Dietary Modulation of Retinal Fatty Acid Composition and Function

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The effect of maternal diet on the modulation of n-3 fatty acid in the retina of offspring has been studied in newly hatched chicks (Anderson et al., 1989), juvenile felines (Pawlosky et al., 1997) and piglets (Arbuckle & Innis, 1993). Docosahexaenoic acid appears to be the preferred fatty acid for raising the level of 22:6n-3 in the retina and brain among different sources of n-3 fatty acids (Anderson et al., 1990). Feeding corn oil supplemented with 22:6n-3 is able to restore the 22:6n-3 level in n-3-deficient chicks (Anderson & Connor, 1994) and felines (Pawlosky et al., 1997) probably through replacement of dipolyunsaturated molecular species (22:6n-3 - 22:6n-3) (Lin et al., 1994). These observations imply that 18:3n-3 alone in the diet as the n-3 fatty acid source may not be adequate for meeting the 22:6n-3 requirement for brain and retinal development, because 18:3n-3 is a less efficient precursor for 22:6n-3 when there is low A6desaturase activity (Anderson et al., 1990; Kohn et al., 1994).

Although retina and rod outer segment tenaciously retain 22:6n-3 during essential fatty acid deficiency (Connor et al., 1990, 1991; Wiegand et al., 1991), severe unbalanced n-6/n-3 diets or depleted n-3 fatty acid levels in membrane can cause abnormal change in biochemical and physiological membrane function. The level of 22:6n-3 in n-3 fatty acid-deficient chick brain and retina is restored by a diet containing 22:6n-3 (Anderson & Conner, 1994) and also after n-3 deficiency in the rhesus monkey (Neuringer et al., 1986; Neuringer & Connor, 1986). Functionally, n-3 fatty acid-deficient monkeys show delayed recovery of the dark adapted electroretinogram and impaired visual acuity at an early age (Neuringer et al., 1986), suggesting that n-6 fatty acids are not interchangeable with n-3 fatty acid in maintaining normal retinal function. After repletion with fatty acids from fish oil, the 22:6n-3 level increased rapidly after feeding, but no improvement in the electroretinogram occurred (Neuringer & Conner, 1989). Developing felines fed corn oil from gestation to 8 wk of age are devoid of a dietary source of 22:6n-3 and display increased a- and b-wave implicit time compared to diets containing long-chain polyunsaturated fatty acids (Pawlosky, 1997). Guinea pigs fed safflower oil through three generations also exhibit significantly decreased levels of 22:6n-3 and reduced both peak to peak and a-wave in the electroretinogram (Weisinger et al., 1996a, b, 1999). Rats raised for several generations with a fat-free diet also show decreased amplitude of the a-wave, which reflects the electrical potential of the photoreceptor membrane and altered rod outer segment disk renewal (Anderson et al., 1974). These studies indicate that n-3 fatty acids exert a key role in normal visual function that is dependent on the n-6/n-3 balance in the diet. It also indicates that the most critical time for providing an adequate diet would be during pregnancy and early lactation.

In term and preterm human infants, studies usually compare feeding breast milk versus infant formula with or without long-chain polyunsaturated fatty acids. Leaf et al. (1996) found correlation between percent intake of breast milk (>50% vs <50%) and 22:6n-3 in both plasma and red blood cells. Uauy and co-workers (1990, 1992) have reported that very low-birthweight neonates fed a soybean-oil-based infant formula have poorer early electroretinogram response, visual evoked potential (VEP) and forced choice preferential looking (FPL) compared to neonates fed human milk or marine-oil-containing formula. Preterm infants fed formula with marine oil (0.3% 20:5n-3 and 0.2% 22:6n-3) appear to have enhanced visual acuity measured by the Teller Acuity Card procedure until 4 mo of age (Carlson et al., 1993b). With a similar formula but using different tools

Fig. 1. Metabolic pathway of essential fatty acids. Recent evidence indicated that 22:6n-3 are produced by P-oxidation of 24:6n-3, which is desaturated from 24:5n-3, after elongation from 22:5n-3. Very long-chain fatty acids in the box are found in the retina; however, the metabolism and function is not known.

Fig. 1. Metabolic pathway of essential fatty acids. Recent evidence indicated that 22:6n-3 are produced by P-oxidation of 24:6n-3, which is desaturated from 24:5n-3, after elongation from 22:5n-3. Very long-chain fatty acids in the box are found in the retina; however, the metabolism and function is not known.

of visual function measurement, Werkman and Carlson (1996) have demonstrated the beneficial effect of 22:6n-3 on discrete looks to both novel and familiar stimuli and short overlook duration for novelty preference (visual recognition memory). The preterm infant measured at 52 wk of postconceptual age after feeding long-chain polyunsaturated fatty acid (0.3%, 22:6n-3) show the maturation pattern of visual evoked potential similar to breast-fed infants (Faldella et al., 1996). Jongmans et al. (1996) found that infants born prematurely with the absence of other major neurological symptoms are at risk for abnormal visual function and perceptual-motor difficulties at earlier school life. From the above studies, it is clear that an appropriately balanced diet can enhance visual function in infants.

Term infants fed breast milk for the first 4 mo of life increase visual acuity, measured by the Teller Acuity Card, more rapidly compared to formula-fed infants (Jorgensen et al., 1996). Carlson et al. (1996) have reported that term infants fed formula with 2% 18:3n-3 and 0.1% 22:6n-3 show better 2-mo visual acuity than infants fed formula. Makrides et al. (1995) also have found that visual evoked potential and visual acuity of breast-fed neonates and infants fed a formula supplemented with 22:6n-3 are better than those of placebo-formula-fed infants at 16 and 30 wk of age. Although these results show an effect of diet on the development of the visual function in different groups of infants, other studies have provided comparable results (Jorgensen et al., 1998).

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