Docosahexaenoic acid

Fatty acids have been shown to influence function of the retina and to play a role in many pathological eye conditions. The bilayer membranes of the outer segments of the light-sensitive rod and cone photoreceptors in the retina contain membrane phospholipids. These are primarily esterified with high concentrations of docosahexaenoic acid (DHA), a long-chain n-3 polyunsaturated fatty acid (PUFA).26 The essential n-3 fatty acid a-linolenic acid can be converted to DHA in the liver or retinal epithelium.

Evidence for the role of DHA in the development of visual function has been found from the study of premature babies.26 Accumulation of DHA in retinal tissue is highest during the last three months of pregnancy (when retinal membranes develop) and until six months after birth. It has been shown that premature babies fed formula milk lacking DHA have compromised visual acuity compared with those fed formula milk enriched with fish oil containing DHA at levels approximating those found in breast milk. This poor visual acuity was related to an immaturity in rod cell function. For breastfed babies DHA intake will depend on the mother's diet.

The best dietary sources of DHA are coldwater fish, such as mackerel and tuna. Supplementation of DHA may be suitable for pregnant or breastfeeding women whose diets may be low in DHA, but more research is needed.

The role of fatty acids in retinitis pigmentosa has also been researched. Patients with this condition are consistently found to have abnormal blood levels of fatty acids. In particular, DHA is reduced and this might be due to an impairment in the final steps of DHA synthesis.26 There may be a role for DHA in maintaining optimal visual function, and supplementation with DHA may be of value. People with high intakes of DHA have also been shown to be less likely to suffer from dry eye syndrome (when not enough tears are produced), which can lead to corneal scarring and vision loss.27

A further application of the n-3 PUFAs DHA and eicosapentaenoic acid (EPA) is in the common eye condition called dry eye syndrome.28 Dry eye syndrome leads to lower visual acuity and difficulty in reading/writing and night vision. Inflammation or blockage of the lacrimal duct is frequently responsible, and artificial tears provide incomplete symptomatic relief. A sample (1546 patients) out of 39 876 female health professionals who were affected by the syndrome had their dietary fat intakes investigated using a food frequency questionnaire. A high ratio of n-6 to n-3 fatty acid consumption was associated with an increased risk of the syndrome, and tuna consumption was inversely associated with risk.28

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