Excessive extradermal water loss, one of the first symptoms described of essential fatty acid deficiency, has been explained. A major specific function of linoleate is in skin ceramides, where the linoleate is incorporated into acylglucosylceramides and acylceramides (Hansen and Jensen 1985). Linoleate is the only fatty acid substantially incorporated into these sphingolipids. On another front, the details of control of elongation and desaturation and interrelationships among the families of fatty acids are still being investigated (Cook et al. 1991; Cunnane et al. 1995; Sprecher et al. 1995).
Since the 1970s, an enormous literature about prostaglandin metabolism has accumulated. The term “eicosanoids” was introduced in 1980 to describe the class of substances having 20 carbon atoms derived from n-6 and n-3 fatty acids. Because eicosanoids are regulators of a large array of physiological functions, there are many possible manifestations of deficiency of their precursors. The attention given to the very long-chain fatty acids from marine animals (fish oils) has created another large body of literature about the interplay between metabolism of omega-3 (n-3) and omega-6 (n-6) families of fatty acids. The competition between the two families for enzymes of chain elongation, desaturation, and conversion to active metabolites first demonstrated by Holman (1964) has introduced additional complexity into the attempt to define the dietary requirements for the two families. We know that a source of 18 carbon n-3 and n-6 fatty acids is a dietary necessity, but whether there may be health benefits from consumption of the longer chain products - 20:4n-6, 22:5n-3, and 22:6n-3 - is a current topic of intensive research. Infant requirements for brain development are important considerations for assuring appropriate recommendations for infant formulas and supplements (Carlson et al. 1993).
A broad range of ongoing research is aimed at defining the functions of n-3 fatty acids, and a deficiency has been associated with the function of rhodopsin (Bush et al. 1994). Many aspects of brain and behavioral development are linked to the availability of linolenic acid and its products (Neuringer, Reisbick, and Janowsky 1994).The effects of the ratio of dietary linoleic to linolenic acids is the subject of current research (Jensen et al. 1996), and immune functions and cell signaling are also current topics of study of fatty acid functions and metabolism (Hayek et al. 1997; Hwang in press).
Summary
The evolution of an understanding of essential fatty acids has progressed through several scientific developmental phases. The earliest was the ability to prepare diets of sufficient purity to exclude lipids, which led to the conclusion that some fat was essential. The substantive proof of a human requirement - that is, that the fatty acids were essential to the diet - had to await the advent of total parenteral feeding. In about the same era, eicosanoids were discovered, which opened a new world for understanding the functions of essential fatty acids. With the tools of science today, the profound participation of fatty acids in all aspects of cellular life and function should provide great excitement as well as great challenges to all scientists interested in the expansion of the science of nutrition.
Jacqueline L. Dupont
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World History
