Nutritional Deprivation of α‐Linolenic Acid Decreases but Does Not Abolish Turnover and Availability of Unacylated Docosahexaenoic Acid and Docosahexaenoyl‐CoA in Rat Brain

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Abstract
We applied our in vivo fatty acid method to examine concentrations, incorporation, and turnover rates of docosahexaenoic acid (22:6 n-3) in brains of rats subject to a dietary deficiency of α-linolenic acid (18:3 n-3) for three generations. Adult deficient and adequate rats of the F3 generation were infused intravenously with [4,5-3H]docosahexaenoic acid over 5 min, after which brain uptake and distribution of tracer were measured. Before infusion, the plasma 22:6 n-3 level was 0.2 nmol ml-1 in 18:3 n-3-deficient compared with 10.6 nmol ml-1 in control rats. Brain unesterified 22:6 n-3 was not detectable, whereas docosahexaenoyl-CoA content was reduced by 95%, and 22:6 n-3 content in different phospholipid classes was reduced by 83-88% in deficient rats. Neither plasma or brain arachidonic acid (20:4 n-6) level was significantly changed with diet. Docosapentaenoic acid (22:5 n-6) reciprocally replaced 22:6 n-3 in brain phospholipids. Calculations using operational equations from our model indicated that 22:6 n-3 incorporation from plasma into brain was reduced 40-fold by 18:3 n-3 deficiency. Recycling of 22:6 n-3 due to deacylation-reacylation within phospholipids was reduced by 30-70% with the deficient diet, but animals nevertheless continued to produce 22:6 n-3 and docosahexaenoyl-CoA for brain function. We propose that functional brain effects of n-3 deficiency reflect altered ratios of n-6 to n-3 fatty acids.

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