I. Early development of the central retina

Infants typically experience a broad range of sensory input. This is fortuitous since it is clear that even subtle deficiencies can have significant effects on visual development. Sugita,¹ for instance, demonstrated the effects of varying ambient illumination on the visual system of four Japanese monkeys (macaca fuscata). This species typically has trichromatic vision similar to humans. Sugita, however, raised the monkeys (beginning at one month) in an environment where the ambient illumination randomly switched between four monochromatic conditions: 465, 517, 592, and 641 nm. These wavelengths were chosen to stimulate the three cone types but never simultaneously as would occur with the type of broad-band light to which an infant is normally exposed. Such rearing produced highly abnormal colour vision. For example, simple colour matching tasks that normal monkeys could master quickly took thousands of trials. Each cone type received sufficient stimulation in isolation in order to develop normally, but the post-receptoral pathways necessary for categorizing colours and generalizing across categories did not develop. These results are consistent with a widely known phenomenon: species-typical sensory input is necessary early in life in order for the visual system to develop properly.

The visual system develops rapidly during the first years of life. For example, the size of the retina increases by a factor of 1.8 every year from birth until the age of six years.² Cone density in the macula (the central region of the retina, about 5-6 mm in diameter) increases by about 80% from birth to 4-5 years.³ In adults, the central portion of the macula (about 1.5 mm in diameter) contains a well-defined area called the fovea centralis which serves as the focal point of vision (e.g., whenever one is viewing an object they are lining that item up with their fovea) and contains mostly cone photoreceptors. Morphologically, the adult fovea is characterized by a small depression created by the centrifugal displacement of inner retinal cells. This displacement serves to reduce the amount of visual clutter between the incoming light and the photoreceptors most responsible for our keenest visual capabilities (fine resolution, colour vision, etc). The peripheral retina of the newborn infant, rich in rods, is relatively similar in structure to the adult. The infant fovea, however, is still quite immature with cones that are very broadly spaced and poorly developed. The outersegments (the portion containing the discs of photopigment) are thin and short in relation to the innersegments which are quite broad (in the adult, the inner-and outer-segments are similar in diameter). The large outersegments create a coarse receptoral lattice (covering only 2% of the newborn fovea compared to about 68% in the adult fovea⁴ which produces correspondingly low visual acuity (a Snellen acuity of about 20/1000). Despite such large inchoate anatomical differences, some visual capabilities (e.g., dark adaptation thresholds) reach adult levels as early as three months. Other visual performance indices (e.g., hyperacuity) do not appear to reach adult levels until much later (e.g., 5-6 years).⁵

As noted, it is quite clear that normal visual development requires species-typical visual input. A related question is whether it also requires specific nutritional input. Obviously, the visual system, like most bodily systems, requires sufficient micro-and- macro nutrients in order to prevent deficiency diseases. For example, retinol (Vitamin A) helps form photopigment and therefore insufficient intake of retinol will result in a lack of photopigment and blindness. If, however, an infant has sufficient intake to prevent gross deficiency, does increased intake of certain nutrients provide additional benefit? If so, what nutrients should be targeted? These questions are only beginning to be raised. Two classes of nutrients have received some studies for their roles in vision: long-chain polyunsaturated fatty acids like docosahexaenoic acid (DHA) and the carotenoids lutein (L) and zeaxanthin (Z). Recent evidence has indicated that, in fact, the two nutrient classes may operate synergistically. For example, when DHA, L and Z are taken together, serum and retinal levels of L and Z are higher than if taken without DHA.⁶

II. The anatomical characteristics of retinal lutein and zeaxanthin

Long-chain polyunsaturated fatty acids like DHA and Arachidonic acid (AA) are essential structural components of the lipid membranes of receptors (and other neural cells) and are known to influence neuronal growth and communication.⁷ DHA is derived from dietary intake of omega-3s which are obtained mostly from food sources such as fatty fish. When ingested by the lactating mother, omega-3s are delivered to newborns directly through breast milk.⁸ A wide array of evidence suggests that insufficient intake of DHA has direct effects both on receptoral physiology and subsequent visual function.^7,9 Taken together, the evidence was sufficient to motivate producers to add DHA to infant formula in amounts similar to those found in breast milk. There is some evidence that such supplementation does, in fact, relate to improved visual function.¹⁰

Accumulating evidence also suggests another class of nutrients that might be vital to the developing retina, namely the two primary carotenoids found within the retina (and other visual structures such as the lens, retinal pigment epithelium, RPE, ciliary body, iridial tissue, visual cortex, etc.), L and Z. There are about 600 carotenoids found in nature, but only about 20 in human serum, and only two in the eye.¹¹ L and Z cannot be synthesized de novo and therefore they must be obtained from the diet. L and Z are primarily found in dark green leafy vegetables such as kale and spinach and some coloured fruits (e.g., Wolfberry). Once ingested, L and Z are incorporated into lipid micelles within the gut and are transported to the liver for further packaging into lipoproteins.¹² High-density lipoproteins and, to a lesser extent, low-density and very low-density lipoproteins transport L and Z to the retina.¹³ Within the eye, L and Z are especially dense in the inner retinal layers around the cone axons.¹⁴ The intense deposition of these yellow-coloured pigments in this region (in and around the fovea shown in Figure 1) has provided the basis for the clinical description of this area as the macula lutea (yellow spot). Macroscopically, this spot is only about 18 degrees in visual angle (5-6 mm) but has dramatic importance in vision. For example, the area of the cortex subserving this region is greatly magnified to reflect the fact that a majority of the brain's visual information processing is due to input from this small region. Degeneration of this area, as in the disease age-related macular degeneration (AMD), causes legal blindness.

L and Z within the macular region are also referred to as the macular pigment (MP), and individual differences in their levels are related to a variety of visual outcomes.¹⁵ Unlike many nutrients within tissue, MP optical density can be measured reliably and accurately using non-invasive psychophysical methods (see review by Hammond et al., 2005).¹⁶ Use of such technology on adult populations has revealed that MP is one of the most variable features of the fovea ranging from near undetectable levels (using optical methods) to densities as high as 1.5 OD at 460 nm.^17-20 Since light must pass through the MP before reaching the receptors, it screens the vulnerable, lipid-rich outer retinal layers (e.g., the receptoral outersegments and RPE) according to its spectral absorption profile. See Figure 2. The wide variability of MP levels across individuals is similar to that seen when examining individual differences in serum levels and dietary intake of L and Z.¹⁸ As with dietary patterns, which are relatively stable, individual differences in MP optical density (OD) also tend to persist over time. Hammond et al. (1997)²⁰ measured the MP OD of ten subjects over periods ranging from 1-16 years. They found that the MP of these subjects changed very little suggesting that, in the absence of significant dietary change, individual differences in MP OD are stable.

Bone et al. (1988)¹⁹ originally showed that MP levels in the infant retina were highly variable across infants (ranging from 4.5 to 66.2 ng within the first 1.5 yrs) although the pattern of carotenoid deposition differs from that of adults. Z is the dominant carotenoid in the centre of the adult retina and L predominates in the periphery. In contrast, L dominates in the centre of the newborn retina (at this point, of course, the macula is quite immature and similar to the periphery). Unlike in adults, the factors that explain individual differences in infant MP have not yet been studied; however, dietary intake of L and Z is clearly still necessary. Whereas MP can be manipulated in the adult via intake of carotenoid-rich foods,²⁰ the obvious concern with infants is that food options in the first few months are limited to breast milk or manufactured infant formulas. Breast milk contains at least 300 defined nutrients, whereas most infant formulas contain approximately 60-70 defined nutrients.²¹ Currently, most commercially-available infant formulas do not contain L and Z in other than trace amounts, and many formulas are completely devoid of L. In contrast, breast milk contains L and Z in concentrations that are approximately proportional to maternal intake of these carotenoids.²² These observations are important since many infants are exclusively formula-fed (e.g., the American Academy of Pediatrics reports that only half of U.S. infants are exclusively breastfed at birth; Section on Breastfeeding, 2005). Johnson et al. (1995)²³ showed that breast-fed infants had significantly higher plasma L and Z than formula-fed infants when tested one month after birth. This implies that retinal levels in formula-fed infants are also low.

There is currently only limited evidence regarding the effects of low levels of L and Z on the developing retina. At least three effects are feasible, however. Those effects are maturational, protective, and optical; evidence for each will now be considered.

III. Effects of low levels of lutein and zeaxanthin on the maturation of the retina and retinal pigment epithelium

Direct effects of nutritional deprivation on the retina are not easy to quantify: both the extent of the deprivation and the anatomical changes are difficult to measure. With respect to the latter, for example, functional measures are difficult since it is possible that architectural changes within the retina could be compensated for by post-receptoral mechanisms. The nervous system is, after all, very plastic early in life. For these reasons, animal models are often selected to study the retina in isolation. These studies are very expensive and a model must be selected that has a macula and nutritional characteristics that resemble the human norm. Leung et al. (2004)²⁴ chose the Rhesus monkey for this reason. Lower than average intake of DHA has similar effects on the maturation of the visual system in humans²⁵ and rhesus and the rhesus is often used as an animal model to study macular pigment. Leung et al. (2004)²⁴ raised 18 monkeys on semi-purified chow containing no L or Z (and varying amounts of omega-fatty acids) and these monkeys were compared to 15 monkeys raised on chow containing a normal amount of L and Z and omega-fatty acids. Six monkeys remained on carotenoid-free chow throughout their entire life, and six were supplemented with pure L and pure Z for 6-24 months prior to death. After the monkeys were sacrificed, the retina was dissected and cell counts were obtained in serial sections. The authors found that an absence of L and Z and omega-fatty acids created gross anatomical changes in the RPE that could be partially reversed by later supplementation of these nutrients. One example of the deficits they observed is the obvious architectural changes shown graphically in Figure 3. Another major difference is the overall increase in cell density which may also characterize the ageing retina.²⁶ As noted by the authors, "Increased RPE density may be induced by stress on the photoreceptor-RPE complex due to altered metabolism or other unfavourable factors. Similar mechanisms may underlie the increased density of RPE cells with normal ageing." The authors concluded that L and Z act synergistically with DHA and that both are essential for the normal development of the RPE and overlying retina.

IV. Effects of decreased protection of the retina due to low levels of lutein and zeaxanthin early in life

Empirical data indicate that light from 400-500 nm is particularly damaging to retinal tissue,²⁷ which has led to this region of the visible spectrum being referred to as the "blue light hazard" (IESNA Photobiology Committee for ANSI; ANSI/IESNA RP-27.1-05). In the adult retina, light shorter than about 400 nm is largely screened by the cornea and lens. Light longer than about 500 nm can damage the retina, but primarily through thermal mechanisms, which require relatively high intensities. "Blue light," however, reaches the retina and is energetic enough to initiate photochemical damage. By absorbing actinic short-wave light before it damages the outer retina, MP would reduce this damage. Given equivalent light histories, an individual with high MP would be expected to suffer less light-initiated damage over many years when compared to an individual with low MP.

There are several nonexclusive models describing the general ageing of the retina and the development of diseases such as AMD. These models include factors such as genetic susceptibility, impairment in choroidal circulation, degradation of Bruch's membrane, and oxidative stress. The idea that oxidative stress is a primary feature of AMD development has evoked wide scientific interest and is supported by an extensive, and diverse, set of studies.²⁸ According to this model of AMD development, degeneration of the retina is cumulative (i.e., starts at birth and continues throughout life) and factors that increase oxidative stress (e.g., blue light) accelerate the ageing of the retina, while factors that decrease oxidative stress (e.g., antioxidants) retard progression.²⁹ If this model is accurate, it follows that by reducing damage early in life, the probability of disease in later life is similarly reduced. This may be particularly significant since a good proportion of the damage to the retina that occurs over the lifespan may, in fact, occur very early.

For example, actinic damage to the retina may be more significant for infants since the crystalline lens of infants transmits an even higher proportion of ultraviolet and blue light.³⁰ This would not be meaningful if children were not exposed to significant amounts of light stress. However, many children are clearly exposed to significant periods of outside light without the benefit of sunglasses or other forms of protection (e.g., during recess or other outside play times). A recent study³¹ of children in Australia (n = 71, 3-15 years of age) showed that fully one-third of the subjects had clinically significant penguecula. These areas of thickened conjunctiva often occur due to extended exposure to the sun.

Exposure to highly-energetic light, in the presence of a photosensitizer, is known to initiate oxidative damage. In addition to their ability to absorb blue light before this damage can begin, L and Z are also well known for their ability to quench the kind of reactive oxygen species generated within the retina. Due to its high metabolic requirements, oxygen tension in the outer retina is high. Oxygen in its ground-state or triplet form (³O₂) is a diradical; meaning that it possesses two unpaired electrons spinning in coordinated parallel orbits. In this form, oxygen is relatively stable. If triplet oxygen, however, absorbs enough energy to reverse the spin of one of its unpaired electron orbits (e.g., by absorbing short-wave light), it can convert to a more reactive singlet form (¹O₂). This reactive species of oxygen is particularly damaging within a lipid-rich cellular environment such as that found within the retina. Receptoral outersegments contain high quantities of polyunsaturated fatty acids (PUFA). If reactive oxygen species are not quenched, they can peroxidize membrane lipids. For instance, reactive oxygen species can abstract hydrogen atoms from PUFA-rich receptoral membranes. The withdrawal of hydrogen will convert the PUFA into an organic radical that may then react in a similar manner with adjacent PUFA molecules.

Carotenoids are carbon ring compounds linked to chains that possess alternating single and double bonds. This extended conjugated double bond system allows these compounds to form extremely stable peroxy radicals. Consequently, carotenoids can tolerate the loss of an electron since this loss is distributed throughout the poly-isoprenyl chain. The "excited" carotenoid can easily relax into its ground state by dissipating the excess energy as heat. L and Z may prevent peroxidation by both screening the receptors from SW light and by quenching the peroxidation process before it completely oxidizes receptoral membranes. L and Z are optimally placed within lipid-rich receptoral outersegments to accomplish this purpose.³²

The antioxidant function of L and Z might be particularly meaningful to children. The metabolic activity of the developing nervous system is even higher than that of the adult. The infant retina is therefore under even higher levels of oxidative stress.³³ The combination of increased light and oxidative stress may be one reason why the accumulation of lipofuscin is so rapid in childhood. See Figure 4. Lipofuscin is an autoflourescent pigment that is often used as an assay of retinal ageing. It is thought to accumulate in the RPE, eventually disrupting the interaction between the overlying retina and RPE which results in retinal diseases such as age-related macular degeneration.¹ L and Z have been shown to prevent photooxidation of A2-PE/A2E,³⁴ a major and toxic component of lipofuscin. A dramatic amount of retinal "ageing" appears to occur very early in life. Protection of the developing retina by L and Z would theoretically prevent much of this ageing, thus presumably retarding the development of age-related diseases much later in life.

V. Optical effects of low levels of lutein and zeaxanthin

As noted earlier, proper maturation of the visual system requires sufficient sensory input. For example, early errors in refraction (e.g., anisometropia) can lead to later amblyopic deficits. It may therefore be significant that so much light entering the eye is poorly focused on the plane of the retina. When an eye of normal axial length is in focus for middle-wave or green light (as it would be for most phases of natural sun light), short-wave or blue light will be focused significantly anterior to the retina, and long-wave or red light slightly posterior to the retina. This effect is known as longitudinal chromatic aberration. For 460 nm blue light (the dominant wavelength of typical phase of daylight, and peak absorption of MP; see Figure 2), the magnitude of this focusing error is approximately -1.2 diopters. When an individual is viewing a natural scene illuminated by broad-band light the accommodative signal is at about 550 nm which means that much of the short-wave region would be seriously out of focus. MP would absorb this short-wave light in the inner layers of the retina before this light is transduced in the outer layers of the retina. Recent evidence³⁵ has shown that this effect does not improve acuity in adults. Nonetheless, the effects of the differential absorption of blurred light (i.e., some infants with high MP may absorb substantial amounts of blurred short-wave light, others with low levels may not) on the infant retina is not known.

In addition to reducing blur from chromatic aberration, MP could improve visibility outdoors by absorbing short-wave dominated skylight. The earth's atmosphere almost always contains small suspended particles from both natural and man-made sources called haze aerosol. Haze aerosol scatters short-wave light more than other wavelengths and results in a bluish veiling luminance. Blue haze, as it is sometimes called, is a major factor that degrades visibility, i.e., how well and how far we can see objects in the outdoors. MP may improve vision through the atmosphere by preferentially absorbing the SW energy produced by blue haze.³⁶ This would effectively increase both the contrast within objects and the contrast of distant objects with respect to their backgrounds.

Finally, MP could reduce the discomfort and disability caused by viewing objects under glare conditions. As a filter, MP would certainly reduce the intraocular scatter associated with a glaring light source. Stringham et al. (2004)³⁷ characterized visual discomfort using EMG recordings of the squint response. From this they generated a photophobia (acute glare discomfort) action spectrum, which peaked at 460 nm. Interestingly, Stringham et al. showed that MP reduced photophobia in a direct linear fashion. More recently, Stringham et al. (2007)³⁸ has shown that MP decreases photostress and increases the visibility of a target when veiled by a glare source. These effects were quite strong (correlation values around 0.7-0.8) and also strongly linear. To the extent that infants are bothered by bright lights, MP could reduce this discomfort and theoretically improve visibility under bright-light conditions.

Taken together, high MP density could improve the quality of the visual input that is processed by the developing infant brain. Such improvements could have lasting effects. More data is needed to address this important possibility.

VI. Conclusion and clinical implications

It is currently not clear what effects either high or low levels of L and Z have on the developing retina. Data from animal models suggest that dietary deficiencies of L and Z can cause distinct architectural changes to the RPE.²⁴ Humans evolved in an environment where dietary intake of carotenoid-rich foods was high, and where infants were routinely breast-fed for long periods of time. Deviations from this "natural" pattern may be meaningful. What we do know is that MP is a major component of the macula. We also know that this area is in a period of rapid development early in the lifespan. There is also a very large body of evidence that suggests that MP protects the retina and that the infant retina is very susceptible to the type of damage that MP protects against. Until weaning, breast milk is the only source of L and Z to the newborn and the amount of L and Z within breastmilk (or available to the foetus in utero) is dependent upon maternal intake. Advising pregnant and breast-feeding mothers to have sufficient intake of L and Z therefore seems warranted. Ultimately, infancy and late adulthood may represent vulnerable periods. Teller and Movshon³⁹ wrote that, for vision, "things start out badly, then they get better; then, after a long time, they get worse again." A large body of evidence suggests that L and Z may be particularly important during these vulnerable periods.

1. Lutein and zeaxanthin are major components of the human retina that are derived solely from the diet.

2. These pigments protect the eye from degenerative eye diseases like macular degeneration and cataract, and may also be critical to the proper development of the central retina and retinal pigment epithelium.

3. The primary source of lutein and zeaxanthin to newborns is breast milk, which contains these pigments in direct proportion to maternal intake.

4. Infant formulas that are currently available contain only trace amounts of lutein and zeaxanthin.

5. This article provides a plausible argument for how sufficient dietary intake of lutein and zeaxanthin may support optimal infant retinal development.