Flax (seed). Flax seeds are the seeds of the common flax, or linseed plant, Linum usitatissimum. They contain high levels of lignans, polyphenolic compounds that have been shown to be beneficial to human health. Flax seeds also contain a significant amount of protein and the essential fatty acid, alpha-linolenic acid (ALA), a biological precursor to omega-3 fatty acids such as eicosapentanoic acid (EPA) 1. Flax seed appears to be well-tolerated, and there is long-standing historical use of flax seed products in dietary supplements 2.