Skin lipids (ceramides) that contain 18:2o6 perform a specific function in preventing transepidermal water loss. Dietary vegetable oils that are rich in 18:2o6 tend to reduce plasma cholesterol, probably by stimulating the hepatic uptake of low-density lipoprotein. Poly-unsaturated fatty acids perform major roles as components of membrane phospholipids, where the degree of unsaturation is a key determinant of the biophysical properties of the membrane. Very high proportions of 22:6o3 are present in the phospholipids of neuronal cells of the brain and of the rod photoreceptor cells of the retina. Optimal functional development of the neural tissues is dependent on adequate provision of 22:6o3 during fetal and neonatal life. Mammalian sperm phospholipids also display high proportions of 22:6o3, presumably to enhance the flexibility of the sperm tail membranes.
Polyunsaturates, particularly 20:5o3 and 22:6o3, are powerful regulatory molecules. By interacting with specific transcription factors (PPARa, SREBP-1, NF-Y), they alter the pattern of gene expression in liver cells, profoundly altering the concentrations of key metabolic enzymes. These changes increase the p-oxidation of fatty acids and simultaneously inhibit the synthesis of fatty acids and triacylglycerol, thereby reducing
Table 1 Omega 6 and 3 fatty acids: Dietary sources and metabolic functions
Linoleic a linolenic Arachidonic
18:2o6 18:3o3 20:4o6
Plant seed oils
(e.g., sunflower, safflower, maize) Green leaves, flaxseeds, linseed oil Meat, eggs
Oily fish, seafood
Oily fish, seafood
Energy source; skin lipids; precursor of 20:4o6
Energy source; precursor of 20:5o3 and 22:6o3
Membrane structure; signal transduction; gene expression; fuel partitioning; precursor of eicosanoids; proinflammatory Gene expression; fuel partitioning; precursor of eicosanoids; anti inflammatory; cardiovascular protection
Membrane structure; brain, retina and sperm function; gene expression; fuel partitioning; anti inflammatory; cardiovascular protection
(Based on information from Refs. 1 5.)
lipoprotein secretion. Thus, dietary fish oils can reduce plasma lipids and inhibit the accumulation of body fat. Polyunsaturates also regulate cell function as a result of their conversion to eicosanoids (prostaglandins, thromboxanes, leukotrienes). Eicosanoids derived from 20:4o6 are generally more proinflammatory and pro-thrombotic than those derived from 20:5o3. Dietary fish oil reduces inflammation and thrombosis by antagonising the production and action of the o6-derived eicosa-noids.
The hypolipidemic, anti-inflammatory and antithrom-botic effects of o3 fatty acids, plus their antiarrhythmic and antihypertensive properties, explain the protective effects of dietary fish oil against cardiovascular disease. Beneficial effects of o3 fatty acids in the prevention or treatment of rheumatoid arthritis, autoimmune diseases, cancers, and mental disorders have also been reported.
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