Omega 3 fatty acids in flax seed (as well as in Hemp Protein) are found in the form of Alpha-Linoleic Acid (ALA). Not only is ALA not sufficient to supplement on its own, but ALA has to be converted by the body into a usable form, and the ratio of conversion from unusable form to usable is rather poor, somewhere in the range of 5-15%. Omega 3 supplements in the form of EPA and DHA are what the body tends to use for many of the benefits associated with Fish Oil.
For vegetarians and vegans, supplementing with DHA from algae can "markedly [enhance] the DHA status (of serum and platelets)" and "[provide] for the formation of substantial EPA". Supplementation of ALA and/or GLA is not enough.
Scientific Support & Reference Citations
Wang C, et al n-3 Fatty acids from fish or fish-oil supplements, but not alpha-linolenic acid, benefit cardiovascular disease outcomes in primary- and secondary-prevention studies: a systematic review
. Am J Clin Nutr. (2006)
Gerster H Can adults adequately convert alpha-linolenic acid (18:3n-3) to eicosapentaenoic acid (20:5n-3) and docosahexaenoic acid (22:6n-3)
. Int J Vitam Nutr Res. (1998)
Brenna JT Efficiency of conversion of alpha-linolenic acid to long chain n-3 fatty acids in man
. Curr Opin Clin Nutr Metab Care. (2002)
Conquer JA, Holub BJ Supplementation with an algae source of docosahexaenoic acid increases (n-3) fatty acid status and alters selected risk factors for heart disease in vegetarian subjects
. J Nutr. (1996)
Fokkema MR, et al Short-term supplementation of low-dose gamma-linolenic acid (GLA), alpha-linolenic acid (ALA), or GLA plus ALA does not augment LCP omega 3 status of Dutch vegans to an appreciable extent
. Prostaglandins Leukot Essent Fatty Acids. (2000)