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There is little doubt regarding the essential nature of alpha-linolenic acid (ALA), yet the capacity of dietary ALA to maintain adequate tissue levels of long chain n-3 fatty acids remains quite controversial. This simple point remains highly debated despite evidence that removal of dietary ALA promotes n-3 fatty acid inadequacy, including that of docosahexaenoic acid (DHA), and that many experiments demonstrate that dietary inclusion of ALA raises n-3 tissue fatty acid content, including DHA. Herein we propose, based upon our previous work and that of others, that ALA is elongated and desaturated in a tissue-dependent manner. One important concept is to recognize that ALA, like many other fatty acids, rapidly undergoes beta-oxidation and that the carbons are conserved and reused for synthesis of other products including cholesterol and fatty acids. This process and the differences between utilization of dietary DHA or liver-derived DHA as compared to ALA have led to the dogma that ALA is not a useful fatty acid for maintaining tissue long chain n-3 fatty acids, including DHA. Herein, we propose that indeed dietary ALA is a crucial dietary source of n-3 fatty acids and its dietary inclusion is critical for maintaining tissue long chain n-3 levels.

Background: Maternal omega-3 fatty acid supplementation has been suggested to provide docosahexaenoic acid (DHA) for the normal brain development during gestation. DHA can be given as such (preformed) or through the omega-3 precursor alpha-linolenic acid (LNA) which is transformed into DHA by elongation and desaturation reactions. Western diet provides low amounts of LNA and DHA; therefore, supplementation with these omega-3 fatty acids has been suggested for pregnant women. However, the bioequivalence of LNA ingestion to DHA supplementation has not been established., Methods: Recently weaning female Wistar rats were fed a diet containing a small amount of LNA and no DHA. The animals were daily supplemented 40 days before mating, during pregnancy, and until delivery with 60 mg/kg of LNA or 6 mg/kg of DHA dissolved in coconut oil. Fatty acids were given as ethyl ester derivatives. Controls received coconut oil. The fatty acid composition of blood plasma, erythrocytes, liver, visceral adipose tissue, and brain segments (frontal cortex, hippocampus, and cerebellum) was analyzed. Brain segments obtained from 16- and 19-day-old fetuses and from 2- and 21-day-old rats were also analyzed for fatty acid composition., Results: Supplementation with LNA and DHA induced a similar accretion of DHA in plasma, erythrocytes, liver, and brain segments of the mothers. The adipose tissue showed a higher DHA accretion after DHA-supplementation. The DHA accretion in frontal cortex, hippocampus, and cerebellum obtained from the fetuses and the newborn rats was similar when the mothers were supplemented with LNA and DHA. Our results show that under our experimental conditions a similar accretion of DHA in the different tissues of the mothers and in the brain segments of fetuses and newborn rats is obtained after LNA and DHA supplementation., Conclusion: LNA and DHA, at the amounts given in this study, show a similar bioequivalence for DHA accretion in different tissues of the mother and in brain segments of fetuses and newborn rats., (Copyright 2004 S. Karger AG, Basel)

This article describes the application of in vivo 13C-nuclear magnetic resonance (NMR) spectroscopy and gas chromatography (GC)-combustion-isotope ratio mass spectrometry to the study of brain uptake and metabolism of polyunsaturated fatty acids in the suckling rat model. NMR spectroscopy is uniquely suited to the non-invasive detection of nonradioactive metabolites in living animals. We applied this approach to the noninvasive detection of 13C-arachidonate in brain and liver of living suckling rats but found that technical limitations in our model, mainly poor signal-to-noise, largely prevent useful results at this time. However, in a tracer study using simultaneous doses of 13C-gamma-linolenate and 13C-arachidonate, 13C-NMR of tissue lipid extracts quantitatively demonstrated a 10-fold greater (liver) or 17-fold greater (brain) accumulation of pre-formed vs newly synthesized arachidonate. GC-combustion-isotope ratio mass spectrometry was used to trace the utilization of [U-13C]-alpha-linolenate into three products in the brain: docosahexaenoate, cholesterol, and palmitate. The rationale was that although alpha-linolenate is used in de novo lipogenesis, the quantitative importance of this pathway is unknown. Our results in the suckling rat show that 2-13% of carbon from [U-13C]-alpha-linolenate appearing in brain lipids is in docosahexaenoate while the rest is in brain lipids synthesized de novo. Overall, these results indicate that the suckling rat brain prefers pre-formed to newly synthesized arachidonate and that alpha-linolenate is readily utilized in brain lipid synthesis. These methods are suited to comparative studies of the metabolism of polyunsaturates and they support previous observations that the metabolism of some polyunsaturates such as alpha-linolenate extends well beyond the traditional desaturation-chain elongation pathway.

The dietary bioequivalence during the brain growth spurt of alpha-linolenic (LNA) and docosahexaenoic acids (DHA) as substrates for brain and retinal n-3 fatty acid accretion is reported for the fetal baboons, whose mothers consumed a long chain polyunsaturate-free diet with a n-6/n-3 ratio of 10:1. Pregnant baboons received i.v. doses of U-13C-labeled fatty acids (LNA or DHA), plasma was collected from mother and fetus, and fetal brain (occipital cortex), retina, and liver were analyzed at various times post-dose. Fetal brain DHA plateaued 15-35 d post-dose with 1.6% of the preformed [U-13C-]DHA dose recovered in the brain. In contrast, LNA-derived DHA accretion also plateaued but was 20-fold lower. Liver and retinal results were of the same order of magnitude, but showed evidence of peaks and decline. Conversion products to n-3 long chain polyunsaturate were observed in the maternal circulation at 1 h after administration, as was transfer of both fatty acids to the fetus. From these measurements we estimate that a dietary level of about 0.45% of energy as LNA is sufficient to meet the requirements of the growing fetal brain, whereas 0.03% of energy as DHA would suffice. These data are the first direct measurements of the bioequivalence of DHA and LNA in developing primates and imply that n-3 fatty acid requirements for the developing fetal brain can be met by attainable dietary LNA for diets low in long chain polyunsaturates.