Abstract: Docosahexaenoic acid (DHA) is an omega-3 long chain polyunsaturated essential fatty acid which is important constituent of the phospholipid membrane of neuronal cells. In young children it enhances the neuronal development and cognitive functions whereas in older age it decreases progression of neurodegenerative disorders. The most important source of DHA is fish oil whereas infants depend on maternal DHA. However, diet of children is usually poor in DHA and DHA fortified food supplements become essential to maintain the DHA content in the body.
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).
Effect of DHA on brain: DHA is found in three phospholipids: phosphatidylethanolamine, ethanolamine plasmalogens and phosphatidyl serine (PS). These phospholipids are important constituents of plasma membrane of the neurons. DHA modulates the carrier-mediated transport of choline, glycine and taurine and the function of delayed rectifier potassium channels (4). Low DHA levels lower neural cell phosphatidyl serine and increase neural cell death (5).
It has been found that total n-3 and n-6 polyunsaturated fatty acids increase in the whole forebrain during the prenatal and post natal periods upto at least 2 years of age (6). Low maternal DHA is linked to poor child neural development and improving maternal DHA by supplementation decreases that risk (7).
In preterm and term babies, provision of n-3 LC PUFA is important for brain development. Human milk is a good source of DHA provided maternal DHA levels are normal. In young children, neurodevelopment and cognitive abilities are also enhanced by early provision of n-3 LC PUFA through milk or DHA fortified foods (8). Evidence supporting the potential importance of n-3 LC PUFA consumption for good cognitive health in older age is now emerging. Recent cross-sectional surveys have reported that higher n-3 LC PUFA consumption are associated with reduced risk of impaired cognitive function and prospective cohort studies have shown decreased risk of dementia in older people (2, 8, 9, 10). Recent evidence also indicates that in addition to the positive effects seen in chronic neurodegenerative conditions, omega-3 PUFA may also have significant neuro protective potential in acute neurological injury (11).
Sources of DHA: The DHA status of the newborn and breast-fed infant depends on the maternal intake of DHA and varies widely (7). DHA is enzymatically synthesized from ALA in liver but this synthesis is very limited. Also there is age-related reduction of hepatic desaturase activity (which participate in synthesis of LC-PUFA) (3). Thus, DHA is an essential fatty acid that is required from dietary sources. The main source of DHA are fish oils. Vegetable oils like flaxseed or linseed oil, rapseseed oil or canola oil, peanut oil, olive oil, soya oil, walnut oil, green leafy vegetables, fenugreek seeds, kidney beans, dry fruits are also sources of DHA (12). Inadequate intake of n-3 PUFA decrease DHA and increase n-6 PUFA in the brain. Western diets as well as diet in developing countries are low in n-3 PUFA (7) whereas consumption of n-6 fatty acids is higher. The ideal ratio of n-6 and n-3 PUFA in diet should be 5-10:1 for optimal health benefits (13). DHA is also commercially manufactured from microalgae; crypthe codinium cohnii and schizochytrium (14) which is vegetarian.
Brain growth in children: The human brain begins forming by 3rd week of gestation and 100 billion neurons are formed during first 5 months of gestation. Recent evidence suggests that new neurons are produced throughout life though far less rapidly and probably in numbers sufficient only to replace those that gradually die off (15). However brain size increases more gradually; a newborn's brain is only quarter the size of an adult (bout 360-380 gm), grows to about 80% of adult size by three years of age (1090-1270 gms) and 90% by 5 years of age. Adult brain size is (1170-1310 gms) (16). Brain growth is largely due to growth of the dendrites of the neurons. Myelination begins around birth and is most rapid in the first 2 years but continues as late as 30 years. The number of synapses in the cerebral cortex peaks within first few years of life but then declines by about one third between early childhood and adolescence (15). Thus brain development is most sensitive to a child's nutrition in fetal period, in first 2 years of life and early childhood. Because of rapid pace of myelination in early life, children need a higher level of fat and fatty acids in their diet (15). With 2/3rd of the human brain weight being phospholipids and DHA being an important part of phospholipids, adequate DHA content is crucial for brain development (17).
Conclusion: Omega-3 fatty acids especially DHA are essential for brain development in infants and children and for decreasing neuropsychiatric disorders and cognitive dysfunction in older age.
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