Alpha linolenic acid (ALA, C18:3 (n-3)) is a member of the omega-3 family of fatty acids. Alpha linolenic acid is important in human nutrition because it is considered an essential nutrient, and therefore a deficiency of ALA in the human diet causes a deficiency disease that ultimately results in death. In humans, ALA is metabolised through a series of elongation and desaturation reactions to form the long chain fatty acids eicosapentaenoic acid (EPA, C20:5 (n-3)) and docosahexaenoic acid (DHA, C22:6 (n-3)) which have important physiological roles in humans. In particular EPA and DHA form a series of short lived signal molecules called eicosanoids and docosanoids, respectively. These signal molecule regulate all aspects of cellular function but have been researched most extensively for their role in the regulation of the inflammatory reaction. In addition, the long chain fatty acids derived from ALA play important roles in the central nervous system where they may be required as structural components of neurones.
The history of ALA dates back to the 1920’s when Burr and Evans discovered that disease could be induced by withdrawal of polyunsaturated fatty acids from the diets of animals. They tentatively identified the substance as vitamin F, but subsequent work revealed that there were in fact two substances one being the omega-3 fatty acid ALA, the other being the omega-6 fatty acid linoleic acid (LA, C18:2 (n-6)). The realisation that the substances were fatty acids resulted in the vitamin F name being dropped and the substances were reclassified as essential fatty acids. Despite the obvious link between ALA and LA deficiencies and disease, it was not until the 1980’s that Bergstrom, Samuelsson and Vane identified the mechanism for this association. They subsequently received the Nobel Prize for their identification of a series of messenger molecules including thromboxanes, leukotrienes, prostaglandins and prostacyclins (eicosanoids) that regulate cellular function and are required in specific ratios for health.
However, despite obvious clinical evidence that ALA was an essential fatty acid in humans and other animals, information regarding the importance of ALA has been slow to permeate to the mainstream. In particular, medicine did not understand the importance of ALA well into the latter part of the twentieth century. For example, in the prestigious American Journal of Clinical Nutrition1, an article in 1982 listed 13 vitamins, 15 minerals and 1 fatty acid (linoleic acid) as essential to human health. In the paper the authors state that ‘the classic experimental approach to determining essentiality and requirement of a nutrient involves feeding purified diets containing all known required nutrients except the one under study. If the nutrient is essential in the diet, the experimental animal develops a unique deficiency syndrome that can be defined in terms of the effect on growth, signs and symptoms of illness, changes in body composition, changes in physiological function of tissues and organs, or biochemical alterations’.
Clearly the authors were not aware of the research in the 1920’s by Burr and Evans and the subsequent work in the 1930’s showing that withdrawal of ALA was able to induce unique deficiency symptoms, including effects on growth, changes in body composition, changes in physiology and changes in biochemistry. Interesting also that the authors comment on the speed at which an essential fatty acid deficient state develops when total parenteral nutrition (feeding a nutritionally controlled solution through a catheter placed into a vein) is administered. This is hardly surprising based on evidence that total parenteral nutrition was administered without the addition of ALA even into the 1980’s2 despite evidence showing its essentiality dating back almost 60 years. Cases like this illustrate the incredible compartmentalisation within science that prevents the free flow of ideas between different disciplines and also leads one to question if the list of essential nutrient we have today is complete.
Dr Robert Barrington’s comments: A number of nutrients are known to be essential in animals but essentiality has not been proven in humans. Arsenic and vanadium are for example essential in rats. The case of ALA is interesting because it took almost 100 years for the work of Burr and Evans to permeate to the mainstream and into the public consciousness. The fact that total parenteral nutrition was still administered without controlled levels of ALA well into the end part of the twentieth century shows the ignorance of solid nutritional research shown by the mainstream medical community.
RdB
