Can the rat liver maintain normal brain DHA metabolism in the absence of dietary DHA?

Background: Docosahexaenoic acid (DHA) is required for normal brain function. The concentration of DHA in the brain depends on both diet and liver metabolism.

Objective: To determine rat brain DHA concentration and consumption in relation to dietary n-3 (omega-3) polyunsaturated fatty acid (PUFA) content and liver secretion of DHA derived from circulating alpha-linolenic acid (alpha-LNA).

Methods: Following weaning, male rats were fed for 15 weeks either: (1) a diet with a high DHA and alpha-LNA content, (2) an n-3 PUFA "adequate" diet containing 4.6% alpha-LNA but no DHA, or (3) an n-3 PUFA "deficient" diet containing 0.2% alpha-LNA and no DHA. Brain DHA consumption rates were measured following intravenous infusion in unanesthetized rats of [1-14C]DHA, whereas liver and brain DHA synthesis rates were measured by infusing [1-14C]alpha-LNA.

Results: Brain DHA concentrations equaled 17.6, 11.4 and 7.14 microm/g in rats on diets 1, 2 and 3, respectively. With each diet, the rate of brain DHA synthesis from alpha-LNA was much less than the brain DHA consumption rate, whereas the liver synthesis-secretion rate was 5-10 fold higher. Higher elongase 2 and 5 and desaturase Delta5 and Delta6 activities in liver than in brain accounted for the higher liver DHA synthesis rates. Furthermore, these enzymes were transcriptionally upregulated in liver but not in brain of rats fed the deficient diet.

Conclusions: While DHA is essential to normal brain function, this need might be covered by dietary alpha-LNA when liver metabolic conversion machinery is intact and the diet has a high alpha-LNA content.