Although the underlying physiological mechanisms of AMD are not completely understood, available evidence suggests there may be considerable scope for preventing and treating this syndrome with rationally designed nutraceuticals. AREDS 1 has established that supplemental zinc, and the combination of vitamin E, vitamin C and beta carotene, have some modest utility, most notably in later-stage disease. But there is good reason to suspect—as is being assessed in AREDS 2—that long-chain omega-3s and xanthophyll carotenoids may be at least as useful in this regard, and quite likely beneficial for primary prevention. Additionally, evidence of varying degrees of cogency suggests that a wide range of additional agents—including phase-2 inducers, melatonin, N-acetylcysteine, spirulina, coenzyme Q10 and soy isoflavones—may merit consideration for AMD prevention and control.
This article focuses on better understanding the potential role of nutrition in AMD prevention and management. In the first section, Pedro F. Lopez, MD, reviews the underlying science behind the development and progression of AMD. This is followed by an interview with several prominent nutrition experts intended to guide us through the maze of scientific evidence that validates the use of nutritional regimens for controlling AMD. The interview centers on those supplements currently evidenced to slow progression and effect possible prevention of macular degeneration, and on measuring and monitoring the presumed effects of supplementation on the eye, enabling integration of a medical model of nutritional therapy into clinical practice.
The Underlying Science Behind the Development and Progression of AMD
Pedro F. Lopez, MD
To understand why nutraceuticals may be beneficial for the prevention and control of AMD, we need to understand the pathophysiology behind AMD.
Typically, AMD has a preclinical, asymptomatic phase, in which extracellular waste material accumulates in the space between the basement membrane (Bruch's membrane) and the pigmented epithelial layer (comprised of retinal pigmented epithelial cells) that overlies it. The result is a formation of yellow-white spots we all know as drusen.
Loss of RPE Tight Junctions
RPE cells are remarkably versatile in their contribution to healthful retinal function, and it is generally believed that dysfunction or death of these cells is crucial to AMD pathogenesis. The retinal pigmented epithelium serves as the blood-retinal barrier and regulates the transport of oxygen, substrates and waste products between the choroidal vasculature and the retina. So, loss of RPE tight junctions—which evidently compromises this barrier function—is a hallmark of AMD.
Decreased Phagocytosis of Photoreceptor Outer Segments
RPE cells also generate growth factors that aid the survival of retinal photoreceptors and other cellular constituents of the inner retina, while suppressing pathological choroidal neovascularization.
The most curious function of RPE cells is to phagocytize photoreceptor outer segments (POS). While doing this, they isomerize all-trans retinyl esters derived from POS back to 11-cis-retinal, which is then returned to the photoreceptors for use in rhodopsin synthesis. This is a crucial function, as photoreceptors are incapable of performing this isomerization themselves. The phagocytic function of RPE cells entails continual lysosomal activity.
When this activity is sub-optimally efficient, as it appears to be in AMD, lipofuscin deposits accumulate in these cells. Lipofuscin has photosensitizing activity, and components of lipofuscin—notably A2-E, a retinoid metabolite—can impair lysosomal function, in part by blocking the proton pump that acidifies lysosomes. (This raises the specter of a vicious cycle in which lipofuscin accumulation encourages further accumulation of lipofuscin.) Drusen that characterizes AMD is derived largely from inadequately degraded POS components as well as lysosomal proteins—suggesting that stressed RPE cells are disgorging half-digested lysosomal contents into the sub-epithelial space. Hence, drusen formation begets further drusen formation, the progression of which culminates in an atrophic macula.
Although the pathogenesis of AMD remains murky, there is a growing consensus that oxidative stress plays a key role in driving this syndrome.1-3 Chemical markers of oxidative stress have consistently been found to be elevated in the maculas of patients with AMD.4 In cell culture studies, exposure of RPE cells to oxidative stress has been shown to impair the function and survival of these cells—inducing a loss of tight junctions, impairing production of pigmented epithelium-derived factor (PEDF, which is a growth factor that provides trophic support for photoreceptors while inhibiting choroidal neovascularization) and of complement factor H (impaired function of which has been linked to increased AMD risk 5,6), and promoting apoptosis. All of these effects are consistent with the pathology of AMD.7-10
Moreover, oxidative stress can also induce apoptosis in photoreceptors, an effect which seems likely to contribute to the photoreceptor loss characteristic of dry AMD.11 Of course, the most well- characterized risk factor for AMD is cigarette smoking, which is well known to promote oxidative stress in tissues.12-15
Exposure to oxidative stress in the macula is high for a number of reasons. The rate of oxygen uptake by the retina is higher than in any other of the body's tissues. High mitochondrial respiration implies high potential for mitochondrial superoxide generation. RPE cells are phagocytic, and like other phagocytes, possess NAPDH oxidase that generates superoxide during phagocytosis.16-18 Most obviously, the macula is exposed to blue light that generates singlet oxygen and other reactive oxygen species when it interacts with photosensitizing chemicals. The lipofuscin that accumulates in stressed RPE cells is rich in such photosensitizers, some of which, such as A2-E, are derived from all-trans-retinal.19-21
A further line of evidence implicating oxidative stress in AMD is the fact that transgenic mice lacking superoxide dismutase activity have been shown to develop a syndrome very analogous to AMD as they age. Drusen, thickened Bruch's membrane and choroidal neovascularization are all observed, and the junctions linking retinal pigment endothelium are disrupted—features characteristic of human AMD.22 Increased light exposure accelerates accumulation of drusen in these mice, presumably because light promotes retinal oxidative stress.
In summary, degeneration of the RPE cells is considered the most important hallmark of AMD. This is clinically characterized by pigment mottling, accumulation of intercellular lysosomal lipofuscin and extracellular drusen, loss of tight junctions, and apoptotic cell death. Chronic oxidative stress and inflammation are strongly linked to RPE senescence and the pathogenesis of AMD. Subjects with a family history of AMD, smokers, and people who are obese or sedentary are at increased risk for this syndrome. People who are genetically prone to increased activation of the alternative complement pathway—such as those carrying certain alleles of the gene for complement factor H—are at greatly increased risk, perhaps reflecting a role for complement in drusen formation.23,24
Contributing factors for AMD include light-induced toxic effects, especially from blue light and ultraviolet light and the exogenous chromophores—amiodarone, chloroquine, phenothiazines, lithium and herbs such as St. John's wort. These exogenous chromophores can increase susceptibility to light-induced toxic effects.
However, subjects who have a favorable risk profile may also develop AMD. Clearly, the most important risk factor for AMD is simply getting older. (Domestically, the vast numbers of baby boomers are approaching the age most affected by AMD.) However, the advancement in understanding nutrients and the possible role they play in protecting the macula in the past two decades gives hope to the millions of individuals worldwide afflicted with this irreversible, disabling condition.
Implementing, Measuring, and Monitoring Nutritional Intervention
An interview with Sheldon Saul Hendler, MD, PhD, Mark F. McCarty and Richard A. Bone, PhD
Personally, I derive a great deal of professional satisfaction from drawing on the array of medicinal and surgical treatments to help patients with advancing wet AMD. However, it is frustrating to me, as I am sure it is for my fellow retinologists and every general ophthalmologist, not to have clinical solutions for either preventing or halting the progression of AMD. It appears that part of the solution to this dilemma may reside in the field of nutrition. Review of Ophthalmology offers a perfect venue for an on-going dialogue between researchers in the field of nutrition and the practicing ophthalmologist. Hence, I asked the following panel of nutrition experts to begin this dialogue by summarizing the science behind a nutraceuticals approach to managing AMD. Future issues will offer further dialogue on this important, burgeoning field.
Let's start with what you consider to be the strongest evidence of an association between diet and AMD.
Dr. Hendler: The best-established association between diet and AMD risk is the finding that those who consume diets high in EPA/DHA or in oily fish (the richest food source of these fatty acids) are at decreased risk for both geographic atrophy (dry) and neovascular (wet) AMD.25-29 In the AREDS cohort, after adjustment for appropriate covariants, those in the upper quintile of EPA/DHA intake (averaging 0.11 percent of total dietary energy) had an odds ratio of 0.65 and 0.68 for progression to geographic atrophy or neovascular AMD, respectively, over 12 years of follow-up.28 Note that, in an individual consuming 2,400 kcals daily, this intake of long-chain omega-3 amounts to about 300 mg daily, suggesting that the protective impact of these fatty acids is quite powerful and achievable with modest intakes. In the cross-sectional EUREYE study, those who consumed oily fish at least once per week, as contrasted to those who ate oily fish less frequently, were at only half the risk for neovascular AMD.29 In a recent study involving a murine model for AMD (Ccl2-/-, Cx3cr1-/-),30 a diet enriched in EPA/DHA was associated with a slower progression of retinal lesions and even a degree of lesion reversion. Additionally, markers of retinal inflammation were substantially reduced in the omega-3-fed mice.31
Although no completed clinical studies have yet evaluated the impact of omega-3 supplementation alone in AMD, an intriguing double-blind study examined a regimen providing daily intakes of 100 mg acetyl-L-carnitine, 530 mg of long-chain omega-3s, and 10 mg of coenzyme Q10 in patients with early AMD over 12 months.32 In the three measures of visual function employed—visual field mean defect, visual acuity, and foveal sensitivity—changes in the treated group were significantly better than those in the placebo group, and on average reflected net improvement. Active treatment also performed better than placebo with respect to fundus alterations, with the treated group achieving a statistically significant reduction in the drusen-covered area.
Although the authors of this study attributed the improvements in the treated group to an allegedly favorable impact of the supplement on retinal mitochondrial function (they refer to the supplement as "mitotropic"), the doses of coenzyme Q10 and acetyl-L-carnitine were so low relative to dose ranges generally thought to be clinically relevant, that it is reasonable to conclude that the chief benefits of the supplementation were conferred by the omega-3 component (well within the dose range associated with marked protection in epidemiological studies). If this analysis is correct, it bodes well for the ultimate results of the ongoing AREDS 2 study, and moreover suggests that good omega-3 nutrition is likely to have a favorable impact on early AMD.
Dr. Bone: In the early '80s, I teamed with Dr. John Landrum at Florida International University to identify the chemical composition of the macular pigment. It turned out to be a mixture of the oxygenated carotenoids (xanthophylls) lutein and zeaxanthin, and in a later study we found out that the zeaxanthin was present as two stereoisomers. One was the common stereoisomer found in many plants and referred to as zeaxanthin; the other was meso-zeaxanthin, which is found in very few food sources and is present in the macula as a conversion product of lutein.
The zeaxanthin-rich berry, Lycium barbarum or "Gou Qi Zi," has been a constituent of traditional Chinese herbal medicine for the treatment of visual disorders. However, probably the first real evidence for a protective effect of lutein and zeaxanthin against AMD appeared in the early '90s in publications of the Eye Disease Case-Control Study Group. The group reported significant associations between either high dietary intakes or high blood serum levels of these carotenoids and a reduced risk of advanced neovascular AMD.
More recently, the Age-Related Eye Disease Study reported that subjects with a high dietary intake of lutein and zeaxanthin had a significantly lower risk of developing both the wet and dry forms of AMD. The current AREDS 2 will specifically examine the efficacy of lutein with regard to AMD. Anecdotally, there have been a number of reports recently of regression of drusen accompanied by improvement in visual function in AMD patients taking a supplement containing all three macular carotenoids, but mostly meso-zeaxanthin.
Dr. Lopez: How do nutrients specifically alter the pathophysiologic processes underlying macular degeneration?
Dr. Hendler: The brain is rich in long-chain omega-3 fatty acids—most notably docosahexaenoic acid—and the retina has the highest omega-3 content in the body.
DHA can constitute up to 50 percent of the fatty acids in rod photoreceptor outer segments.59 This evidently bespeaks a vital structural or functional role for DHA in retinal neurons, and indeed, the DHA-rich phospholipids in outer segment membranes appear to be crucial for efficient function of the rhodopsin that resides in those membranes.66 Moreover, in vitro, rat photoreceptor neurons undergo apoptosis if deprived of DHA.67,68 With respect to DHA's biosynthetic precursor, eicosapentaenoic acid (EPA), while it is a less- prominent structural component of the retina, it has important anti-inflammatory potential owing to its ability to competitively inhibit production of arachidonate-derived pro-inflammatory prostanoids, and to give rise to other mediators (resolvins, lipoxins), which play a role in the resolution of inflammation.69-72 Furthermore, EPA has anti-angiogenic potential reflecting decreased endothelial expression of Flk-1, a key receptor for vascular endothelial growth factor;73 DHA likewise can suppress endothelial VEGF signaling.74 Long-chain omega-3s are highly susceptible to oxidative damage; one of the key roles of retinal xanthophyll carotenoids, present in high concentrations in photoreceptor outer segments, may be to preserve the native structure of DHA in these photoreceptors.60
In foods and in supplements, lutein and zeaxanthin are found mainly in their ester forms, which undergo intestinal hydrolysis to their free forms before being absorbed and ultimately incorporated into the macula. Xanthophyll uptake by RPE cells—which constitute the blood-retinal barrier, and hence regulate the availability of xanthophylls to photoreceptors—is mediated by scavenger receptor class B type I, which has relatively low affinity for beta-carotene.33 The characteristically high concentration of xanthophylls in the macula may also reflect the presence of a xanthophyll-binding protein (XBP). This protein protects xanthophylls from degradation by reactive oxygen species, and, in particular, the XBP-xanthophyll complex may have better antioxidant activity than the free xanthophyll.34-36 A retinal protein that specifically binds lutein has also been recently characterized.37
Dr. Bone: We understand the processes by which the carotenoids can exert their protective influences against AMD. Firstly, they are potent antioxidants and secondly, they screen vulnerable tissues from actinic blue light that can promote photo-oxidation.
The accumulation of oxidation products as drusen in the retina is an indicator of early age-related maculopathy. Thus a group of experts recently concluded in a Vision Research article that xanthophyll levels in the eye "potentially serve as a biomarker not only for predicting the risk for eye disease but also for visual function." Another group, writing in the Journal of Food Science, concluded: "It seems clear that MP [macular pigment] does influence visual performance through, at least, a few optical mechanisms. The most robust effects appear to be related to its actions as an optical filter."
I would like to add, with important emphasis, the macula is not the only eye tissue where lutein and zeaxanthin are sequestered. They have also been detected in the human lens, and their occurrence in this tissue may be related to the observation in large epidemiological studies of a lower incidence of cataract among both men and women having a relatively high dietary intake of these carotenoids.
Mr. McCarty: Lutein and zeaxanthin are natural fat-soluble yellowish pigments found in some plants, most notably green leafy vegetables (spinach, collard greens, etc.), algae and photosynthetic bacteria. Humans are unable to synthesize these substances and must obtain them from their diets; hence, they are essential nutrients. In the organisms that make them, they serve as accessory light-gathering pigments and provide protection from the harmful impact of ultraviolet radiation and the reactive oxygen species that it gives rise to. The light energy that they collect is used to drive photosynthesis; in this respect their function is similar to that of chlorophyll. In humans, these pigments are found in extraordinarily high concentrations in the macula. They are also found in the crystalline lens.
Their physiological location, as well as their ability to absorb blue light and quench dangerous singlet oxygen, suggest that they function to protect the macula and the lens from light-induced oxidative stress.38-39 Indeed, it is now strongly suspected that carotenoids play an important role in providing protection against not only AMD, but possibly cataracts as well—a conclusion that is concordant with reports that macular pigment density tends to be lower in patients with AMD.40-44 Macular pigment also functions to reduce the impact of light scatter and chromatic aberration on visual performance.45,46
Photoreceptors are exceptionally susceptible to oxidative damage, as their membranes are extremely rich in the long-chain omega-3 polyunsaturate docosahexaenoic acid. DHA can constitute up to half of the fatty acids in photoreceptor outer segments.47 This presumably explains why POS membranes are also unusually rich in xanthophyll carotenoids capable of quenching the singlet oxygen that otherwise might peroxidize DHA.11,48 These xanthophylls also are found in photoreceptor axons, where their presumed key function is to absorb blue light before it can penetrate to deeper levels of the macula and generate oxidants. The fact that the xanthophyll content of the macula is subnormal in patients with AMD might in some measure be a consequence of the disease, but it is also reasonable to suspect that this deficit contributes to disease progression by lessening the macula's antioxidant protection.42-44
Dr. Hendler: It should be noted that, unlike beta-carotene, the xanthophyll carotenoids lack pro-vitamin A activity. In other words, they cannot give rise to the 11-cis-retinal required for rhodopsin synthesis. Dietary carotene or pre-formed retinol is required for this purpose. In light of epidemiological evidence that dietary pre-formed retinol has the potential to compromise bone density, modest intakes of dietary beta-carotene, from foods or supplements, may be the most prudent way to meet the retina's requirement for retinol.49 However, there is no evidence that beta-carotene contributes notably to antioxidant protection of the retina. I predict that this observation will likely be confirmed in the AREDS 2 Trials.
Dr. Lopez: You have all concen-trated on xanthophylls (lutein, zeaxanthin and meso-zeaxanthin) and long-chain fatty acids, namely ome-ga-3 or DHA. Do other nutrients, perhaps not well-publicized, show potential for slowing AMD ?
Dr. Hendler: Another intriguing natural xanthophyll is astaxanthin, produced by certain types of algae; this is what makes flamingoes pink!50,51 Astaxanthin has more versatile antioxidant activity than any of the other xanthophylls, but it is not a significant component of most human diets, and its impact on macular pigment or retinal function remains to be assessed.
A very wide range of dietary phytochemicals can act directly as oxidant scavengers. But far more potent antioxidant protection can be achieved with phytochemicals, which also can stimulate a so-called "phase-2" response.52,53 Phase-2-inducing phytochemicals are prominent components of common foods such as green tea, allium vegetables (garlic, onions) and cruciferous vegetables (broccoli, cabbage, cauliflower, kale, etc.) They are also found in many commonly used herbal preparations, including some that have traditionally been used for ocular protection (e.g., extracts of bilberry, grape seed and green tea).
The phase 2-inducing components of some of these foods or herbs have shown protective antioxidant activity in RPE cells in vitro, and oral administration of green tea polyphenols to albino rats prevents light-induced damage to photoreceptors.54-57 (In light of the heavy consumption of green tea in East Asia, it is striking that so far no published epidemiological studies have attempted to correlate habitual green tea intake with AMD risk or prognosis.) However, the dose-dependency of the clinical phase-2-inducing effect achieved by ingestion of these foods or herbs requires further clarification, and the concentration of the active inductive components is likely to vary as a function of cultivar and method of preparation.
Mr. McCarty: Alpha-lipoic acid, an endogenously synthesized cofactor for several alpha-keto decarboxylase enzymes, has excellent antioxidant activity in many rodent studies.58 Although its direct oxidant scavenging activity is versatile, it also has been shown to be a phase-2 inducer.59-61 Lipoic acid may have good potential for use in AMD, as in rodent studies it has shown central neuroprotective effects, suggesting it has good access to the brain parenchyma (and likely the retina as well).62,63 Moreover, in an oral dose of 600 mg one to three times daily, lipoic acid has been shown to be therapeutically useful in diabetic neuropathy; hence, clinical doses within this range may have potential for protecting the retina.64
Whether the lower physiological levels achievable through supplementation can likewise provide protection to the retina in vivo, is not yet clear, but the utility of oral lipoic acid in the management of diabetic neuropathy suggests that this may be a reasonable possibility. To date, there do not appear to be any published clinical studies in which the impact of lipoic acid on AMD progression has been assessed.
Dr. Hendler: The small tryptophan-derived hormone melatonin is synthesized primarily within the pineal gland, though it is produced in more minor quantities by other tissues, including the retina. The nocturnal burst of melatonin secretion, regulated in part by retinal light exposure, plays a role in promoting appropriate circadian physiological rhythms.65 However, acting via membrane as well as nuclear receptors, melatonin also exerts potent antioxidant effects on many tissues, via up-regulated expression of several key antioxidant enzymes (superoxide dismutase, glutathione peroxidase, glutathione reductase, catalase) as well as of gamma-glutamylcysteine synthetase.66
In a recent study patients at various stages of AMD were treated with 3-mg melatonin nightly for six to 24 months.67 Fifty-five patients received melatonin for at least six months. The authors noted that visual acuity remained stable in the large majority of patients; they also noted that "change in the fundus picture is also promising … Only eight eyes showed an increase in retinal blood and six eyes showed an increase in retinal exudates. The majority had a reduction compared to pathologic changes before treatment in these patients, although atrophy was slightly increased." The authors conclude that, relative to the clinical course that could be expected based on literature reports, the response of their patients was favorable, and that melatonin merits more formal study in the prevention and management of AMD.
A more recent study has attempted to assess nocturnal melatonin production in AMD patients by measuring the levels of the chief melatonin metabolite 6-sulfatoxymelatonin in morning urine (expressed as its ratio to creatinine).68 This ratio was found to be about 40 percent lower in AMD patients than in age- and gender-matched controls. Whether this striking disparity is a cause or an effect of AMD (bearing in mind that light reception regulates pineal melatonin release) is open to question, although the authors note that, among their patients with AMD, sulfatoxymelatonin levels were no lower in those with poor visual acuity than in those with better acuity. In any case, even if the pathologic process in AMD does somehow decrease pineal melatonin production, it could be argued that, owing to melatonin's key physiological/antioxidant roles, this decline should be compensated by supplementation. Moreover, pineal melatonin production declines during healthy aging,69 and it is conceivable that this phenomenon contributes to the higher risk for AMD in the elderly.
Mr. McCarty: Whereas gamma-glutamylcysteine synthetase is the rate-limiting enzyme for glutathione synthesis, intracellular cysteine is the rate-limiting substrate for this process. The antioxidant effects of N-acetylcysteine (NAC) supplementation have been traced to its ability to boost intracellular levels of free cysteine, and thereby boost cellular glutathione production.70-72 These considerations suggest that concurrent supplementation with NAC might amplify the ability of phase-2 inducers or of melatonin to increase glutathione in RPE cells or retinal photoreceptors. In divided doses providing 600 to 1,800 mg daily, NAC is typically well-tolerated and has shown clinical potential as an antioxidant.71,72 Although there do not appear to be any published clinical studies that have evaluated NAC in AMD, the exposure of RPE cells to NAC in vitro has been reported to protect them from apoptosis when they are subjected to hypoxia or toxic carotenoid-derived aldehydes.73,74 Moreover, NAC was found to have a favorable impact on the efficiency of lysosomal function in RPE cells pre-loaded with oxidized photoreceptor outer segments, such that accumulation of lipofuscin was decreased.75 In rats subjected to intraocular hypertension, systemic administration of NAC prevented the induced increase in retinal oxidative stress.76 These considerations suggest that NAC may have important potential for lessening the contribution of oxidative stress to the pathogenesis of AMD. It is reasonable to expect that this potential will be best actualized if induction of gamma-glutamylcysteine synthetase is achieved concurrently.
Dr. Hendler: Coenzyme Q10 functions as an obligate mediator of electron transport in mitochondrial respiration. In the mitochondrial inner membrane, it can function as either an antioxidant or a pro-oxidant, depending on the circumstances. A recent study concludes that coenzyme Q10 content of the human retina and choroid declines by about 40 percent during the aging process; the authors suggest that this may compromise the bioenergetics and antioxidant protection of the aging retina, possibly contributing to the risk for AMD.77 Moreover, plasma levels of coenzyme Q10 were found to be lower in patients with wet AMD than in age-matched controls.78 Several studies demonstrate that pre-administration of coenzyme Q10 to rats protects the retina from excitotoxicity induced by transient ischemia or NMDA administration and also can protect retinal ganglion cells from hydrogen peroxide in vitro.77,79,80 These considerations suggest that coenzyme Q supplementation may provide worthwhile antioxidant protection for the retina, and merits evaluation for control of AMD.
Unfortunately, the only published clinical study to assess a regimen containing coenzyme Q10 in patients with AMD provided such a low dose (10 mg daily) that it seems most unlikely that the favorable results achieved in this study reflected the impact of this agent.32 Effective oral coenzyme Q10 therapy is now more feasible than previously, owing to the development of micellized forms of ubiquinol, which have increased oral bioavailability.81-83
Mr. McCarty: There is some reason to suspect that NADPH [nicotinamide adenine dinucleotide phosphate] oxidase may be a key source of oxidant stress in RPE cells. RPE cells act as phagocytes, engulfing outer segments of ret-inal rod photoreceptors. Like other phagocytes, RPE possess NADPH activity that increases during phagocytosis.16,17 More recent work establishes that RPE expresses p22phox, a key membrane component of NADPH oxidase complexes.18 Intriguingly, viral delivery of small interfering RNA for p22phox to the subretinal space prevents choroidal neovascularization in a mouse model of AMD (involving laser disruption of Bruch's membrane).84 The authors conclude that "NADPH oxidase-mediated ROS production in RPE cells may play an important role in the genesis of neovascular AMD, and this pathway may represent a new target for therapeutic intervention in AMD."
Additionally, NAPDH oxidase activity is expressed by retinal neurons. This activity increases when growth factor support is withdrawn (as might be expected if death or dysfunction of retinal pigment epithelium impairs its trophic function), and there is suggestive evidence that NADPH oxidase activation may drive the apoptotic death of cone cells in retinitis pigmentosa.85,86
These findings are of particular interest in light of the recent discoveries that intracellular free bilirubin functions as an inhibitor of NAPDH oxidase87-89 (thereby rationalizing the antioxidant activity of heme oxygenase), and that phycocyanobilin (PhyCB), a bilirubin homolog that functions as a chromophore in spirulina and other microalgae, can mimic this activity.90
Since orally administered spirulina or phycocyanin—the spirulina protein that contains PhyCB as a chromophore—exerts neuroprotective effects in rodent studies, there is reason to suspect that spirulina or PhyCB-enriched spirulina extracts may have the potential to confer antioxidant protection to the retina.91,92 So far, however, no studies have examined the impact of spirulina or phycocyanin on retinal function or pathology. It would be of particular interest to determine whether individuals with Gilbert's syndrome,93 an innocuous genetic variant associated with chronically elevated plasma levels of free bilirubin and reduced risk for cardiovascular disease, are at decreased risk for AMD.
Dr. Lopez: Fascinating. But, getting back to current possible effective therapeutic regimens, can macular protective pigment density be increased by dietary supplements?
Dr. Hendler: In 1994, Dr. Johanna Seddon and co-workers at Harvard published a case-control study that linked a high intake of xanthophyll-rich food, particularly dark green leafy vegetables such as spinach, with notably reduced risk for AMD.94 This finding accorded well with the recent discovery that xanthophylls constitute the macular pigment, and triggered tremendous interest in the potential of supplemental or dietary xanthophylls to protect the retina.
A number of studies have demonstrated that the amount of the macular pigment can be increased by dietary supplementation with lutein or zeaxanthin or by ingestion of xanthophyll-rich foods.95-99 Lutein and zeaxanthin, as well as their esters, are efficiently absorbed—more avidly so than beta-carotene—and ultimately wind up in the macula; their trans isomers are most available.
Dr. Bone: At about the time Dr. Landrum and I discovered meso-zeaxanthin, interest in the macular pigment was growing because of emerging evidence of its potentially beneficial influence on eye health. Dr. Landrum and I obtained NIH funding to compare macular pigment levels in autopsy eyes from donors with and without AMD, and we found, on average, lower levels in the diseased eyes. We also conducted the first supplementation study with lutein and were able to demonstrate up to 40-percent increases in macular pigment density in our subjects over a 120-day supplementation period. The 30-mg daily lutein dose in this case was suspended in a few milliliters of vegetable oil, which is probably the reason for its dramatic effect.
As might be expected, serum levels of lutein and zeaxanthin have been found generally to be positively correlated with high levels of macular pigment, so our study with the autopsy eyes is consistent with the large epidemiological studies.
Dr. Hendler: I might add that, although little meso-zeaxanthin is found in natural diets (it is found in seafood, but not in vegetables) or in plasma, supplementation studies show that orally administered meso-zeaxanthin is well-absorbed and can improve macular pigment density.97,100
Dr. Bone: Correct. Dr. Landrum and I have conducted many such supplementation studies, with lutein, zeaxanthin as well as meso-zeaxanthin, and the results have been similar. Furthermore, as a biophysicist, I have also been very interested in developing techniques for non-invasively measuring macular pigment optical density in the retina so that the effects of supplementation can be readily measured. These techniques are being designed for cost-effective usage in the clinical setting.
In our studies we have always found a very strong association between macular pigment optical density and the subject's diet. Those whose diet includes frequent consumption of green, leafy vegetables tend to have average to high MPOD, whereas the vegetable avoiders tend to be on the low side.
Dr. Lopez: Is evaluation of macular pigment levels important prior to and during dietary supplementation with carotenoids?
Dr. Hendler: In a number of subsequent studies, oral administration of xanthophylls for three months or longer, either in supplements or natural foods, was shown to increase macular pigment density.95-99 However, the response of macular pigment to xanthophyll intake was quite variable from person to person. In particular, a lesser response was noted in overweight subjects. Baseline body fat also tends to correlate negatively with macular pigment density, and perhaps an adverse effect of obesity on macular xanthophyll uptake is at least partially responsible for the increase in AMD risk associated with obesity noted in some epidemiological studies.101
Dr. Bone: We have found that the average MPOD for the population is about 0.4 absorbance units (AU), but it can range from essentially zero to more than 1.0 AU. Subjects in the below average category would be those for whom dietary supplementation with the macular carotenoids would be advisable. Since subject response to supplementation is quite variable, monitoring the subject's MPOD at, say, six-month intervals would allow clinicians to reach decisions concerning the continued use of the supplement at the same dose or possibly at an increased or, for that matter, decreased dose.
Dr. Lopez: What are the methods of measuring MPOD and what are their advantages and disadvantages?
Dr. Bone: The methods of measuring MPOD fall into two categories, the so-called psychophysical methods and the objective physical methods.
Many investigators consider the psychophysical method of heterochromatic flicker photometry (HFP) to be the gold standard against which other methods are evaluated. In this method, the subject views a small visual stimulus that alternates between two colors, blue that is absorbed by the macular pigment and green, which is not absorbed. The subject operates a control to adjust the brightness of the blue until the perception of flicker is minimized. The second part of the test, which eliminates the effects of light absorption in the lens, requires the subject to minimize flicker while directing his or her gaze at a peripheral fixation mark so that the stimulus is seen "out of the corner of the eye." Some subjects find this latter task to be difficult, so I have recently been directing my efforts towards the development of a user-friendly HFP system. To date, this new system, which will hopefully be available on the market in the near future, has been described by the subjects who have used it as much easier than traditional HFP. The uncertainties associated with their measurements bear this out.
I also have had experience with one of the physical methods, fundus reflectometry using a retinal camera. The advantage of this method is that it provides a profile of MPOD across the retina. The disadvantage is that the clarity of the optic media decreases with age, resulting in low contrast images and a concomitant underestimate of the MPOD that gets worse with age. In addition, depending on the type of retinal camera, pupil dilation may be necessary.
The problem of decreasing lens clarity with age also appears to pose a problem in the method of resonance Raman spectroscopy. In this method, carotenoid molecules in the retina are stimulated by light of one wavelength and emit light at a slightly different wavelength which is detected and quantified. Rather than providing the MPOD, this method provides "Raman counts," which are related to the amount of carotenoid present.
The final physical method is based on lipofuscin hyperfluorescence. Lipo-fuscin is a fluorescent material that accumulates in the RPE anterior to the macular pigment layer. It can be stimulated by a range of wavelengths and fluoresces at longer wavelengths in the yellow part of the spectrum. This fluorescence is measured when the stimulating light is firstly a wavelength that is strongly absorbed by the macular pigment (blue) and secondly by one that is not absorbed (green). The differential lipofuscin fluorescence due to these two wavelengths provides a means of calculating the MPOD. The advantage of this method is that it can provide a profile of MPOD across the retina; the disadvantage is that it requires specialized and expensive instrumentation such as a scanning laser ophthalmoscope.
Dr. Lopez: Finally, considering the protective role of carotenoids, omega-3 and antioxidants, what guidelines should ophthalmologists follow in recommending consumption amounts to their patients?
Dr. Hendler: The foregoing discussion strongly suggests that supplementation with xanthophyll carotenoids, long-chain omega-3 fatty acids, and various adjunctive antioxidants may have important potential for preventing and controlling AMD. The on-going AREDS 2 study, as well as other studies in progress or in planning, should provide more definitive information in this regard.
Nonetheless, the pathogenesis of AMD remains poorly understood, and it seems likely that a point of diminishing returns will be reached as antioxidant measures are piled atop each other. Hence, it may be worthwhile to identify additional targets that can be addressed in AMD management—strategies with potential for complementing the efficacy of antioxidants and omega-3s. A further examination of the epidemiology of AMD may provide some useful clues in this regard.
I might add that it is possible but very difficult to modify one's diet with increased real foods in order to enhance MPOD significantly. Realistically, the supplement market is far more likely to offer concentrated forms of pure ingredients that will successfully provide adequate protection. This is true for everyone, including allegedly low-risk groups, though in particular high-risk persons with AMD or with a family history of AMD.
Dr. Lopez: As we have found in the epidemiologic studies, certain non-modifiable risk factors are associated with AMD. The primary risk factor is, of course, age. The Eye Disease Prevalence Research Group as well as the AREDS Study found a lower incidence of AMD in blacks, while the Beaver Dam Eye Study and the Blue Mountains Eye Study demonstrated women to be more likely to develop early and late AMD. Sadly, we cannot prevent aging, nor choose our race or gender, but we can modify behavior. Please summarize the modifiable risk factors, other than diet or supplement intake.
Dr. Hendler: The strongest modifiable risk factor is cigarette smoking. The epidemiologic studies have not proven that stopping smoking prevents the development of AMD, but the epidemiologic data give yet another reason to advise patients not to smoke. Blood pressure, dyslipidemia and obesity are other modifiable risk factors, that though not confirmed to be directly causal, should be addressed with patients. Also, there may be an association between cataract surgery and an increase in frequency of development and progression of AMD. This latter risk factor may be due to the spurious association of both early AMD and cataract in elderly patients, so I would not alarm patients with this concern, especially when the benefits of cataract surgery outweigh this risk. Surgeons might consider the use of ultra-violet light protection either with the choice of implant at the time of surgery or the use of blue-blocker sunglasses following surgery.
Dr. Lopez: And, finally, with regard to supplement intake, what would you recommend?
Dr. Hendler: With regard to practical advice for one's patients, I would encourage AMD patients to take a general multi-vitamin as well as a product that contains lutein, zeaxanthin, meso-zeaxanthin, and omega-3 (in particular DHA). Soon, through Dr. Bone's work, we will be able to cost-effectively measure, with accuracy and repeatability, the effects of diet modification on the protective macular pigment of the retina. The physician will have the scientific justification for recommending supplements, then measure the effects on the macular pigment or the effects on the lens, and then subsequently alter those recommendations based on serial clinical findings/measurements. Recom-mendations for non-AMD patients may then be defensible and prudent.
Dr. Hendler, Dr. Bone and Mr. McCarty are paid members of the Scientific Advisory team at Guardion Health Sciences, a newly formed nutraceutical company based in California. The comments in this paper should not be construed as endorsement of any of GHS's current or future products. Dr. Lopez has no management or financial interest in any related products or services.
All citations referred to in this article are available upon request.