Dear Doug
re:Fatty acid analysis in TPN patients
In the situation you describe it is quite probable that the patients are EFA-deficient and the analysis of choice will be red cell EFA’s by FAME (fatty acid methyl ester) analysis using capillary GLC with FID detector. It’s not too difficult a technique although the number of compounds involved can be initially intimidating.
Levels of red cell membrane fatty acids best reflect the efficiency of, and demands on, the metabolic pathways and also on the extent of EFA’s storage. Plasma EFA’s are more useful when it is necessary to look at dietary intake and absorption. So red cell levels (i.e.lysates from EDTA collections, frozen after collection and separation) are the sample of choice.
EFAs include:
- the omega-6 family: derived particularly from plant oils
(such as linoleic acid and arachidonic acid)
- the omega-3 family: derived particularly from fish oils
(such as gamma-linolenic acid, eicosapentaenoic acid (EPA) and
docosahexaenoic acid (DHA)).
The body converts EFAs by lengthening them, the EFAs towards the end of the metabolic process being the long chain polyunsaturated fatty acids. The enzymes involved are delta-5 desaturase, delta-6 desaturase and several different elongases.
Delta-6 desaturase is zinc and vitamin B1 dependent, while delta-5 desaturase is zinc and B3 dependent. Surgical patients can therefore have EFA deficiencies caused by increased utilisation, poor intake, or deficiency of other nutrients. In particular, shortage of zinc exacerbates the problem.
I think that looking at the ratio of C 20:4 omega-6 (arachidonic) acid to C 20:5 omega-3 (eicosapentaenoic) acid is unlikely to be of much use in this situation.
Best wishes,
Nick Miller,
London
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