Omega-3 Fatty Acids: EPA, DHA, ALA, and the Evidence

A 97% Conversion Rate That Never Happens in Practice

Alpha-linolenic acid (ALA) — found in flaxseed, chia seeds, and walnuts — is technically an essential omega-3 fatty acid. The body can convert it to EPA and then DHA. In practice, that conversion is catastrophically inefficient: studies consistently find only 5–10% of ALA converts to EPA, and less than 0.5% becomes DHA. The enzymes responsible (delta-6 and delta-5 desaturase) are also used to process omega-6 fatty acids, and Western diets — with omega-6 to omega-3 ratios of 15:1 to 20:1 — keep those enzymes occupied. Relying on ALA as an omega-3 source is, metabolically speaking, wishful thinking.

EPA and DHA: What They Actually Do

Eicosapentaenoic acid (EPA, 20:5 n-3) and docosahexaenoic acid (DHA, 22:6 n-3) are the long-chain omega-3s with documented physiological roles. DHA is a structural component of neuronal membranes and retinal photoreceptors, comprising roughly 40% of brain polyunsaturated fatty acids. EPA is primarily a signaling molecule: it competes with arachidonic acid (AA) for cyclooxygenase and lipoxygenase enzymes, reducing production of pro-inflammatory eicosanoids including leukotriene B4 and prostaglandin E2. Neither function can be replicated by dietary ALA at realistic consumption levels.

REDUCE-IT: Spectacular Results, Serious Controversy

The REDUCE-IT trial (2018, Bhatt et al., New England Journal of Medicine) randomized 8,179 statin-treated patients with elevated triglycerides to 4 g/day of icosapentaenoic acid (Vascepa, purified EPA) or a mineral oil placebo. Results were dramatic: a 25% relative risk reduction in major adverse cardiovascular events (MACE). The FDA approved Vascepa for cardiovascular risk reduction in 2019 based largely on this trial.

Then the controversy began. Mineral oil — the placebo — raises LDL cholesterol, may increase inflammatory markers, and is not a metabolically inert substance. Critics including Steven Nissen at the Cleveland Clinic argued the mineral oil placebo inflated the treatment benefit by making the control group sicker. A reanalysis found that nearly half the relative risk reduction disappeared when accounting for placebo-arm LDL increases.

STRENGTH Trial: The Failure That Clarified Everything

The STRENGTH trial (2020, Nissen et al., JAMA) randomized 13,078 patients to 4 g/day of a mixed EPA+DHA supplement (Epanova) or corn oil placebo. It was stopped early for futility: no cardiovascular benefit whatsoever. The contrast with REDUCE-IT was stark, and it ignited a debate about whether:

• Purified EPA (as in REDUCE-IT) has uniquely different cardiovascular effects than EPA+DHA combinations.
• The mineral oil placebo in REDUCE-IT created an artificial benefit signal.
• Higher baseline triglycerides are required for fish oil benefits (both trials required >150 mg/dL, but patient populations differed).

The honest answer: the cardiovascular benefit of fish oil supplements, except in very-high-dose purified EPA for high-triglyceride statin patients, remains genuinely uncertain.

Algae-Based DHA: Where Fish Get It Anyway

Fish do not produce omega-3s. They accumulate EPA and DHA by eating microalgae and phytoplankton. Algae-based DHA supplements — derived from Schizochytrium and Crypthecodinium cohnii species — bypass the fish entirely. Bioavailability studies show algae-derived DHA is equivalent to fish-derived DHA (Geppert et al., British Journal of Nutrition 2005). For vegans and vegetarians, this is the only direct source. Algae-derived EPA remains less commercially available, though dual EPA+DHA algae oils are increasingly common.

Anti-Inflammatory Mechanisms: More Than Eicosanoids

The anti-inflammatory effects of EPA and DHA operate through several mechanisms beyond eicosanoid competition:

• Specialized pro-resolving mediators (SPMs): EPA generates E-series resolvins; DHA generates D-series resolvins and protectins. These actively resolve inflammation rather than simply suppressing it — a mechanistically distinct action from NSAIDs.
• NF-κB suppression: DHA inhibits activation of nuclear factor kappa-B, the master transcription factor for pro-inflammatory gene expression.
• Membrane fluidity: DHA incorporation into immune cell membranes alters lipid raft structure, reducing toll-like receptor 4 signaling efficiency.

For most healthy adults, two servings of fatty fish per week (salmon, mackerel, sardines) delivers approximately 500–1,000 mg combined EPA+DHA — the target most professional guidelines endorse. For those who cannot or will not eat fish, algae-based DHA at 400–600 mg/day achieves comparable blood DHA levels.

This article is for informational purposes only. Consult a qualified healthcare professional.