Just to recap, omega 3 and omega 6 are 'essential' fatty acids because your body cannot produce them. So they must be ingested as part of a diet regimen. Typical Western diets are incredibly rich in omega 6 owing to vegetable oil abuse and grain fed animals with omega 3 almost nonexistent. The British (and American) diet may have in excess of 20 parts omega 6 to 1 part omega 3 (>20:1).
Dietary bodies and vocal internet groups seem to be at odds as to the correct ratio of omega 6 to omega 3. Some nutritionists state a 4:1 ratio whilst others are happy to push a 1:1 ratio. However a 2:1 ratio of omega 6 to omega 3 seems favourable here.
Why all the strife over ratios?
Omega 3 tends to keep Omega 6 in check otherwise omega 6 on its own causes mayhem, including the oxidisation of LDL cholesterol. Lower ratios have been associated with a decreased risk of diabetes and heart disease. In fact a 2:1 ratio of omega 3 to omega 6 was found to reduce inflammation associated with arthritis.
Recommended Daily Allowance
The World Health Organisation recommends 1-2 servings of fatty fish (e.g. salmon, sardines, pilchards, mackerel and herring) coupled with oils containing omega 3 per week. The British Heart Foundation says pretty much the same. There isn't a recommended daily allowance set in stone for those wishing to supplement with omega 3 oils but an approximated 2g of EPA and DHA seems good.
EPA, DHA and Linseeds (flax)
Linseeds are a great source of omega 3 but in the wrong form of it. They contribute an omega 3 called Alpha-Linolenic acid (ALA). This ALA is a 'short chain' fatty acid and must be converted to 'long chain' fatty acids known as Eicosapentaenoic acid (EPA) and Docosahexaenoic acid (DHA). The process of converting ALA to EPA and DHA is inefficient. In women this conversion is about 36% and only 16% in men. Thus recommended dosages of Linseed oil for women would be approximately 10g for women and 20g for men daily. Those wishing to ingest blitzed powered seed would need to double those amounts.
How does ALA make EPA & DHA?
I'll try and keep this simple without too much technical jargon. When you take ALA the body has to convert it to EPA and DHA as mentioned earlier. This is done through several long winded rounds of elongating and desaturating. See the flowchart below for the steps involved:
In the top left corner, you have ALA. This is acted upon by delta-6 desaturase (an enzyme) to form Stearidonic acid. This is another 18-carbon omega 3 like ALA. This fatty acid is then elongated by the injection of an ethyl group to form Eicosatetraenoic acid. This is now a 20-carbon omega 3. It's then acted upon by delta-5 desaturase (another enzyme) and EPA is made.
EPA is still 2 carbon atoms short for DHA though. So it's elongated again by an ethyl group to form Docosapentaenoic acid (DPA). This is finally converted by delta-4 desaturase (yet another enzyme) to DHA.
The best sources of EPA and DHA are animal products. Fish, grass-fed meats and eggs (non-grain fed) are ideal sources. To hit your daily quota supplementation is required. If you do decide to pursue plant based omega 3 (e.g. Linseed, walnuts) please bear in mind the aforementioned conversion rates.