SDA (Stearidonic Acid).

Stearidonic acid is the product of delta-6 desaturase acting on alpha-linolenic acid, which is the rate-limiting step flax has to pass. Supplying SDA starts the pathway one step past that bottleneck.

Stearidonic acid is elongated and desaturated to eicosapentaenoic acid far more efficiently than alpha-linolenic acid is. Adding EPA directly covers the same endpoint without relying on that conversion.

Stearidonic acid raises tissue EPA but has little effect on DHA, because the final steps run through a peroxisomal route that stays limited. A preformed DHA source covers the end of the chain that SDA does not reach.

Marine oil delivers EPA and DHA directly while stearidonic acid feeds the endogenous route to EPA. Together they raise the omega-3 index by two independent mechanisms.

Stearidonic acid carries four double bonds and oxidises readily in the bottle and in tissue. Tocopherol is the chain-breaking antioxidant that keeps it intact.

The delta-5 desaturase and elongase steps that carry stearidonic acid onward to EPA depend on zinc. Zinc status therefore sets part of the conversion ceiling.

Gamma-linolenic acid from borage occupies the same elongase and delta-5 desaturase steps that carry stearidonic acid toward EPA. High omega-6 substrate lowers the share of those enzymes available to the omega-3 branch.

Evening primrose loads linoleic and gamma-linolenic acid into the same enzyme set stearidonic acid needs. The two branches compete, so the omega-6 dose lowers omega-3 conversion efficiency.

Astaxanthin sits across the membrane bilayer where highly unsaturated fatty acids are most exposed. It is a common companion antioxidant in omega-3 formulations.

Delta-6 desaturase, the elongases and delta-5 desaturase all handle n-6 and n-3 substrates, and linoleic acid is present in the diet in far larger amounts. High linoleic intake therefore occupies the same enzymes that would carry stearidonic acid onward to EPA. Reducing background n-6 intake changes how much of an 18-carbon n-3 gets through.

Gamma-linolenic acid is the n-6 mirror of stearidonic acid, both sitting immediately past delta-6 desaturation on their respective branches. Seed oils such as echium and blackcurrant naturally carry both. A rodent study of 1,3-diacylglycerols rich in the two reported changes in metabolic parameters and tissue lipid composition; animal data, not human.

The delta-5 and delta-6 desaturases are diiron enzymes that receive electrons through the iron-containing cytochrome b5 system. Iron status therefore sits underneath how efficiently 18-carbon n-3 fatty acids are converted onward. This is a cofactor requirement of the pathway rather than an effect of taking the two together.

Glutathione peroxidases are selenoenzymes that reduce lipid hydroperoxides formed when polyunsaturated fatty acids oxidise in membranes. A fatty acid with four double bonds generates more of those peroxides than a saturated one. Selenium status is part of how the body handles that load.

Alpha-tocopherol is the chain-breaking antioxidant that protects polyunsaturated fatty acids inside membranes, and ascorbate regenerates it from its radical form at the membrane surface. Stearidonic acid, with four double bonds, raises the demand on that system. Ascorbate acts on the tocopherol rather than on the fatty acid itself.

Rosemary extract standardised to carnosic acid and rosmarinic acid is one of the common natural antioxidant systems used to slow oxidation in polyunsaturated seed oils. It protects the oil in the bottle rather than doing anything in the body. Peroxide and anisidine values are how that protection is measured.

Krill oil supplies EPA and DHA already formed and bound to phospholipids, whereas stearidonic acid must be elongated and desaturated to reach EPA. Taking both covers the pathway from two ends. It also means total n-3 intake should be counted across both rather than from one label.

Pyridoxine status has been reported to affect delta-6 desaturase activity in experimental work, though the relationship is not as firmly settled as the iron requirement. Any effect would sit on the same conversion step that stearidonic acid partly bypasses. The support here is mechanistic and largely preclinical.

The elongation cycle that carries 18-carbon n-3 fatty acids to 20 carbons runs on malonyl-CoA and NADPH, and the acetyl-CoA carboxylase step that supplies malonyl-CoA is magnesium-dependent. Magnesium therefore sits under the elongation half of the pathway. The link is biochemical rather than demonstrated by a combination study.