ALA = little or no DHA conversion.  This has been discussed here, so thought
I would post this study.

Theola

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Br J Nutr 2002 Oct;88(4):355-64

Eicosapentaenoic and docosapentaenoic acids are the principal products of
alpha-linolenic acid metabolism in young men*.

Burdge GC, Jones AE, Wootton SA.

The capacity for conversion of alpha-linolenic acid (ALNA) to n-3 long-chain
polyunsaturated fatty acids was investigated in young men. Emulsified
[U-13C]ALNA was administered orally with a mixed meal to six subjects
consuming
their habitual diet. Approximately 33 % of administered [13C]ALNA was
recovered
as 13CO2 on breath over the first 24 h. [13C]ALNA was mobilised from
enterocytes
primarily as chylomicron triacylglycerol (TAG), while [13C]ALNA
incorporation
into plasma phosphatidylcholine (PC) occurred later, probably by the liver.
The
time scale of conversion of [13C]ALNA to eicosapentaenoic acid (EPA) and
docosapentaenoic acid (DPA) suggested that the liver was the principal site
of
ALNA desaturation and elongation, although there was some indication of EPA
and
DPA synthesis by enterocytes. ... There was no apparent 13C enrichment of
docosahexaenoic acid (DHA) in plasma PC, TAG or non-esterified fatty acids
at
any time point measured up to 21 d. This pattern of 13C n-3 fatty acid
labelling
suggests inhibition or restriction of DHA synthesis downstream of DPA. ...
Since
the capacity of adult males to convert ALNA to DHA was either very low or
absent, uptake of pre-formed DHA from the diet may be critical for
maintaining
adequate membrane DHA concentrations in these individuals.

PMID: 12323085