Alpha-Linolenic Acid

Alpha-linolenic acid (ALA, 18:3n-3) is an 18-carbon omega-3 polyunsaturated fatty acid with three cis double bonds at positions 9, 12, and 15. ALA is the essential dietary omega-3 from which humans can synthesize the longer-chain omega-3s (EPA 20:5n-3, DHA 22:6n-3), though at low efficiency. It serves both as a direct metabolic fuel and as the parent compound for the omega-3 lipid mediator family.

Essentiality

Humans cannot introduce the omega-3 double bond into any fatty acid (lacking delta-15 desaturase), so ALA must come from the diet. Frank ALA deficiency in humans is rare but has been documented in cases of long-term fat-free parenteral nutrition, producing neurological symptoms and growth failure that reverse on ALA reintroduction. The 2005 NAM DRI set Adequate Intake at 1.1 g/day for adult women and 1.6 g/day for adult men, based on median US intake and maintenance of normal plasma ALA concentrations.

Dietary sources

Characteristic ALA contents per USDA FoodData Central: flaxseed oil 53 g per 100 g oil, flaxseed (whole) 23 g per 100 g, chia seeds 18 g per 100 g, walnuts 9 g per 100 g, hemp seeds 8 g per 100 g, canola oil 9 g per 100 g, soybean oil 7 g per 100 g, wheat germ 7 g per 100 g. A single tablespoon of flaxseed oil provides approximately 7 g ALA, meeting the AI multiple times over.

Conversion to EPA and DHA

The ALA-to-EPA-to-DHA conversion pathway involves delta-6 desaturase, elongase-5, delta-5 desaturase, elongation to 24:5n-3, delta-6 desaturase again to 24:6n-3, and finally peroxisomal beta-oxidation (the "Sprecher shunt") to yield DHA (22:6n-3). Stable-isotope tracer studies in adults consistently show conversion efficiency of approximately 5-10% ALA → EPA and less than 0.5% ALA → DHA, with conversion rates modestly higher in women (likely due to estrogenic effects on desaturase expression) and in younger individuals. This limited conversion underpins the functional importance of direct dietary EPA and DHA intake beyond ALA alone.

ALA and cardiovascular outcomes

Observational cohort evidence (Pan et al. 2012 meta-analysis) associates higher ALA intake with approximately 10% lower coronary heart disease risk for highest versus lowest intake quintiles, independent of EPA and DHA intake. Randomized trials of ALA supplementation are sparser. The Lyon Diet Heart Study (de Lorgeril, Circulation 1999) used an ALA-rich Mediterranean-style diet (canola-based margarine) and observed marked cardiovascular event reductions, though multiple dietary factors changed simultaneously. The AHA positions ALA as part of a cardioprotective dietary pattern without specifying gram targets beyond the NAM AI.

Flaxseed vs. fish oil

A common question: is plant ALA an adequate substitute for marine EPA/DHA? The honest answer is "partially." Adequate ALA intake prevents frank essential fatty acid deficiency and provides some cardiovascular benefit on its own. However, limited conversion means that populations with elevated EPA/DHA needs — pregnancy, lactation, cardiovascular secondary prevention, specific neurological contexts — generally benefit from direct EPA/DHA intake (fatty fish or algal oil supplementation) beyond ALA alone.

Practical tracking considerations

Most dietary assessment software reports total omega-3 without decomposition into ALA vs. EPA/DHA. Users attempting to verify EPA/DHA adequacy should cross-reference fish intake (2+ servings weekly of fatty fish) or supplementation, rather than relying on ALA-weighted totals that overstate long-chain omega-3 adequacy in plant-based diets.