I found the book. It's called Becoming Vegan by Brenda Davis, RD & Vesanto Melina, MS, RD. (rd = register dietician)
Basically your body can usually convert certain types of fatty acids into othe rtypes of fatty acids, except in the case of DHA and EPA. Vegetarians/vegans usually get enough of the other fatty acids, except DHA, from their daily intake of foods. DHA and EPA are most commonly present in fish, eggs, poultry, seaweed, and microalgae (in the latter two . Since most vegans don't eat microalgae salads or seaweed cookies in the amount that they would need to to gain sufficient amounts of these fatty-acids, it might not be a bad idea to suppliment DHA. (DHA can be converted to EPA by the body, so supplimenting just DHA should be fine).
Here is a snippet from the book on page 65. Comments i added are in parentheses:
[quote] With vegan intakes of EPA and DHA typically being zero and serum levels low, an important question arises: Can vegans sufficiently convert parent fatty acids, LA and LNA to longer chain fatty acids or do they need a direct dietary source?
While conversoin of LA to AA (two essential fatty acids) is rarely a problem for vegans, conversion of LNA to EPA and DHA (three other essential fatty acids) is far less efficient, with rates shown in the chart to the right (chart not included, but you get the idea). DHA consumed directly from microalgae can be converted back to EPA from DHA.
And so forth.
And for what exactly DHA is:
[quote]Docosahexaenoic acid (DHA) is not converted to eicosanoids (umm?) but is an important structural component of the gray matter of the brain, the retina of the eye, and specific cell membranes, and is found in high levels in the testes and sperm. Low levels of DHA have been associated with several neurological and behavioral disorders such as depression, schizophrenia, Alzheimer's disease, and Attention Deficit Hyperactivity Disorder. It is also linked to reduced visual and brain development in infants.
But i think it is important to note that "low levels of DHA associated with several neurological disorders" means that folks with these disorders have been observed to have low levels of DHA. Low levels of DHA is not necesarilly the cause of the disorders, and could simply be a symptom.[/quote]