Abstract: Docosahexaenoic acid (DHA) is an important component of the retinal rods and important for development of visual acuity in infants and children.
Introduction: Essential fatty acids are structural components of all tissues and are indispensable for cell membrane syntheses; the brain, retina and other neural tissues are particularly rich in long-chain polyunsaturated fatty acids (LC-PUFA) (1). DHA (n-3 LC-PUFA) is the most abundant PUFA in the retina and brain and comprises 60% of the PUFA in the retina and 40% of the PUFA in the brain (2). DHA is esterified into phospholipids and is part of rod photoreceptor disc membrane and synaptic terminals (3). Numerous randomized controlled trials have demonstrated that relative sufficiency of n-3 LC PUFA results in improved sensitivity to light in infancy (1).
Development of eye and vision: The eye is derived from 3 types of embryonic tissue: the neural tube (neuroectoderm) from which arise retina proper and its associated pigment cell layer, the mesoderm with produces cornea, sclera and uvea and the ectoderm that produces the lens (4). In the early months of life, the visual system is still maturing; it is not fully developed at birth (and is even less developed in the premature infant). From birth to maturity, the eye increase to three times its size at birth and most of this growth is complete by age 3. (5). In a preterm, the optic nerve fibres are not myelinized and visual system is still not ready to function. In a full term, the retina (especially the macula) is not fully developed. By 2 years of age, myelinization of the optic nerve is completed and by 3 years of age, retinal tissue is mature (5). Children's vision continues to develop throughout their preschool years and eye/hand/body coordination, eye teaming, depth perception continues (6).
DHA and retina: Retinal rod outer segments contain high levels of n-3 PUFA with the outer segments having the highest DHA content of any cell in the body and an unusually efficient DHA retention mechanism (7). These DHA rich photoreceptors have faster electroretinogram (ERG) a-waves and also better amplitudes in human and animal studies (8-10) as well as improvement in ganglion cell function (11). Also beneficial effects have been shown on post-receptoral neurons (9, 12).
Thus, ability of n-3 LC PUFA in rod outer segments of the retina enhance photoreceptor signal transduction processing. Thus, DHA deficient subjects therefore require greater light stimulation to elicit the same level of photoelectric response. Also DHA plays a key role in photoreceptor growth and functional development by effect of DHA on gene expression (13).
Visual acuity in children and DHA: Breast fed infants have well documented increase in visual functions in the first year of life as compared to formula fed infants (14). For preterm infants, consistent results have been shown in various studies that infants who are breast fed had better visual acuity at 2-4 months of age (15) and more advanced retinal development than those who were formula feed (10). Other observational studies showed that breast-fed full term infants had better visual acuity at age 2 months or 4 months or better stereoacuity at 3.5 years (16-18) as compared to children who were fed non-supplemented formula milks. This benefit of breast feeding has been attributed to the presence of DHA (18).
In preterms and also full term infants, several randomized controlled trials have showed that infants who were fed DHA supplemented formula food had higher visual acuity at 2 and or 4 months (in preterms) and 6, 17, 16, 30, 52 weeks (in full terms) though less consistent (10, 16,19,20). This effect is mediated not only by the known effects on membrane biophysical properties, neurotransmitter content and the corresponding electrophysiological correlates but also by a modulating gene expression of the developing retina and brain (1).
DHA and age-related eye problems: In an ageing retina, there is decrease in phototransduction efficiency in rods resulting in decreased sensitivity of rods to light (21). Recent information suggests that higher intake of DHA is associated with lower risk for progression to advanced age-related macular degeneration (AMD) (22, 23).
Conclusion: DHA improves visual acuity in preterms and to some extent in full term infants and also helps to prevent age related macular degeneration due to its action on the photoreceptors in the retina of the eye.
References:
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