The Canadian Society for Nutritional Sciences 1995 Young Scientist Award Lecture: Recent Studies on the Synthesis, Beta oxidation, and Deficiency of Linoleate and Alpha-linolenate: Are Essential Fatty Acids more Aptly Named Indispensable or Conditionally
The Canadian Society for Nutritional Sciences 1995 Young Scientist Award Lecture: Recent Studies on the Synthesis, Beta oxidation, and Deficiency of Linoleate and Alpha-linolenate: Are Essential Fatty Acids more Aptly Named Indispensable or Conditionally
Year: 1996
Authors: S C Cunnane.
Publication Name: Can. J. Physiol. Pharmacol.
Publication Details: Volume 74; page 629.
Abstract:
In this review paper, the author presents the hypothesis that recent research published in the areas of the synthesis, beta-oxidation, and deficiency of LA and ALA suggests that the term essential fatty acid may be outdated. He reviews studies that have indicated LA and ALA can be synthesized from their respective 16-carbon precursors, which are present in the human diet. Retroconversion of DHA to EPA and possibly through to ALA has also been demonstrated in animal models. However, whether the rate of conversion and dietary supply of these precursors as well as retroconversion, is sufficient to meet metabolic requirements for LA and ALA will depend upon the physiological state of the individual. The main route of LA and ALA metabolism, as identified by whole-body fatty acid balances studies, is through beta-oxidation. In situations such as undernutrition and fasting-refeeding, research has shown that oxidation of the EFAs increases in response. The author indicates that a portion of the oxidized carbon from LA and ALA can be recycled and used for de novo synthesis of “non-essential” fatty acids and cholesterol. In the brain of the suckling rat, de novo synthesis of “non-essential” fatty acids and cholesterol can exceed conversion to longer chain PUFAs by as much as 10- to 40-fold. Cunnane suggests that the terms indispensable and conditionally dispensable be used to describe dietary requirements for LA and ALA. The term indispensable is appropriate for use in situations in which LA and ALA requirements are higher than normal such as in pregnancy, lactation and during rapid growth. Dispensable FA would be those for which there are no known limits to endogenous synthesis such as 1 to 14 C SFAs and MUFAs. He emphasizes that his position is supported by research showing the capability of humans to synthesize LA and ALA from their respective 16-carbon precursors, and the absence of clear symptoms of LA and ALA deficiency in healthy adults. Cunnane indicates that better data on actual intakes of dietary LA and ALA as well as on the synthesis, retroconversion and utilization of PUFAs in humans is required. In addition, the protective effect of n-3 PUFAs in cancer and coronary heart disease needs to be considered in determining the dispensability of ALA, EPA and DHA in humans at risk of developing these diseases.
