Nutrition and the eye
Macular pigment
The role of xanthophylls in preventing AMD
Hannah Bartlett BSc (Hons), PhD, MCOptom, FAAO
and Frank Eperjesi BSc (Hons), PhD, DipOrth, MCOptom, FAAO
The carotenoids are a family of pigments that are divided into two maingroups – carotenes and xanthophylls. Although not considered to beessential micronutrients, they have antioxidant and photoprotective
properties, and these functions have prompted interest in their potentialrole in the prevention of disease. The focus of this article will be on therole of the xanthophylls, lutein, zeaxanthin and meso-zeaxanthin, in
preventing the onset or progression of age-related macular degeneration(AMD).
Despite increasing evidence supporting theFurthermore, a negative correlationrole of xanthophylls in preventing disease,between age and serum lutein levels inrecent data suggest that dietary intakeindividuals consuming lutein esters haslevels have declined in Europe and the US1,2
.been reported, which was not found inLutein and zeaxanthin are now “generallypeople supplementing with lutein12. Thisrecognised as safe” (GRAS), which meansmay suggest that the ability to hydrolisethat they can be added to foods such aslutein esters declines with age. The samecereals. This is important, as lutein andgroup reported no difference in the absorp-zeaxanthin are not formed within the bodytion of 6mg lutein from spinach, 6mg luteinand so can only be obtained from the diet.supplements or 10.23mg lutein estersThey are abundant in dark green leafysupplements, but a significantly highervegetables such as spinach and kale3
, as wellserum response following supplementationas yellow and orange fruits and vegetableswith lutein-enriched eggs providing 6mgsuch as peppers4.
lutein. They concluded that the bioavailabil-Dietary supplements containing luteinity of lutein from eggs is higher than thatand zeaxanthin may be produced usingfrom other sources, and that this may bemarigold flowers (Tagetes erecta), which arerelated to the fact that within eggs, lutein isgrown in Asia, Mexico, and Central andlocated in the digestible lipid matrix.
South America5
. In marigold petals, luteinIt should be noted that the eggs used indominates at approximately 93-95%,this study contained approximately fivehowever, lutein can be metabolised totimes the amount of lutein than conven-zeaxanthin within the body6
. In flowertional eggs. Nevertheless, the resultspetals, the pigments are stored as diesters,provide useful information about thewhereas they are found unesterified in mostbioavailability of different sources, suggest-fruits and vegetables7
. In fact, industrialing that a lipid base may be optimum forresearch showed that 93% of the lutein andsupplements.
zeaxanthin found in fruits, vegetables andThe fact that no significant difference ineggs is found as lutein, rather than luteinserum response following supplementationesters8
.
with spinach, lutein supplements or luteinLutein esters contain two fatty acidesters supplements contrasts with othergroups that must be cleaved off before thereports that supplemental lutein is twice asbody can use the lutein9
. The efficacy of thisbioavailable than lutein from spinach13
, andhydrolysis of lutein esters into lutein occursthat supplemental lutein induces a 60%with an efficacy that is well below 5%10,11
.
greater increase in serum concentrations oflutein than a daily vegetable intake contain-About the authors
ing a similar amount of lutein14
.
Dr Hannah Bartlett is Post
Oxidative stress and AMD
Doctoral Research Fellow in AstonUniversity’s Ophthalmic ResearchIt is generally thought that oxidativeGroup. Dr Frank Eperjesi isdamage is responsible for ageing and thatthis process has an important role in theDirector of the Optometrypathogenesis of age-related conditions suchUndergraduate Programme inas AMD15
. Oxidation involves the removal ofthe School of Life and Healthelectrons, and is mediated by reactiveoxygen species (ROS). ROS is an umbrellaSciences, Aston University
term and includes some free radicals, singlet
32| May 19 | 2006 OToxygen and hydrogen peroxide. Freeradicals have an unpaired electron in theirouter orbits, which makes them unstableand harmful to cells of the body. In order toachieve stability, they pluck electrons fromother molecules, producing further ROS andfuelling disease-generating cytotoxic chainreactions16. Examples of free radicals includethe superoxide anion radical, formed fromthe reduction of molecular oxygen to water,and the hydroxyl radical. The hydroxylradical is particularly damaging, as it cantake electrons from almost any organicmolecule.
The reduction of oxygen to water alsoproduces hydrogen peroxide, whichinteracts with the superoxide anion radicalto form the hydroxyl radical, another potentantioxidant. Singlet oxygen does not havean unpaired electron in its outer orbit, but itdoes have a peripheral electron that isexcited to an orbital above that which itnormally occupies, making it highlyreactive16
.
Ocular tissues are particularly susceptibleto oxidative damage. The transparency ofthe cornea, aqueous humour, lens andretina allow continuous exposure to light,which along with ageing, inflammation, airpollutants and cigarette smoke, has beenshown to increase production of ROS17,18
.The role of oxygen in cataract formation hasbeen demonstrated19
, and the retina isparticularly vulnerable for the followingreasons:
1.Polyunsaturated fatty acids are abundantin the retina, particularly the macularregion. They are found in photoreceptorouter membranes and are readilyoxidised17,20,21.
2.The retina is subjected to high levels oflight exposure. Light (particularly bluelight) is a strong oxidising agent. Thesimultaneous presence of light andoxygen promotes production of freeradicals22
.
3.Phagocytosis, which itself produces freeradicals, occurs within the retinalpigment epithelium (RPE).
4.The retina is highly active metabolicallyand has a much higher blood flow thanother tissues22
.
The body has several defence mechanismsagainst the production of ROS. The firstinvolves antioxidant enzymes such ascatalase and peroxidase23
. Other micronutri-CLINICAL
Nutrition and the eye
ents, such as selenium, zinc, manganeseand copper, facilitate these antioxidantenzymes23,24
. The second involves antioxidantnutrients such as vitamin E (alpha-tocopherol)25-29
, beta-carotene30
and vitaminC (ascorbate)23,31-34
and lutein and zeaxan-thin35. Insufficient intake of dietary antioxi-dant vitamins and minerals can decrease theefficiency of the body’s natural antioxidantsystems and may allow cellular damage byROS17,36
. A review of the nutrients consideredbeneficial for ocular disease has beenpublished37
.
Structure and function oflutein, zeaxanthin andmeso-zeaxanthin
Interest has been raised into the protectiverole of the oxygenated xanthophylls groupof carotenoids in the eye, particularly theretina. Lutein and its isomers are the onlycarotenoids present in the lens38
and retina39-42
, and are known as macular pigment (MP).The proposed specific function of xantho-phylls at the macula40
is supported by thefact that macular levels are several thousandtimes higher than serum levels43
. This maybe explained by the discovery of a putativelutein-binding protein in the retinae ofhuman eyes44
, which binds with high affinityand specificity to lutein and other xantho-phylls.
Although possible binding proteins havebeen identified, the mechanism for uptakeof xanthophylls into the bloodstream is stillnot clear. The efficacy of absorption fromthe gut depends on its original source, forexample, lutein from egg yolk45
is morereadily absorbed than that derived fromgreen leafy vegetables46
. This relatively lowabsorption from green leafy vegetables maybe due to complexing to proteins in chloro-plasts within cell structures47
. Xanthophyllsthat are associated with oil or fat may bemore readily extracted during digestion47
. The xanthophylls are packaged as plasmalipoproteins by the liver and released intothe systemic circulation. Their major storagesite is adipose tissue48,49, so much so that anegative correlation between adipose tissuelutein concentration and the amount oflutein and zeaxanthin in the retina (macularpigment optical density, MPOD) has beenreported in women50
.
In the central macula, lutein, zeaxanthinand meso-zeaxanthin are found in equalquantities, but the ratio of meso-zeaxanthinto zeaxanthin decreases with increasingeccentricity51
. Meso-zeaxanthin has beenfound in the human macula, retina and RPE,but most recently has not been detected inthe plasma or liver52
. This forms the basis forthe assumption that meso-zeaxanthin isformed via isomerisation of lutein51
, and it isthought that the conversion mechanism isconcentrated at the macula. The xantho-phyll binding protein mentioned earlier mayalso act as an enzyme for the conversion of
34| May 19 | 2006 OTlutein to meso-zeaxanthin.
In human retinae, the xanthophylls areconcentrated mainly in the inner and outerplexiform layers. The ratio of lutein tozeaxanthin and meso-zeaxanthin within0.25mm of the fovea is approximately1:2.453
, but the situation reverses at theretinal periphery where the ratio is 2:153
.There is a hundred-fold drop in the concen-tration of xanthophylls in the peripheralretina compared with the fovea, althoughlevels vary vastly between donors39,40
. Theratio of lutein:zeaxanthin and meso-zeaxan-thin varies linearly with the ratio ofrods:cones with increasing eccentricity53
.The hypothesis that zeaxanthin is onlyfound in the rods is refuted by the fact thatthe fovea contains predominately cones, aswell as by the fact that squirrel monkey andmacaque retinae have their highest concen-tration of lutein and zeaxanthin in thecentral fovea54
.
Xanthophylls have also been isolated inthe rod outer segment 55,56
where there is ahigh concentration of polyunsaturated fattyacids that are particularly prone to oxidativeattack. Within the rod outer segments, theirhighest concentration is found perifoveally,where it is 2.5 times higher than in theperipheral retina56
.
It has been suggested that xanthophyllsplay a similar role in humans as in plants, asantioxidants and screeners of high-energyblue light57
. The MP may prevent light-initiated oxidative damage to the retina,and therefore protect against subsequentage-related deterioration58
. The presence ofMP in the inner retinal layers59
supports aphotoprotective role. The absorbancespectrum of MP peaks at 460nm and it ispurported to act as a broadband filter,reducing the sensitivity of the macularregion to short wavelength light which ismost damaging in the 440-460nm range60,61.Lutein is reported to be a superior filter62
dueto the fact that it is orientated both paralleland perpendicular to the plane of themembrane63
. Zeaxanthin is orientatedperpendicular to the membrane plane only,and so may not be able to absorb theexcitation beam from all directions.Zeaxanthin, however, is reported to be asuperior photoprotector during prolongedlight exposure; the shorter time-scale ofprotective efficacy of lutein has been attrib-uted to oxidative damage of the carotenoiditself63
.
Carotenoids are also able to quenchsinglet oxygen (a potent oxidant)64
,scavenge reactive oxygen species65
, limitperoxidation of membrane phospholipids66
and reduce lipofuscin formation67
. Thepresence of MP in the rod outer segmentsand RPE55,56
is suggestive of a ROS-quenchingfunction. The fact that lutein and zeaxan-thin have been found in higher concentra-tion in the rod outer segments of theperifoveal retina than the peripheral retina,lends support to their proposed protectiverole in AMD55
.
In vivomeasurementof MPOD
MPOD in the central 1-2˚ of the macula liesin the range 0.1-0.9 for most people68,69
. Fora person with MPOD at the low end of thisrange, structures posterior to the MP will beexposed to approximately six times the bluelight flux, compared to a person with MPODat the higher end of the range70
. It followsthat there is a suspected increased risk ofAMD development for those with lowMPOD levels. It has also been noted thatgeographic atrophy tends to spare the verycentral macula, where MPOD peaks, untilthe disease is well advanced71,72
.
Psychophysical methods
The psychophysical approach to MPODmeasurement is based on the fact that theMPOD acts as a broadband filter in the 440-460nm range. In heterochromic flickerphotometry73-75
, a blue reference light, closeto the optical peak density of MP (450nm) isalternated with a light of variablewavelength. This is set to a value which isnot absorbed by the MP, such as 560nm76
.Whilst viewing this flickering stimulus, theluminance of one of the lights is altereduntil the perceived flicker is minimised. Atthe minimum flicker point, the perceivedluminance of the two lights is equalised.The perceived intensity of the blue referencelight will be relatively low when viewed atthe fovea (where MP is relatively high),compared with a point outside the fovea(where there is less MP). The differencebetween the ratios of the luminance of thetwo lights obtained at foveal and para-foveal points is used to derive the MPOD. Although this technique is reproducibleand exhibits good test-retest reliability77, it isdifficult for the subject to perform78,79
, andrequires good visual acuity. It is also associ-ated with high variability in subjects withlow levels of MPOD80
. A commercial instru-ment that employs this technique formeasurement of MPOD is the MacuScope™
from the Birmingham Optical Group(Figure 1).
Figure 1The MacuScope
(by courtesy of Birmingham Optical Group)
Imaging techniques
Fundus reflectometry involves measuringthe reflectance of short wavelength light(462nm) that has passed through pigmentcontaining layers of the retina twice81
. Adigitised image obtained at an illuminatingwavelength of 559nm is subtracted fromone taken at 462nm in order to correct forthe absorptive effects of melanin andoxyhaemoglobin. This provides the spatialvariation of the MP.
Scanning laser ophthalmoscopy (SLO) canalso be used to produce fundus reflectancemaps, and this method is reported to bemore resistant to light scatter than conven-tional fundus reflectometry82
. Digitalsubtraction of the maps at 488-514nm,with adjustments made for absorption ofthe lens, provides a mean value of MPOD83
.A disadvantage of this technique is that itrequires a normal retinal structure, andtherefore is not suitable for use in patientswith advanced AMD.
Raman spectroscopy
This technique is based on the Ramaneffect, which is the inelastic scattering ofphotons by the molecules under investiga-tion. In other words, the wavelength of asmall fraction of the radiation scattered bycertain molecules differs from that of theincident beam, and the shift in wavelengthdepends on the chemical structure of themolecules responsible for the scattering.This phenomenon has been used in theassessment of MPOD because whencarotenoids are excited with a monochro-matic laser beam, they exhibit characteristicwavelength shifts of the back-scatteredlight. A blue/green argon laser is used toexcite the electronic absorption ofcarotenoid pigments84. The resultant Ramansignals are recorded and analysed by aspectrometer. This technique has theadvantage that it can be used to assessMPOD in AMD-affected eyes. Thistechnique is reported to be highlyreproducible and not subject to meaningfultest-retest variability85
, although it has onlybeen used in a research setting.
Apparent motion photometry
A more recent development in MPODmeasurement is based on an apparentmotion technique86for matching theluminance of different colours. Thistechnique has the advantage of simplicitywhen used for adjusting colour luminanceon television displays. If a red/green square-wave grating is suddenly replaced with adark yellow/light yellow square-wavegrating, which is displaced by one-quarterof a cycle to the right, then the grating willappear to jump to the left if the green barsare lighter than the red bars, or to the rightif the reverse is true86
. If, however, the redand green bars are made equiluminous, noconsistent apparent motion is seen.
A MPOD measurement techniquedeveloped by Cambridge Research Systems
uses a stimulus made up of four consecu-tively presented square wave gratings, each90˚ out of phase with the next (Figure 2). The first grating is a chromatic grating ofred and blue bars. The luminance of theblue is fixed whilst the red luminance can bevaried. The second grating is a purelyluminance modulated grating, modulatedaround the mean luminance of the blue/redchromatic grating. If the luminance of thered component in the chromatic grating isgreater than the blue, the observercorrelates that with the brighter of the barsof the luminance grating when it ispresented. However, if the luminance of thered is less, then it is correlated with thedarker bar in the luminance grating. Thiscontinues in successive grating presenta-tions, so that the sequence of gratingsappears to move in one direction or theother, the direction being solely dependentupon the relative luminance of the twocomponents in the chromatic gratings.
When the red luminance is greater, thegrating appears to drift upwards, when theblue is greater it drifts downwards. Thesubject is simply required to decide whetherthe grating is drifting upwards ordownwards in a 2AFC weighted up/downstaircase procedure.
Epidemiological
and clinical evidence
A cross-sectional sample taken from theNational Health and Examination Survey(NHANES) in the late 1980s revealed aninverse association between fruit andvegetable intake and AMD87
. Serumcarotenoids (lutein, zeaxanthin, beta-carotene, alpha-carotene, cryptoxanthinand lycopene) were associated with reducedrisk for AMD in 199288, and prevalence ofAMD in this sample was 66% lower inthose in the highest quintile of carotenoidintake compared with those in the lowest.In the early 1990s, however, subsequentepidemiological studies reported onlymarginal relationships89,90
.
More recently, serum concentrations oflutein and zeaxanthin and MP density havebeen found to be responsive to dietarymodifications50,75,91
. It has been reported that55% of the variability in serum concentra-tions of lutein and zeaxanthin can beexplained by their dietary intake of thesecarotenoids; 30 % of the variability onMPOD can be explained by serum levels92.Although retinal response to a change indietary intake of lutein and zeaxanthin ismuch slower that the serum response75,93,both serum and dietary lutein were signifi-cantly positively correlated with MPOD in astudy involving 278 healthy volunteers94
.A cross-sectional study reported thatpeople with plasma concentrations of luteinin the lowest third of the distribution have asignificant odds ratio for risk of AMD of 2.0(95% CI: 1.0-4.1) compared with those in
CLINICAL
Nutrition and the eye
the highest third after adjustment for otherrisk factors95
. There were no significanttrends between plasma concentrations oflutein or lutein plus zeaxanthin.
There is evidence for selective depositionof lutein in the retina55,96
, increase of retinaland serum levels of lutein with supplemen-tation75,83,91
and an increased risk of AMDwith low serum97
and retinal98,99
lutein levels.Lutein/zeaxanthin supplementation hasbeen linked with improved visual function inpatients with congenital retinal degenera-tions100
and with AMD101
. Monkeys fed alutein-free diet eventually lost all macularpigment and developed pathology consis-tent with macular degeneration102,103
.Repletion of these monkeys led to restora-tion of macular pigment levels103
.
Studies investigating the effect ofunesterified lutein dosage on MPOD levelsfound a general increase in MPOD responsewith dose104,105
. In one study, those supple-menting with 10mg or 20mg of lutein, butnot 5mg lutein, for 120 days had anincreased response compared with thosetaking a placebo104
. Another study showedthat in patients with varying stages of AMD,doses of 2.5mg, 5mg and 10mg lutein allinduced an increase in serum levels by onemonth, and a peak by three months. Three-month levels ranged from 104% to 339%change from baseline. MPOD levels,however, remained largely unchanged overthe six-month supplementation period. Noadverse reactions to lutein were reported105
.The Lutein and Antioxidant SupplementTrial (LAST) was a 12-month RCT designedto evaluate the effect of 10mg unesterifiedlutein alone, or 10mg lutein combined withadditional carotenoids and antioxidants andminerals, on MPOD and objective visual
Figure 2
Measuring MPOD using wave gratings
(by courtesy of Cambridge Research Systems)
35| May 19 | 2006 OTCLINICAL
Nutrition and the eye
Constituents (mg)per daily doseBeta-caroteneVitamin AVitamin CVitamin EZincCopperVitamin B3LuteinZeaxanthinSeleniumVitamin B2ManganeseOther antioxidants,vitamins and mineralsDosage/dayVitaluxPlus(Novartis)®ICaps(Alcon)®--6020100.251010(unesterified)-----One capsule-0.67400150604-3.6(unesterified)0.40.041010-Two tabletsOcuvitePreserVision(Bausch & Lomb)17.2-45226869.61.6--®®OcuviteLutein(Bausch & Lomb)®Visionace(Vitabiotics)®Retinex(Healthspan)-------10(unesterified)0.44----One tabletRDA (mg)-----Four capsules--6020152-6(unesterified)-----One capsule-0.315060151183.8(esterified)0.20.154.84131.35One tablet0.8-6010151.1518None establishedNone established0.0551.6--Table 1Formulations of a selection of commonly available ocular nutritional supplementsoutcome measures in 90 subjects withatrophic AMD. Glare recovery and contrastsensitivity significantly improved with bothinterventions, although it is worth notingthat the study population was 95.6%male.106possible contraindications and adverseeffects of nutritional supplementshelpful.It is also important to remember thatpatients can modify their diets to increaselutein and zeaxanthin intake, and informa-tion about different foods can be found inTable 2.110FoodKale (cooked)Kale (raw)Turnip Greens (cooked)Collard Greens (cooked)Spinach (cooked)Spinach (fresh, raw)Broccoli (cooked)Corn (cooked)Green peas (canned)Lettuce (Romaine)Corn (canned)Eggs (two)Green beansOrange juice(frozen concentrate)OrangesPapayasTangerines (fresh)mg / serving33.8 / 1 cup22.1 / 1 cup18.1 / 1 cup17.2 / 1 cup15 / 1 cup6.7 / 1 cup3.4 / 1 cup2.9 / 1 cup2.3 / 1 cup1.5 / 1 cup1.4 / 1 cup0.5 / 2 medium0.76 / 1 cup0.50 / 12oz0.49 / 2 medium0.45 / 2 medium0.40 / 2 mediumMacular pigment andnormal visual function With respect to healthy eyes, it has beenhypothesised that the blue-light filter effectof xanthophylls may reduce longitudinalchromatic aberration. The acuity hypothe-sis states that MP may improve visual acuityfor images that are illuminated by whitelight by absorbing poorly focused shortwavelengths before this light is processedby the retina. Despite a lack of empiricalevidence, lutein/zeaxanthin supplementsare being taken by the public in an attemptto improve retinal health and vision. 107108109ConclusionThere is compelling evidence for a role oflutein and zeaxanthin in the protection ofthe retina against oxidative damage. TheLutein Antioxidant Supplementation Trialreported a positive effect of 10mg unesteri-fied lutein supplementation on measures ofvisual function in atrophic AMD patients.Recent work on the optimum dosage levelsof lutein and zeaxanthin suggest that a dailyintake of 10mg or more is required toincrease MPOD levels. In summary, a review of the currentevidence suggests that for optimumbenefit, lutein should be taken in itsunesterified form, dissolved in lipid, and at alevel of at least 10mg per day. Patientsshould be advised to talk to their GP aboutnutritional supplementation if they aretaking prescribed medication. Modification ofxanthophylls intake The characteristics of nutritional ocularsupplements commonly available tooptometrists can be found in Table 1.Practitioners who discuss nutrition withtheir patients may find a review of the
ReferencesVisit www.optometry.co.uk/references
Table 2
Lutein content of various foods
36| May 19 | 2006 OT
因篇幅问题不能全部显示，请点此查看更多更全内容