Aaron is right about EFA in fish oil, but fish oil does contain a good amount of EPA and DHA, both omega 3. Flaxseed also contains Omega three in the form of Linolenic (a true EFA), but conversion in the body to DHA isn't the greatest.
From
http://www.beyondveg.com/billings-t/comp-anat/comp-anat-1a.shtml
Aaron would be the one to comment on the papers cited.
Nettleton [1995] reports that alpha-linolenic acid (LNA) can be converted to EPA and DHA, but the efficiency of the conversion is low [Dyerberg, Bang, and Aagard 1980; Sanders and Younger 1981; as cited in Nettleton 1995]. Emken et al. [1994], as cited in Conquer and Holub [1996], report a conversion rate of LNA to DHA of ~5% in adults. Salem et al. [1996] suggest (Table 1, p. 50; see also p. 51) a minimum conversion rate of ~1% from LNA to DHA in infants (0.9 mg of DHA produced from 100 mg precursor).
Kelley et al. [1993] fed volunteers a diet in which 6.3% of calories were from flaxseed oil. (Given the cost of flaxseed oil, this is a large amount and would be expensive.) They observed a statistically significant increase in EPA levels in peripheral blood mononuclear cells (PBMNC) in those receiving flaxseed oil. EPA levels in serum were not affected by flaxseed oil. Regarding DHA levels, no increases (in PBMNC or serum) were seen from flaxseed oil supplementation.
Gerster [1998] is a review article that looks into the issue of whether adults can adequately convert ALA to EPA and DHA. The tables in Gerster [1998, pp. 165-166, 168] list a number of studies that showed a pattern similar to the one described above. That is, flaxseed oil supplementation may increase EPA, but not DHA.
Studies that found no changes in DHA levels from flaxseed oil supplementation: Dyerberg et al. [1980], Singer et al. [1986], Kelley et al. [1993], Cunnane et al. [1995], Mantzioris et al. [1994], Layne et al. [1996].
Studies that reported DHA decreased with flaxseed oil supplementation: Sanders and Roshanai [1983], Allman et al. [1995].
Studies that reported DHA increased with flaxseed oil supplementation: Kestin et al. [1990], and Sanders and Younger [1981].
Gerster also makes the point that fish oils are superior sources for EPA and DHA, and concludes [1998, pp. 159-160]:
[b said:
Quote[/b] ]These findings indicate that future attention will have to focus on the adequate provison of DHA which can reliably be achieved only with the supply of the preformed long-chain metabolite.
For another view, Nettleton reports [1995, pp. 33-34]:
[b said:
Quote[/b] ]The fact that LNA is less effective than EPA and DHA in enriching tissues with n-3 FA [fatty acids] means that foods rich in LNA, vegetables and seed oils will be less satisfactory sources of n-3 FA for human health than seafoods or other animal foods enriched with EPA and DHA. That is not to say, though, that oils such as canola and soy are not useful sources of n-3 FA...