THE METABOLISM AND AVAILABILITY OF ESSENTIAL FATTY-ACIDS IN ANIMAL AND HUMAN TISSUES

Essential fatty acids (EFA), which are not synthesized in animal and h uman tissues, belong to the n-6 and n-3 families of polyunsaturated fa tty acids (PUFA), derived from linoleic acid (LA, 18:2n-6) acid alpha- linolenic acid (LNA, 18:3n-3). Optimal requirements are 3-6% of ingest ed energy for LA and 0.5-1% for LNA in adults. Requirements in LNA are higher in development. Dietary sources of LA and LNA are principally plants, while arachidonic acid (AA, 20:4n-6) is found in products from terrestrian animals, and eicosapentaenoic acid (EPA) and docosahexaen oic acid (DHA) are found in products from marine animals. EFA are prin cipally present in dietary triacylglycerols, which should be hydrolyze d by lipases in gastric and intestinal lumen. DHA seems to be released more slowly than the others. Its intestinal absorption is delayed but not decreased. Long-chain PUFAs are incorporated in noticeable amount s in chylomicron phospholipids. However, their uptake by tissues is no more rapid than uptake of shorter chain PUFA. In tissues, LA and LNA, which constitute the major part of dietary EFA, should be converted i nto fatty acids of longer and more unsaturated chain by alternate desa turation (Delta 6, Delta 5, Delta 4)-elongation reactions. Animal tiss ues are more active in this biosynthesis than human tissues. Liver is one of the most active organs and its role is critical in providing le ss active tissues, particularly the brain, with long-chain PUFA secret ed in VLDL (very low density lipoprotein). In liver, many nutritional, hormonal and physiological factors act on the PUFA biosynthesis. Diet ary fatty acids exert a great influence and are often inhibitory. Diet ary LNA inhibits Delta 6 desaturation of LA. The desaturation products AA, EPA, and DHA inhibit Delta 6 desaturation of LA and Delta 5 desat uration of DGLA (dihomo-gamma-linolenic acid). With regard to hormones , insulin and thyroxin are necessary to Delta 6 and Delta 5 desaturati on activities, whereas other hormones (glucagon, epinephrine, ACTH, gl ucocorticoids) inhibit desaturation. Concerning the physiological fact ors, the age of individuals is critical. In the fetus, the liver and t he brain are capable of converting LA and LNA into longer-chain EFA, b ut these are also delivered by the mother, after synthesis in the mate rnal liver and placenta. Just after birth, in animals, the Delta 6 des aturation activity increases in the liver and decreases in the brain. In aging, the capacity of the whole liver to desaturate LA and DGLA is equal at 1.5 and 25 months of age in rats fed a balanced diet through out their life and the AA and DHA content of tissue phospholipids is u nchanged in aging. Fatty acid oxidation in the liver is also likely to decrease the availability of EFA in tissues, in particular oxidation of long-chain PUFAs which is high in peroxisomes. However, in healthy individuals, despite the numerous factors likely to influence the avai lability of EFA, in most cases a regular and balanced diet meets the t issue requirements in n-6 and n-3 EFA