|MadSci Network: Biochemistry|
First of all, to address the differences among ALA, EPA, and DHA, we should look at their basic characteristics, such as how many carbons, double bonds, etc. that each contains.
ALA contains 18 carbons and 3 double bonds. The chemical shorthand for this is C18:3. (Forgive me if you already know this, but I want to make sure that I we have a good foundation to build on.) ALA's double bonds occur at carbons 9, 12, and 15.
EPA is a 20 carbon fatty acid with 5 double bonds. C20:5 (5, 8, 11, 14, 17).
DHA is C22:6 (4, 7, 10, 13, 16, 19).
The basic differences among these fatty acids have to do with the fact that they are composed of different numbers of carbons, and have differing numbers of double bonds. These double bonds are also located on different carbons.
ALA, EPA, and DHA are similar in that they are all n-3 fatty acids...meaning that they all possess double bonds 3 carbons from the end of their carbon chains (eg., ALA possesses a double bond at carbon 15 out of the 18 total).
The body can convert ALA to EPA and DHA via elongase and desaturase enzymes.
I think that the fact that they are n-3 fatty acids and that ALA can be converted to the other 2 is why the answer to 1076291010.Bc indicated that ALA, EPA, and DHA are the same.
I have not seen any data on the efficiency of ALA conversion to EPA/DHA or under what circumstances this occurs (eg., if ALA is metabolized for energy production, it cannot be converted to EPA or DHA).
I have not seen any information to indicate that ALA is less bioavailable than salmon oils, as long as you are talking about consuming ground seeds or oil. It's probably true that consuming whole seeds will result in little to no absorption of ALA.
There is theoretical and animal evidence to indicate that an indicate that increasing the n-3/n-6 ratio may be beneficial, but no such evidence exists, to my knowledge, about ratios between EPA and DHA.
Bottom line, I think that both Dr. Weil's site and the answer to 1076291010.Bc make good points, but give incomplete information.
I hope that this filled in some of the gaps.
Thanks for your question!
Try the links in the MadSci Library for more information on Biochemistry.