EPA.

EPA and DHA ride together in fish oil and move through the same fatty-acid pathway, with DHA able to convert back into EPA. DHA embeds in cell membranes, densest in nerve and eye tissue where it supports normal membrane fluidity, while EPA is the preferred raw material for the signaling molecules that shape the body's normal inflammatory response, so the two cover complementary roles.

EPA has five double bonds that oxidize readily, so vitamin E, a fat-soluble antioxidant, hands off a hydrogen atom to quench the peroxyl radicals that would otherwise break it down, both inside the softgel and once EPA is built into cell membranes and lipoproteins. A higher omega-3 intake also lifts the body's vitamin E requirement, which is part of why the two are usually formulated side by side.

GLA and EPA run through the same desaturase and elongase enzymes, and GLA taken alone tends to raise arachidonic acid, the precursor for more inflammatory signaling molecules. EPA blocks the delta-5-desaturase step that produces arachidonic acid, so combining the two holds that rise in check while both shift eicosanoid output toward the less inflammatory series.

Astaxanthin spans the lipid bilayer and quenches radicals at both membrane surfaces, which is where highly unsaturated fatty acids are most exposed. It is added to marine oils to limit peroxidation of the fatty acid itself.

Vitamin D uptake needs fat in the same meal for bile release and micelle formation, and a long-chain fatty acid oil provides it. That is why vitamin D is so often dosed inside a fish oil capsule.

Rosemary diterpenes are a standard in-oil antioxidant system for polyunsaturated oils, which oxidise quickly at five double bonds. This protects the ingredient in the bottle rather than in the body.

Glutathione peroxidase is selenium-dependent and clears lipid hydroperoxides from membranes where EPA is incorporated. Selenium status shapes the fate of those membrane fatty acids.

Ginkgolides antagonise platelet activating factor and EPA shifts thromboxane production toward a weaker form. Both reduce normal platelet aggregation, so the combined effect is larger than either alone.

Garlic organosulfur compounds damp platelet aggregation by a route independent of eicosanoid substrate competition. Stacked with EPA the effect on normal clotting adds.

Gingerols act on thromboxane synthase, the same branch of eicosanoid production EPA competes for as a substrate. The two together have a larger effect on normal platelet function.

Long-chain fatty acids enter mitochondria only as carnitine esters through the carnitine palmitoyltransferase shuttle. Carnitine supply governs how much of a long-chain fatty acid is oxidised.

Phospholipid-bound fatty acids emulsify readily in the gut and route into membrane phospholipid pools. Added phosphatidylcholine improves emulsification of a triglyceride EPA oil.

EPA competes with arachidonic acid at the cyclooxygenase and lipoxygenase step, while curcuminoids act on NF-kB driven expression of those same enzymes. Substrate competition and expression control are distinct levers on one pathway.

Linoleic acid and alpha-linolenic acid are handled by the same delta-6 desaturase and elongase steps, so a high omega-6 load reduces the share of that enzymatic capacity available to the omega-3 branch. Downstream, arachidonic acid from the omega-6 side and eicosapentaenoic acid compete for cyclooxygenase and lipoxygenase and for the sn-2 position of membrane phospholipids. This is settled lipid biochemistry rather than a combination trial result.

Eicosapentaenoic acid is a long chain fatty acid and is absorbed through micelle formation, mucosal re-esterification and chylomicron export, all of which need dietary fat and bile. Medium chain triglycerides are handled differently, moving largely by the portal route, so they act as a carrier matrix rather than a competitor for the same pathway. The rationale is absorption physiology; the magnitude of any gain depends on the meal it is taken with.

Phospholipid emulsifiers reduce droplet size in the gastric and duodenal phase, which increases the surface available to pancreatic lipase. Lecithin is used in softgel and emulsion formats for exactly that reason. This is formulation physiology, and the effect size varies with the format and the meal.

Long chain omega-3 fatty acids shift thromboxane and prostacyclin balance toward less platelet aggregation, an established pharmacological effect at higher intakes. Nattokinase has fibrinolytic activity on a different part of the same system. Stacking them is an additive direction on normal clotting and is a reason for care rather than a benefit claim, particularly around surgery or dental work.

Salicylates inhibit platelet cyclooxygenase-1, reducing thromboxane A2 formation. Eicosapentaenoic acid displaces arachidonic acid as the substrate for that same enzyme and yields a less aggregatory three-series thromboxane. The two push in the same direction on platelet function, which is a caution to note rather than a pairing to promote.

Highly unsaturated fatty acids carry multiple bis-allylic hydrogens and are the most peroxidation-prone lipids in the diet. Iron in its free form catalyses initiation and propagation of that chain reaction, which is textbook radical chemistry. In practice this argues for antioxidant protection in the formulation and against co-dosing a large unbound iron load with a polyunsaturated oil in the same unprotected matrix.

Alpha-tocopherol terminates lipid peroxidation chains and is left as a tocopheryl radical. Ascorbate at the aqueous interface reduces that radical back to tocopherol, a redox couple described in vitro and in membrane models. The pairing is relevant to oxidative stability of a polyunsaturated oil, and no combination trial in the candidate set measured a clinical endpoint.

Reduced coenzyme Q10 acts as a lipid-phase antioxidant and can regenerate tocopherol within membranes and low density lipoprotein particles. Eicosapentaenoic acid is incorporated into those same particles, where it raises the count of oxidisable double bonds. The pairing addresses oxidative stability of the lipid pool, which is a mechanism point and not a measured clinical outcome.

Eicosapentaenoic acid does not circulate mainly as a free acid; it is esterified into phospholipids, and phosphatidylcholine is the dominant species. Choline supplies the headgroup for that synthesis through the CDP-choline pathway. Adequate choline is therefore part of the machinery that handles an omega-3 load, which is established biochemistry.

Dihydrolipoic acid can reduce oxidised glutathione and ascorbate, feeding the network that ultimately regenerates tocopherol in membranes. That places it upstream of the antioxidant defence around polyunsaturated lipid. The connection is network chemistry; no combination trial in the candidate set measured the pair.

Catechins bind transition metals and scavenge lipid radicals in vitro, both of which slow peroxidation initiation. That is a plausible complement to a highly unsaturated fatty acid, in the formulation and in circulating lipoproteins. It has not been tested as a combination for a clinical endpoint, so read it as chemistry.

An isolated eicosapentaenoic acid concentrate and a whole fish oil are not the same material: whole oil carries a mixed fatty acid profile, natural tocopherols and a triglyceride backbone. Delivering the fatty acid in that triglyceride matrix with dietary fat is the standard route for absorption. This is a formulation relationship, stated so the two are not read as interchangeable label entries.

Bile acids are conjugated with taurine or glycine before secretion, and the resulting salts form the micelles that solubilise dietary fat. Taurine availability is one input to that conjugation pool. The step is established physiology, but no study in the candidate set shows that supplemental taurine changes omega-3 absorption in people.