In a double-blind trial, 72 vegans and non-seafood-eating vegetarians took 0 to 1,000 mg/d of algal DHA for 8 weeks. Plasma DHA and EPA rose with dose, while DPAn-3 fell and the EPA:DPAn-3 ratio climbed, the signature of DHA blocking EPA elongation, a key step in its own synthesis. Carbon isotope tracing showed that at 132 mg/d, plasma DHA came equally from the supplement and from ALA conversion, implying roughly 264 mg/d of net DHA-equivalent supply and up to 251 mg/d made endogenously.
The finding challenges the common assumption that plant-based eaters convert ALA to DHA too poorly to matter. If the body can synthesize close to 250 mg/d when dietary DHA is absent, the gap between vegan intake and typical recommendations of 250 to 500 mg/d of EPA plus DHA narrows considerably. The feedback inhibition also suggests diminishing returns: once supplemental DHA arrives, the body throttles back its own production, so total status rises less than the dose implies. Important caveats apply. This is a preprint and has not been peer reviewed, so the numbers may shift. The study measured blood fatty acid levels and synthesis kinetics, not brain, heart, or mortality outcomes, so it cannot say what intake is clinically adequate. Participants were a small, diet-restricted group, and the isotope method relies on assumptions about ALA sources. Treat it as a solid mechanistic advance that sharpens the DHA requirement question rather than answering it, and one that gives vegans reason to discuss, not abandon, algal DHA with their doctors.