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Department of Human Ecology, University of Texas at Austin, Austin, TX 78712, E-mail: mtnakamura{at}mail.utexas.edu
World Review of Nutrition and Dietetics, Vol 83, edited by AP Simopoulos, 1998, 240 pages, hardcover, $215.00. Karger, Basel, Switzerland.
Fatty acids of the 3 (n-3) and 6 (n-6) series are 2 distinct groups of fatty acids, both of which are essential for humans. These fatty acids are essential because humans cannot synthesize them de novo and because they play essential physiologic roles. Therefore, at least 2 fatty acids, -linolenic acid (18:3n-3) and linoleic acid (18:2n-6), must be supplied by the diet as precursors. Humans can synthesize other essential fatty acids, such as docosahexaenoic acid (DHA; 22:6n-3), eicosapentaenoic acid (EPA; 20:5n-3), and arachidonic acid (20:4n-6), from these precursor fatty acids via a desaturation and elongation pathway shared by both n-3 and n-6 fatty acids. n-3 Fatty acids are involved in a variety of physiologic processes, including retina and brain function, modulation of eicosanoid action, and transcriptional regulation of gene expression. Many of these functions are the mechanism by which n-3 fatty acids reduce the risk of cardiovascular diseases. Despite the importance of n-3 fatty acids in health, especially in preventing chronic diseases, many questions remain unanswered about the precise mechanisms of n-3 fatty acid functions, the metabolism of n-3 fatty acids in the body, dietary recommendations, therapeutic value of preformed desaturation products, and how to supply n-3 fatty acids to large populations.
This volume, which addresses some of the above questions, is a compilation of the work presented in a conference entitled "International Conference on the Return of 3 Fatty Acids into the Food Supply: I. Land-Based Animal Food Products," which was held in September 1997 in Bethesda, MD. This book consists of 4 parts: 1) "3 Fatty Acids and Health," 2) "3 Fatty Acids in Land-Based Animal Food Products," 3) "Companion Animal Nutrition," and 4) "Scientific and Policy Aspects."
Part 1 presents selected topics on dietary requirements and the physiologic role of n-3 fatty acids. The first 2 chapters summarize anthropologic studies on human nutrition, implying that diets of preagricultural humans had higher ratios of n-3 to n-6 fatty acids (1:1) than do contemporary Western diets (1:10). Another chapter summarizes current understanding of how n-3 fatty acids prevent cardiac arrhythmia. Other chapters present studies on roles of n-3 fatty acids in bone metabolism and retinitis pigmentosa. Both studies are still in their early stages and are not conclusive.
Part 2 offers technologies emerging to deliver n-3 fatty acid–enriched foods to the table, and is the major strength of the book. Two chapters describe successful production of DHA in algae for use as a component of animal or human diets. Five chapters are allocated to enriching chicken eggs with n-3 fatty acids. These studies show that n-3-enriched eggs have the potential to become an alternative to fish consumption as an n-3 fatty acid source. The other chapters describe attempts to enrich meat and milk with n-3 fatty acids. In contrast with eggs, the enrichment of meat and milk seems to be more difficult because of changes in taste and (in the case of ruminants) disturbance of the rumen fermentation.
Part 3 is a brief summary of pet food nutrition that suggests a potential benefit of increasing n-3 fatty acids in the diet to prevent chronic diseases of cats and dogs. Part 4 mostly addresses dietary recommendations for n-3 fatty acids in several countries. Although the concern regarding low ratios of n-3 to n-6 fatty acids in Western diets is sensible, there is insufficient scientific evidence for this to alter the dietary recommendations for n-3 fatty acids.
The questions this book left unanswered reflect the status of research in the n-3 fatty acid field, indicating that we need more basic research on the metabolism and functions of n-3 fatty acids to provide clear directions to dietary recommendation, therapeutic use of n-3 fatty acids, and the production of n-3-enriched food. For example, some authors in this book assume that conversion of -linolenic acid to EPA and DHA is limited in humans, necessitating the supply of EPA and DHA in the diet. New data show the expression of the messenger RNA of 6-desaturase, the rate-limiting enzyme for the synthetic pathway, in many human tissues. The highest expression has been found in the brain, suggesting active synthesis of EPA and DHA in human tissues, including fetuses (1). Fortunately, explosive advances in molecular biology and genetics provided tools to rapidly move this field forward and have yielded promising results, such as the cloning of human 6-desaturase (1), the determination of the molecular mechanism of gene regulation by fatty acids (2), the role of DHA in the transport of retinal in retina (3), and the direct interaction of fatty acids with ion channels (4). In conclusion, this book provides a timely introduction to the topic of n-3 fatty acids, especially n-3-enriched animal food production.
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