids compete for the same enzymes to produce eicosanoids and DHA-derived signaling molecules with opposing physiological functions. While omega-6 derived signaling molecules are pro-inflammatory, omega-3 derived are anti-inflammatory. These two types of PUFAs and their metabolites also interact with transcription factors to modulate the target gene regulation. Moreover, omega-3 and omega-6 fatty acids also compete to incorporate into cell membranes, directly impacting the function of the membrane. The opposing effects of omega-3 and omega-6 fatty acids lead to the omega-6:omega-3 ratio theory, advocated actively by Artemis Simopoulos through many reviews and dietary books (Simopoulos & Robinson, 1999; Simopoulos, 2006; 2008).

Based on evidence from studies on the evolutionary aspects of diet, modern day hunter-gatherers, and traditional diets, the omega-6:omega-3 ratio theory proposes that the genetic makeup of human beings is adapted to a diet in which the ratio of omega-6:omega-3 was about 1. In today’s Western diets the ratio is 15:1 to 17:1. Many of the chronic conditions—cardiovascular disease, diabetes, cancer, obesity, autoimmune diseases, rheumatoid arthritis, asthma and depression—are associated with increased production of thromboxane A2 (TXA2), leukotriene B4 (LTB4), IL-1β, IL-6, TNF and CRP. The increase of these factors is due to the high ratio of omega-6:omega-3 in dietary fat that is incompatible to our genetic makeup. Therefore, reducing your omega-6:omega-3 ratio to roughly 1:1 by including healthy oils or an omega-3 dietary supplement in your diet can help promote optimal health.

The omega-3 side effects

No side effect has been associated with omega-3 rich food. However, high intake of omega-3 supplements, such as more than 3 grams of fish oil daily, may increase the risk of bleeding. Higher doses of omega-3 dietary supplements may also compromise your immune function. Most of the side effects and precautions for fish oil also pertain to cod liver oil with a few exceptions. Cod liver oil contains both vitamin A and D. Consuming excessive amounts of these two vitamins can cause toxicity and dangerous side effects. Moreover, certain medications and mineral oil may interfere with the absorption of vitamin A. In addition, using vitamin A at higher dosages in conjunction with synthetic vitamin A derivatives can result in an increased risk of toxicity.

References

1. Chung S.W. Kang B.Y. Kim S.H. Pak Y.K. Cho D. Trinchieri G. Kim T.S (2000). Oxidized low density lipoprotein inhibits interleukin-12 production in lipopolysaccharide-activated mouse macrophages via direct interactions between peroxisome proliferator-activated receptor-gamma and nuclear factor-kappa B. J. Biol. Chem. 275:32681–32687. PMID:10934192
2. Delerive P. De Bosscher K. Besnard S. Vanden Berghe W. Peters J.M. Gonzalez F.J. Fruchart J.C. Tedgui A. Haegeman G. Staels B (1999). Peroxisome proliferator-activated receptor alpha negatively regulates the vascular inflammatory gene response by negative cross-talk with transcription factors NF-kappaB and AP-1. J. Biol. Chem. 274:32048–32054. PMID:10542237.
3. Desvergne B, Wahli W (1999). Peroxisome proliferator-activated receptors: nuclear control of metabolism. Endocr Rev 20:649–688. PMID:10529898
4. Krey G, Braissant O, L’ Horset F, Kalkhoven E, Perroud M, Parker MG, Wahli W (1997). Fatty acids, eicosanoids, and hypolipidemic agents identified as ligands of peroxisome proliferator-activated receptors by coactivator-dependent receptor ligand assay. Mol Endocrinol. 11(6):779-91. PMID:9171241
5. Li Y, Kang JX, Leaf A (1997). Differential effects of various eicosanoids on the production or prevention of arrhythmias in cultured neonatal rat cardiac myocytes. Prostaglandins. 54(2):511-30. PMID:9380795
6. Ricote M, Huang JT, Welch JS, Glass CK (1999). The peroxisome proliferator-activated receptor(PPARgamma) as a regulator of monocyte/macrophage function. J Leukoc Biol. 66(5):733–9. PMID:10577502
7. Russo GL (2009). Dietary n-6 and n-3 polyunsaturated fatty acids: from biochemistry to clinical implications in cardiovascular prevention. Biochem Pharmacol. Mar 15;77(6):937-46. PMID:19022225
8. Schmitz G, Ecker J (2008). The opposing effects of n-3 and n-6 fatty acids. Prog Lipid Res. Mar;47(2):147-55. PMID:18198131
9. Simons K, Vaz WL (2004). Model systems, lipid rafts, and cell membranes. Annu Rev Biophys Biomol Struct;33:269–95. PMID:15139814
10. Simopoulos AP (2006). Evolutionary aspects of diet, the omega-6/omega-3 ratio and genetic variation: nutritional implications for chronic diseases. Biomedicine & Pharmacotherapy 60:502–507. PMID:17045449
11. Simopoulos AP (2008). The importance of the omega-6/omega-3 fatty acid ratio in cardiovascular disease and other chronic diseases. Exp Biol Med (Maywood). 233(6):674-88. PMID:18408140.
12. Simopoulos AP and Robinson J (1999). The Omega Diet: The Lifesaving Nutritional Program Based on the Diet of the Island of Crete. HarperCollins. New York.