Introduction: Docosahexaenoic acid (DHA) is the most abundant omega-3 (n-3) long-chain polyunsaturated fatty acid (LC-PUFA) in the brain and retina. It constitutes 40% of the PUFA in the brain & 60% of the PUFA in the retina. Fifty percent of the weight of the neuron's plasma membrane is composed of DHA (1).
Effect of LC-PUFA on brain: Both n-3 and n-6 LC-PUFA play important roles in neuronal growth, development of synaptic processing of neural cell interaction, and expression of genes regulating cell differentiation and growth (2). It has been shown that differentiation and functioning of cultured brain cells requires not only alpha linolenic acid ((ALA) (precursor of DHA) but also very long n-6 and n-3 PUFA. ALA deficiency alters the course of brain development, perturbs the composition and physicochemical properties of brain cell membranes, neurones, oligodendrocytes and astrocytes. This results in neurosensory and behavioral changes. ALA deficiency induces more marked abnormalities in frontal cortex and pituitary gland as compared to other areas of the brain. Thus intellectual abilities, cognition, behavioral aspects, neuropsychiatric disorders are affected by LC-PUFA (3).
Sources of LC-PUFA: Both fetus and newborn infant are dependent on maternal supply of DHA which is in turn related to mother's long-term dietary intake.(8) For healthy infants, breast milk supplies LC-PUFA. If infant formula is given, 0.2% DHA & 0.35% of AA of total fatty acids is useful. For preterm infants, since they are born with much less total DHA & AA, they will need an infant formula that includes at least 0.35% DHA and 0.4% AA.(3)
Role of LC-PUFAs in growing period:Two important LC-PUFA that have been recognized for cognitive development are DHA (a n-3 fatty acid) and Arachidonic acid (a n-6 fatty acid). DHA is a major component of retina reaching concentrations of 30-40% of total fatty acids in the rod photoreceptor in human retina.(3) There is a growth spurt in the human brain during both last trimester of pregnancy and first post natal months with a large increase in the cerebral content of AA & DHA.(3) Thus LC-PUFA are associated with improved visual and cognitive development. It has been found that breast fed children have higher IQ scores compared with children who received on infant formula that did not contain LC-PUFA.(5) Also, mothers whose children were found to be allergic have shown to have lower levels of DHA in their breast milk as compared to milk from mothers of non-allergic children.(6)
A number of studies have shown that consumption of foods such as fish oil containing n-3 fatty acids; eicosapentaenoic acid (EPA) and DHA is associated with decreased risk of coronary heart disease.(1) However, since n-6 and n-3 fatty acids complete for same enzymes and have different biological roles (often opposing effects) on smooth muscle cells, platelet aggregation and vascular permeability, the balance between n-6 and n-3 fatty acids in diet is of considerable importance.(1) Thus for preterm infants, LC-PUFA intake should be similar to that of human milk with AA to DHA ratio of 1.5 and a DHA content of 0.4%.(2) Addition of LC-PUFA in infant formulas with appropriate regard for quantitative and qualitative quality enables the formula fed infant to achieve the same blood LC-PUFA status as that of breast fed infants.(2)
A cochrane analysis of 11 randomised trials comparing LC-PUFA supplemented formula milk Vs non-supplemented formula milk in term infants found that 3 studies found beneficial effect of LC-PUFA on visual acuity whereas no beneficial effect was seen on physical growth.(7) However no adverse effect of LC-PUFA have been noted when supplemented to formula.
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