Essential fatty acids
ESSENTIAL FATTY ACIDSThree polyunsaturated fatty acids, linoleic acid, linolenic acid and arachidonic acid are called "essential fatty acids" (EFA). They cannot be synthesised in the body andmust be provided in the diet. Lack of EFA in the diet can produce growth retardation and other deficiency manifestation symptoms.Which EFA is Impoant?Linoleic acid is most impoant as, arachidonic acid can be synthesised from linoleic acid by a three-stage reaction by addition of acetyl-CoA. Pyridoxal phosphate is necessary for this conversion. Biologically arachidonic acid is very impoant as it is precursor from which prostaglandins and leukotrienes are synthesised in the body.Why EFA cannot be Synthesised?Introduction of additional double bonds in unsaturated fatty acid is limited to the area between - COOH group and the existing double bond and that it is not possible to introduce a double bond between the - CH3 group at the opposite end of the molecule and the first unsaturated linkage. This would explain body's inability to synthesise an EFA from oleic acid.Functions of EFA: (Biomedical Impoance)Structural elements of tissues: Polyunsaturated fatty acids occur in higher concentration in lipids associated with structural elements of tissues.Structural element of gonads: Lipids of gonads also contain a high concentration of polyunsaturated fatty acids, which suggests impoance of these compounds in reproductive function.Synthesis of prostaglandins and other compounds: Prostaglandins are synthesised from Arachidonic acid by cyclooxygenase enzyme system. Leucotrienes are conjugated trienes formed from arachidonic acid in leucocytes by the Lipoxygenase pathway.Structural element of mitochondrial membrane: A deficiency of EFA causes swelling of mitochondrial membrane and reduction in efficiency of oxidative phosphorylation. This may explain for increased heat production noted in EFA deficient animals.Serum level of cholesterol: Fats with high content of polyunsaturated fatty acids tends to lower serum level of cholesterol.Effect on clotting time: Prolongation of clotting time is noted in ingestion of fats rich in EFA.Effect on fibrinolytic activity: An increase in fibrinolytic activity follows the ingestion of fats rich in EFA.Role of EFA in fatty liver: Deficiency of EFA produces fatty liver.Role in vision: Docosahexaenoic acid (22:6n-3) is the most abundant polyenoic fatty acids present in retinal photoreceptor membranes. Docosahexaenoic acid is formed from dietary linolenic acid. It enhances the electrical response of the photoreceptors to illumination. Hence linolenic acid is necessary in the diet for optimal vision.Deficiency manifestations: A deficiency of EFA has not yet been unequivocally demonstrated in humans. In weaning animals, symptoms of EFA deficiency are readily produced. They are Cessation of growth.Skin lesions: Acanthosis (hyperophy of prickle cells) and hyperkeratosis (hyperophy of stratum corneum). Skin becomes abnormally permeable to water. Increased loss of water increases BMR.Abnormalities of pregnancy and lactation in adult females.Fatty liver accompanied by increased rates of fatty acids synthesis lessened resistance to stress.Kidney damage.CLINICAL ASPECTHuman deficiency: Some cases ofEczema like dermatitis,Degenerative changes in aerial wall andFatty liver in man may be due to EFA deficiency.There are also some repos that administration of EFA in such cases may produce:Some improvement of eczema in children kept on skimmed milk,Prevent fatty liver (some cases) andLowering of cholesterol levels.Infants and babies with low-fat diet develop typical skin lesions which have shown to be improved with EFA (linoleic acid).Fate of EFA: EFA undergoes b-oxidation after necessary isomerisation and epimerisation, like other unsaturated fatty acids.CLINICAL ASPECTAbnormal Metabolism of EFAAbnormal metabolism of EFA, which may be concerned with dietary insufficiency, has been noted in a number of disease like cystic fibrosis, hepatorenal syndrome, Crohn's disease, acrodermatitis enteropathica, Sjogren's syndrome, Cirrhosis and Reye's syndrome.Ref: M.N. Chatterjee - Textbook of Biochemistry, 8th edition, page no: 48-49
Linoeic acid
Linolenic acid
Arachiconic acid Palmitic acid
Plamitic acid
Metabolism of lipid