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(Investigative Ophthalmology and Visual Science. 2000;41:1492-1497.)
© 2000 by The Association for Research in Vision and Ophthalmology, Inc.

Macular Pigment Optical Density in a Southwestern Sample

Billy R. Hammond, Jr.1 and Mary Caruso–Avery2

1 From the Department of Psychology, University of Georgia, Athens, Georgia; and 2 Vision Science Laboratory, Arizona State University West, Phoenix, Arizona.


   Abstract
Top
 Abstract
Introduction
 Methods
 Results
 Discussion
 References
 
PURPOSE. Increasing evidence implicates macular pigment in protecting the retina and retinal pigment epithelium from light-initiated oxidative damage. Little information, however, is available regarding "average" levels of macular pigment in the general population. This study was designed to assess macular pigment in a high-light environment and to determine what personal characteristics influence macular pigment density in that sample.

METHODS. Macular pigment optical density was measured psychophysically using a 1°, 460-nm test stimulus. Personal data were collected using a questionnaire.

RESULTS. 217 subjects (79 men, 138 women) were recruited from the Phoenix metropolitan area (age range = 17–92 years). The average macular pigment density was 0.22 ± 0.13. There was a slight tendency for macular pigment density in this sample to decline with age (r = -0.14, P < 0.02). Average macular pigment density was significantly lower in women versus men (P < 0.05), lower in individuals with light-colored irises versus dark-colored irises (P < 0.009), and lower in heavy smokers compared to light (P < 0.0045) and never (P < 0.034) smokers.

CONCLUSIONS. Macular pigment density was lower than average levels obtained from the Northeast but similar to average values obtained in a recent study of adults recruited from Indianapolis. Consistent with past studies, MP density was 13% lower in women and 18% lower in individuals with light- versus dark-colored irises. The relation of smoking to macular pigment density was only significant for those current smokers who smoked more than 10 cigarettes per day (about a 25% reduction). The large number of individuals in this sample with low macular pigment density motivates the need for population-based assessment of the possibly poor nutritional state of the average American’s retina.


   Introduction
Top
 Abstract
 Introduction
Methods
 Results
 Discussion
 References
 
Concentrated within the inner layers of the primate fovea is a yellow pigment that is derived from the dietary carotenoids lutein and zeaxanthin. Two main hypotheses have been advanced to account for the presence of these macular pigments in primate retinas. The "protection hypothesis" proposes that macular pigment (MP) protects the retina and retinal pigment epithelium from oxidative damage leading to age-related loss of function and macular disease.1 2 Although direct evidence for this hypothesis is difficult to obtain and would require longitudinal data, a variety of indirect evidence is available to support this hypothesis. For example, epidemiologic evidence has indicated that high intake of lutein and zeaxanthin and high blood levels are related to reduced risk of age-related macular degeneration (AMD).3 4 5 Damage leading to geographic AMD tends to be paracentral, with a relative sparing of the retinal area where MP can be found in its highest concentration,6 and monkeys raised on carotenoid-depleted diets show macular anomalies.7 There are also directional parallels between factors influencing the risk of AMD1 and factors that tend to predict individual differences in MP density. Lower MP density tends to be related to factors that may increase risk of AMD (e.g., smoking,8 light irises,9 female gender,10 ), whereas higher MP density tends to be related to factors that may decrease risk of AMD (e.g., improved diet, male gender, dark irises). Finally, individual differences in MP density are related to individual differences in age-related losses in visual sensitivity, with those having the lowest MP density having the highest amount of age-related loss.11 Although the confluence of this evidence is compelling, the evidence is largely correlational in nature, and the magnitude of the effect is far from clear.

The other main functional hypothesis for MP is based on the possibility that the pigments improve visual resolution. The "acuity hypothesis" proposes that MP improves acuity by absorbing short-wave light, which is easily scattered and poorly focused.12 The acuity hypothesis is based on the problem that the optics of the eye create rather severe chromatic aberrations in the very spectral region that MP maximally absorbs (ca. 400–490 nm).13 The possibility that MP improves visual performance is consistent with preliminary data showing that supplementing the MP carotenoid lutein (L) may improve visual function. For example, Zorge et al.14 recently reported that L supplements significantly improved visual function (e.g., acuity) in 20 patients with congenital retinal degenerations, such as retinitis pigmentosa. Similarly, Richer15 has shown that dietary supplementation of patients with AMD (n = 14) caused dramatic improvements in a number of visual function tests (e.g., 92% had significant improvements in contrast sensitivity). Richer supplemented using 5 ounces of spinach, which he suggested increased the patient’s MP density,16 leading to the improvements in visual function. Such studies have not addressed whether MP is improving the optics of the eye (the acuity hypothesis) and/or treating the underlying disease (the protection hypothesis). No direct empiric test of whether MP actually improves acuity is yet available.

Based on the available evidence, it is reasonable to conclude that MP does serve some function within the eye rather than simply being an imperfection in the eye’s optics. Thus, the fact that MP density varies so dramatically between individuals is also meaningful. If this premise is correct, then information regarding "average" levels of MP density in the general population is needed. Although a number of large epidemiologic studies are available showing average levels of dietary carotenoid intake17 18 and blood levels of lutein and zeaxanthin,19 few large studies are available showing variation in retinal carotenoid levels within the normal population. The lack of a representative database is at least partially due to the advanced optics required to measure MP in the traditional manner (e.g., Maxwellian view optical systems). This has limited study of the MP carotenoids to smaller samples that may not be representative of the larger population. The recent availability of simplified optics for measuring MP in natural view20 has provided the means for larger studies on MP density to be conducted. In the present study, we report MP density in a large urban sample recruited from the Southwest region of the United States.


   Methods
Top
 Abstract
 Introduction
 Methods
Results
 Discussion
 References
 
Subjects
Two hundred seventeen subjects (79 men, 138 women) from the Phoenix metropolitan area were tested. All subjects were naïve to the purpose of the study and were not experienced in psychophysical tasks. Informed consent was obtained from all subjects, and the tenets of the Declaration of Helsinki were followed. The age distribution of this sample is shown in Table 1 . Eighty-nine subjects (41%) were recruited from the local community through the use of newspaper advertisements and word of mouth. Forty-one subjects (18.9%) were recruited from the west campus of Arizona State University and were either staff or students. An additional 41 students (18.9%) were recruited from a local community college (Glendale Community). Finally, 46 subjects (21%) were recruited from a local senior center (Los Olivos).


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  		Table 1. Descriptive Statistics for Selected Study Variables

 
Each subject filled out a personal data questionnaire that was used to obtain personal and medical information. Subjects used in the study reported good ocular health. Three subjects (recruited from the senior center) were excluded on the basis of reported ocular problems, and two subjects with type II diabetes were excluded. The personal data questionnaire also included questions regarding iris color ("what color are your eyes, i.e., iris color"), smoking status, and two ordinal-ranked questions (Likert-scale) regarding dietary intake of fat, fruits, and vegetables. Fifty-two of the subjects were current smokers, 53 were past smokers, and 112 of the subjects had never smoked. Seventy-two of the subjects reported blue-gray irises, 60 subjects reported green-hazel irises, and 82 subjects reported brown-black irises. Some subjects were also asked how long they had resided in Arizona, and this information is provided in Table 1 .

Measurement of MP Optical Density
MP optical density was measured psychophysically using flicker photometry (for a review of this procedure and the underlying assumptions, see Snodderly and Hammond21 ). Only the right eye of each subject was measured. A circular test stimulus was presented near the center of a 6°, 1.5 log Td, 470-nm circular background. The size of the test stimulus was 1°. We also measured MP density in some subjects (n = 171) with a 2° test and a 2° annular field to check within-session consistency (as described more fully later). The wavelength composition of the test stimulus alternated between a 460-nm measuring field (peak MP absorbance) and a 570-nm, 1.7 log Td reference field (minimal MP absorbance). The measuring and reference fields were superposed and presented out of phase at an alternation rate of 11 to 12 Hz in the foveal condition and 6 to 7 Hz in the parafoveal condition. Subjects adjusted the radiance of the 460-nm measuring field to achieve minimal flicker with the 570 nm reference. This measurement was done in the fovea (where MP is the most dense) and 4° in the parafovea (where light absorption by MP is negligible).22 23 A tiny (5 minute) opaque fixation point was located on the left edge of the background and subjects fixated this point when making the parafoveal measurement. Subtracting the foveal from the parafoveal log sensitivity measurement yields an optical density measure of MP.

Light for the 10° background was produced by three LEDs (packed tightly in a triangular array) with peak energy at 470 nm and half-widths of approximately 20 nm. Light for the 570-nm reference field was produced by an LED with peak energy at 570 nm (half-width = 20 nm). Light for the 460 nm measuring field was produced by two LEDs with peak energy at 458 nm (half-width = 20 nm). Light from the LED sources was collimated with planoconvex lenses and was then passed through polycabonate diffusers (high-efficiency, holographic type; Physical Optics Co., Torrance, CA), which served essentially as back projection screens.

The size of the background and test stimulus was defined by circular apertures (constructed by computer-generated images exposed on high-density, photographic mylar film) placed after the collimating lenses. The background and test stimuli were then combined and reflected to the subject by a 2-inch beamsplitter whose front surface was located 16 inches from the subject’s eye. The entire optical system was contained in a rectangular, black Plexiglas box. One side of the box contained a one-inch hole centered on the subject’s optical axis through which the stimulus could be viewed. Head alignment was accomplished by the use of an adjustable head and chin rest assembly and, when properly aligned, the subject viewed the hole in the box as slightly larger and concentric with the background field.

Stimuli were calibrated using a photocell (PIN-10, UDT Sensors, Inc., Hawthorne, CA). The LEDS were driven by a constant current power supply. Radiance variation was achieved by varying the frequency of a 1.5-msec pulse over a range of 300 to 300,000 Hz. Our calibration of the high-frequency pulse rate shows that the frequency delivery is nearly perfectly proportional to the radiance output. Thus, MP density values could be derived by simply calculating the log ratio of the frequencies of the 460-nm measuring field at the foveal and parafoveal eccentricities, respectively.

The apparatus used for the MP measurement delivered the stimulus in natural view, but used a stimulus configuration that was similar to configurations used in past studies, where the stimulus was presented in Maxwellian view.8 9 10 11 16 22 24 Recent data on 32 subjects (age range = 16–60 years) has shown, however, that MP density measured in natural view and with slight differences in stimulus configuration (e.g., this study used a 4° rather than a 6° parafoveal reference) provides the same values as MP measured in Maxwellian view (range of MP values = 0.0–0.60).20 As an additional check, we measured the MP density of two highly experienced investigators using the Maxwellian systems in Boston8 11 16 and Phoenix20 and the natural view optical system used in the present study in Phoenix and a similar system in Indianapolis.25 The different systems yield the same values at the different sites (0.40 ± 0.05 and 0.64 ± 0.03).

Given the reliability of the MP measurement technique,22 we elected to limit subject assessment to only one experimental session. Eight subjects, however, with no previous experience with psychophysical tasks, were measured in 10 separate sessions spaced over 2 to 4 weeks to check the reliability of our current instrument. The range of MP values across experimental sessions was 0.07 for the best subject and 0.27 for the worst subject (average range = 0.166). The values had strong central tendencies, however, and were reliable (Cronbach’s {alpha} = 0.97).

Because the spatial density distribution of MP is well known,22 26 subject accuracy can be checked by changing the spatial configuration of the stimulus and checking the resultant value against the known spatial density distribution of the pigments. To this end, we measured MP density using a 2° solid test field and a 2° annulus (see Table 1 ). Past work22 has suggested that MP density declines exponentially when it is measured at increasing distances from the center of the fovea. Consistent with this prediction, the average MP density at 2° (0.13) is what would be predicted based on the average MP density value at 1° (0.22). The edge hypothesis24 predicts that when MP density is measured using flicker photometry that the derived optical density value is largely determined at the edge of the flickering test stimulus rather than averaged across the entire test field. Although the average MP density obtained with the 2° annulus (0.10) was slightly lower than the value obtained with the solid 2° field (0.13), the correlation was high (n = 171, Y = 0.007 + 0.71X, r = 0.80). Consistent with past studies,22 these analyses suggest that the technique we used for measuring MP density provided reliable data.

Statistics
Results are expressed as mean ± SDs. P values for inferential statistics were calculated using a one-tailed, independent-groups Student’s t-test. Relationships were analyzed using Pearson product moment correlational coefficients.


   Results
Top
 Abstract
 Introduction
 Methods
 Results
Discussion
 References
 
Table 1 shows the mean and SD of MP density for the entire sample. A histogram of these data are provided in Figure 1 . As shown in the figure, the data are normally distributed and do not show the bimodal pattern seen in some other samples.10 Figure 2 shows the relationship between age and MP density. As seen in this figure, the MP density of this sample shows a slight decline with age (Y = 0.26 - 0.001X, r = -0.14, P < 0.02). There was a relationship between age and years spent living in Arizona (r = 0.53). There was not, however, a significant relationship between MP density and years spent living in Arizona (n = 128, r = -0.12, P < 0.09).



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  		Figure 1. Macular pigment optical density for the entire sample (n = 217). The smooth line represents the best fit to a normal curve.

 


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  		Figure 2. The relationship between macular pigment and age.

 
A comparison of the data for the men (n = 79) and women (n = 138) is provided in Table 1 . As shown in the table, MP density at 1° is approximately 13% higher for men compared to women (P < 0.05). This difference is shown graphically in Figure 3 . As shown in the figure, most of the sex difference is due to the lack of women in the highest MP range. This pattern is similar to that seen in past studies.10 If the top 5% of men with the highest MP density are removed from the sample, the sex difference in MP density is minimal (0.21 and 0.22 for women and men, respectively).



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  		Figure 3. A histogram showing the difference in macular pigment optical density between men and women.

 
The relationship that we found between MP density and iris color also was similar to past studies.9 As shown in Figure 4 , subjects with light irises (n = 104, blue, gray, green, MP = 0.199) had 18% lower MP density on average compared to subjects with dark irises (n = 113, brown, black, hazel, MP = 0.242). This difference was statistically significant (P < 0.009), but was largely driven by those individuals with the lightest iris color. Subjects with blue-gray irises (n = 69) had lower (P < 0.003) MP density (0.186 ± 0.11) than either individuals with green-hazel irises (n = 65, 0.23 ± 0.12) or brown-black irises (n = 83, 0.244 ± 0.146).



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  		Figure 4. A histogram showing the difference in macular pigment optical density between individuals with light- and dark-colored irises.

 
We also analyzed the relationship between smoking status and MP density. The current (n = 52) and past smokers (n = 53) had lower average MP density (0.21 ± 0.11 and 0.21 ± 0.13, respectively) than the never smokers (0.23 ± 0.14), but this difference was not significant (P < 0.19). There was, however, a significant relationship between smoking frequency for the current smokers (cigarettes per day) and MP density (r = -0.29, P < 0.015; see Fig. 5 ), suggesting that heavier smoking may be related to lower MP density. Consequently, we compared heavier smokers (>10 cigarettes per day, n = 29) to lighter (<10 cigarettes per day, n = 25) and never smokers. Heavier smokers had significantly (P < 0.0045) less MP density than light smokers (0.18 ± 0.09 and 0.25 ± 0.09, respectively) and never smokers (P < 0.034, 0.23 ± 0.09). No relationship between cumulative exposure to cigarettes (packs per day multiplied by years smoked) and MP density was found.



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  		Figure 5. The dose–response relationship between macular pigment optical density and the number of cigarettes current smokers smoked per day.

 

   Discussion
Top
 Abstract
 Introduction
 Methods
 Results
 Discussion
References
 
The present study reports on MP density in 217 subjects from the Phoenix metropolitan area. In contrast to past studies using similar stimulus conditions, this sample had relatively low MP density. For example, our average MP density value was approximately 40% lower than the average obtained using similar measurement procedures on samples recruited from the Northeast (assuming an average of 0.35).8 9 10 16 22 Our value was similar, however, to the value (0.21 ± 0.13) recently obtained from a large sample (n = 280) recruited from the Indianapolis area.25 The Indianapolis sample recruited subjects from the general population and excluded individuals who were affiliated with the University or Medical center.

One limitation of the present study is that the reference for the MP measurement was obtained at a retinal eccentricity of 4°. Some preliminary data27 have suggested the possibility that MP might have a secondary spatial density peak at 4° in older individuals with high MP density (The Indianapolis population is not subject to this possible artifact because only subjects younger than 50 years were tested). This possibility is consistent with the age-related decline we found in MP density. Because the average MP density of individuals less than 50 years (n = 149, 0.229 ± 0.12) was only slightly lower (11%) than the average MP density of individuals more than 50 years (n = 68, 0.204 ± 0.148), any underestimation is probably small.

Similar to past studies,8 9 10 we found lower MP density in women compared to men (13%) and lower MP densityin subjects with light-colored irises compared to subjects with dark-colored irises (18%). We did not find a relationship between light smoking (>10 cigarettes per day) and MP density. Heavier smokers, however, did have significantly less MP density (approximately 25%) than either light smokers or never smokers. In contrast to earlier studies,8 9 10 all the relationships we found in this study were muted, probably because of the relatively low average MP levels within this population. No factor can deplete MP if an individual has no MP to deplete. Moreover, it is possible that depletion of MP is not linear and differs between subjects (similar to increases in MP in response to dietary supplementation).16 For example, equal exposure to a negative factor might lower MP density more in individuals with high MP levels compared to low MP levels.

For individuals with an average MP optical density of 0.22, 70% of the light at 460 nm is reaching the vulnerable outer segments of their macular cones. In contrast, we have measured individuals with MP density exceeding one log unit, which translates to only 2% to 3% of this damaging short-wave light reaching the cones. It is likely that this type of difference, considered over time, would produce meaningful differences in the aging of an individual’s retina. This may be particularly meaningful in areas such as Arizona, where annual light levels are so high. Recent data11 comparing younger and older individuals suggests that to retard age-related loss of retinal sensitivity (measured as isolated sensitivity to short-wave light), MP density should be at least twice as high as the average in the present sample. Data from the US Department of Agriculture17 18 on dietary intake of fruits and vegetables indicate that the most frequently ingested fruits and vegetables in the United States contain little L and zeaxanthin. Future efforts should be directed toward increasing individual’s intake of carotenoid-rich foods such as spinach.28


   Footnotes
 
Supported by grants from the Visual Science Laboratory at Arizona State University.

Submitted for publication August 5, 1999; revised November 2 and November 30, 1999; accepted December 9, 1999.

Commercial relationships policy: N.

Corresponding author: Billy R. Hammond, Jr., Department of Psychology, Franklin College of Arts and Sciences, Psychology Building, University of Georgia, Athens, GA 30677. bhammond{at}egon.psy.uga.edu


   References
Top
 Abstract
 Introduction
 Methods
 Results
 Discussion
 References
 

  1. Snodderly, DM (1995) Evidence for protection against age-related macular degeneration by carotenoids and antioxidant vitamins Am J Clin Nutr 62,1448S-1461S[Abstract/Free Full Text]
  2. Schalch, W, Dayhaw-Barker, P, Barker, FM, II (1999) The carotenoids of the human retina Taylor, AJ eds. Nutritional and Environmental Influences on Vision ,215-250 CRC Press Boca Raton.
  3. . Eye isease Case Control Study Group (1992) Risk factors for neovascular age-related macular degeneration Arch Opthalmol 110,1701-1708[Abstract/Free Full Text]
  4. . Eye isease Case Control Study Group (1993) Antioxidant status and neovascular age-related macular degeneration Arch Opthalmol 111,104-109[Abstract/Free Full Text]
  5. Seddon, JM, Umed, AA, Sperduto, RD, et al (1994) Dietary carotenoids, vitamins A, C, and E, and advanced age-related macular degeneration JAMA 272,1413-1420[Abstract/Free Full Text]
  6. Weiter, JJ, Delori, FC, Dorey, K. (1988) Central sparing in annular macular degeneration Am J Opthalmol 106,286-292[Medline][Order article via Infotrieve]
  7. Neuringer, M, Klein, ML, Snodderly, DM, Johnso, EJ, Feeney–Burns, L, Kittleson, C. (1999) Effects of carotenoid depletion and N-3 fatty acid status on macular defects in rhesus monkeys [ARVO Abstract] Invest Ophthalmol Vis Sci 40(4),S164Abstract nr 882
  8. Hammond, BR, Jr, Wooten, BR, Snodderly, DM (1996) Cigarette smoking and retinal carotenoids: implications for age-related macular degeneration Vis Res 36,3003-3009[Medline][Order article via Infotrieve]
  9. Hammond, BR, Jr, Fuld, K, Snodderly, DM (1996) Iris color and macular pigment optical density Exp Eye Res 62,715-720
  10. Hammond, BR, Curran–Celentano, J, Judd, S, et al (1996) Sex differences in macular pigment optical density: relation to plasma carotenoid concentrations and dietary patterns Vis Res 36,2001-2012[Medline][Order article via Infotrieve]
  11. Hammond, BR, Jr, Wooten, BR, Snodderly, DM (1998) Preservation of visual sensitivity of older subjects: association with macular pigment density Invest Opthalmol Vis Sci 36,1-10[Medline][Order article via Infotrieve]
  12. Reading, VM, Weale, RA (1974) Macular pigment and chromatic aberration J Am Optom Assoc 64,231-234
  13. Thibos, LN, Bradley, A, Zhang, X. (1991) Effect of ocular chromatic aberration on monocular visual performance Opt Vis Sci 68,599-601
  14. Zorge, I, McDonald, G, Dagnelie, G. (1999) Lutein improves visual function in some patients with congenital retinal degenerations—a pilot study via internet. [ARVO Abstract] Invest Ophthalmol Vis Sci 40(4),S697Abstract nr 3680
  15. Richer, S. (1999) Part II. ARMD—Pilot (case series) environmental intervention data J Am Optom Assoc 70,24-36[Medline][Order article via Infotrieve]
  16. Hammond, BR, Johnson, EJ, Russell, RM, et al (1997) Dietary modification of human macular pigment density Invest Ophthalmol Vis Sci 38,1795-1801[Abstract/Free Full Text]
  17. Nebeling, LC, Forman, MR, Graubard, BI, Snyder, RA (1997) The impact of lifestyle characteristics on carotenoid intake in the United States: the 1987 National Health Interview Survey Am J Pub Health 87,268-271[Abstract/Free Full Text]
  18. LaChance, P. (1988) Dietary intake of carotenes and the carotene gap Clin Nutr 7,118-122
  19. Stacewicz–Sapuntzakis, M, Bowen, PE, Mares–Perlman, JA (1993) Serum reference values for lutein and zeaxanthin using a rapid separation technique Ann NY Acad Sci 691,207-209[Medline][Order article via Infotrieve]
  20. Wooten, BR, Hammond, BR, Land, R, Snodderly, DM (1999) A practical method of measuring macular pigment optical density Invest Ophthalmol Vis Sci 40,2481-2489[Abstract/Free Full Text]
  21. Snodderly, DM, Hammond, BR (1999) In vivo psychophysical assessment of nutritional and environmental influences on human ocular tissues: lens and macular pigment Taylor, AJ eds. Nutritional and Environmental Influences on Vision ,251-273 CRC Press Boca Raton.
  22. Hammond, BR, Wooten, BR, Snodderly, DM. (1997) Individual variations in the spatial profile of macular pigment JOSA A 6,187-1196
  23. Snodderly, DM, Brown, PK, Delori, FC, Auran, JD (1984) The macular pigment. I. Absorbance spectra, localization, and discrimination from other yellow pigments in primate retinas Invest Ophthalmol Vis Sci 25,660-673[Abstract/Free Full Text]
  24. Werner, JS, Donnelly, SK, Kliegl, R (1987) Aging and human macular pigment density Vis Res 27,257-268[Medline][Order article via Infotrieve]
  25. Curran–Celentano, J, Hammond, BR, Cooper, DA, Ciulla, TA. (1999) The relationship between dietary intake, blood and retinal concentrations of carotenoids in adult volunteers in the Indianapolis area [ARVO Abstract] Invest Opthalmol Vis Sci 40(4),S568Abstract nr 2993
  26. Snodderly, DM, Auran, JD, Delori, FC (1984) The macular pigment. II. Spatial distribution in primate retinas Invest Ophthalmol Vis Sci 25,674-685[Abstract/Free Full Text]
  27. Bieber, ML, Werner, JS. (1999) Assumptions underlying the psychophysical estimation of the macular pigment density [ARVO Abstract] Invest Opthalmol Vis Sci 40(4),S353Abstract nr 1873
  28. Sommerburg, O, Keunen, JEE, Bird, AC, van Kuijk, FJGM (1998) Fruits and vegetables that are sources for lutein and zeaxanthin: the macular pigment in human eyes Br J Ophthalmol 82,907-910[Abstract/Free Full Text]



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