Measurement of Macular Pigment Optical Density in a Retina

Retina
Measurement of Macular Pigment Optical Density in a
Healthy Chinese Population Sample
Jie Yu,1 Elizabeth J. Johnson,2 Fu Shang,2 Apiradee Lim,3 Haiying Zhou,1 Lei Cui,1 Jun Xu,1
Torkel Snellingen,4 Xipu Liu,4 Ningli Wang,1 and Ningpu Liu1
PURPOSE. Macular pigment may protect against age-related
macular degeneration (AMD) by its capacity to absorb blue
light and scavenge free radicals. Current information on human
macular pigment density has been largely from studies on
Caucasian populations. The purpose of this study was to assess
macular pigment density and its determinant factors in a
Chinese population sample.
METHODS. Macular pigment optical density (MPOD) was
measured in a healthy Chinese population using heterochromatic flicker photometry (HFP). Participants received a
standard ophthalmic examination, and only subjects who
were confirmed not to have any eye diseases except mild agerelated cataract were included in the study. Demographic and
lifestyle data and general health status were recorded by
questionnaire.
RESULTS. A total of 281 unrelated healthy Chinese individuals,
including 96 males and 185 females, with ages ranging from 17
to 85 years, participated in the study. The mean and standard
deviation of MPOD levels were 0.56 6 0.19, 0.49 6 0.18, 0.36
6 0.15, and 0.19 6 0.12, respectively, at 0.258, 0.58, 1.08, and
1.758 eccentricity points. A significant age-related decline in
MPOD was observed at 0.258 (P ¼ 0.014). Females tended to
have relatively lower levels of MPOD than males at 0.258 (P ¼
0.21), 0.58 (P ¼ 0.025), and 1.08 (P ¼ 0.16). No statistically
significant association of MPOD was observed with body mass
index or smoking status.
CONCLUSIONS. Macular pigment density measured by HFP
tended to decline with aging in this healthy Chinese
From the 1 Beijing Tongren Eye Center, Beijing Tongren
Hospital, Capital Medical University, Beijing Ophthalmology and
Visual Sciences Key Laboratory, Beijing, China; the 2Jean Mayer US
Department of Agriculture Human Nutrition Research Center on
Aging, Tufts University, Boston, Massachusetts; the 3Department of
Mathematics and Computer Science, Faculty of Science and
Technology, Prince of Songkla University, Pattani Campus, Muang
Pattani, Thailand; and the 4Sekwa Eye Hospital, Beijing, China.
Supported in part by the National Basic Research Program of
China (973 Program) Grant 2007CB512201, the Beijing Municipal
Health Bureau Grant 2009208, the National Natural Science
Foundation of China Grant 81070734, and an unrestricted research
fund from Kemin Japan. Kemin Health USA kindly provided the
heterochromatic flicker photometer for the study.
Submitted for publication September 2, 2011; revised December 30, 2011 and January 26, 2012; accepted February 20, 2012.
Disclosure: J. Yu, None; E.J. Johnson, None; F. Shang, None;
A. Lim, None; H. Zhou, None; L. Cui, None; J. Xu, None; T.
Snellingen, None; X. Liu, None; N. Wang, None; N. Liu, Kemin
(F), Bausch & Lomb (C)
Corresponding author: Ningpu Liu, Beijing Tongren Eye Center,
Beijing Tongren Hospital, Capital Medical University, No. 1 Dong
Jiao Min Xiang, Dongcheng District, Beijing 100730, China;
[email protected].
2106
population sample. Females may have lower levels of MPOD
than males. (Invest Ophthalmol Vis Sci. 2012;53:2106–2111)
DOI:10.1167/iovs.11-8518
he macula is the center of the posterior retina and can be
discerned clinically as the area of yellowish pigmentation.
Macular pigment, composed of three carotenoids—lutein,
zeaxanthin, and meso-zeaxanthin1—is believed to improve
visual performance by reducing chromatic aberration and glare
sensitivity.2–4 Through its ability to absorb high-energy shortwave blue light and its antioxidative property, macular pigment
is also hypothesized to protect the macula from light-induced
and oxidative damage, which may lead to age-related loss of
function and macular diseases.5,6 Although obtaining direct
evidence for this hypothesis is difficult, a variety of indirect
evidence supports this hypothesis. For example, epidemiologic
studies suggest that a high intake of lutein and zeaxanthin and
high blood levels of these carotenoids are related to a reduced
risk of age-related macular degeneration (AMD).7–9 Lutein and
zeaxanthin are entirely of dietary origin and cannot be
synthesized by the human body. Foods such as green leafy
vegetables, corn, squash, broccoli, peas, and egg yolks are a
rich source of lutein and zeaxanthin.10 In healthy subjects, it
has been shown that blood or retinal levels of carotenoids can
be modified by dietary intake or supplementation.11–13
However, whether increasing blood or retinal levels of
carotenoids can offer additional protection from degenerative
changes of the retina remains to be proven.
It is well-known that the prevalence of AMD varies among
different populations. Prevalence of early and late AMD in a
Chinese population, for example, was reported to be lower
than that in Caucasians.14,15 In addition to the differences in
genetic susceptibility,16–18 traditional Chinese foods with
higher concentrations of carotenoids19 may translate to higher
macular pigment and contribute to the relatively lower
prevalence of AMD in the Chinese population. Current
information regarding human macular pigment optical density
(MPOD), however, is mainly from studies on Caucasian
populations. To the best of the authors’ knowledge, only two
studies reported MPOD in small samples of Hong Kong
Chinese.20,21 The present study aimed to measure MPOD in a
relatively larger sample of a healthy Chinese population in
urban Beijing and to evaluate its determinant factors.
T
SUBJECTS
AND
METHODS
Study Participants and Clinical Evaluation
The study was performed under approval of the Ethics Committee of
Beijing Tongren Hospital. Informed consent was obtained from
participants after explanation of the purpose and procedure and
possible consequences of the study. The research procedures followed
Investigative Ophthalmology & Visual Science, April 2012, Vol. 53, No. 4
Copyright 2012 The Association for Research in Vision and Ophthalmology, Inc.
Measurement of Macular Pigment
IOVS, April 2012, Vol. 53, No. 4
TABLE 1. Characteristics of the Study Subjects (n ¼ 281)
Subjects
Number
%
54
48
46
64
69
19.2
17.1
16.4
22.8
24.6
96
185
34.2
65.8
121
123
37
43.1
43.8
13.2
226
55
80.4
19.6
Age, years
30
31–40
41–50
51–60
>60
Sex
Male
Female
BMI, kg/m2
23
23–27
‡27
Smoking status
Nonsmokers
Smokers
the tenets of the Declaration of Helsinki. Healthy Chinese volunteers
were recruited to the study by poster and oral advertisement in the
local community of downtown Beijing. All study participants
underwent a standard ophthalmic examination, including visual acuity,
intraocular pressure, slit-lamp biomicroscopy, and fundus examinations
through dilated pupil. Color fundus photographs centered on the
fovea of both eyes were taken for each subject. Body mass index (BMI,
kg/m2) was calculated according to the height and weight of the
participant. Personal and medical information was obtained using a
questionnaire. Only those subjects with self-reported healthy status
and normal fundus in both eyes were included in the study. Exclusion
criteria included history of diabetes or any life-threatening disease such
as myocardial infarction or cancer, or subjects with ocular disease
including corneal scar, dense cataract, glaucoma, uveitis, vitreous
hemorrhage, chorioretinal and optic nerve diseases, or any maculopathy.
2107
Throughout testing, the subject was reminded to gaze at the
fixation point. They were also advised to blink frequently or to close
the testing eye for a short while to minimize the influence of fatigue.
Both eyes were measured in a randomized order to avoid the potential
order effect.
Statistical Analysis
Data analyses were performed using the open-source R statistical
software package.23 Mean and standard deviation of MPOD at each
eccentricity were calculated. Paired t-test was performed to compare
MPOD levels between the right and left eyes at each eccentricity.
MPOD levels between males and females or between smokers and
nonsmokers were compared using t-test for two independent samples
or Wilcoxon rank sum test for non-normal distributed data. Multiple
comparisons were made using ANOVA followed by Tukey’s test for
normal distributed data or Kruskal-Wallis test for non-normal distributed data. Correlations between continuous variables were evaluated
using Pearson’s correlation coefficient (r). Relationships between
MPOD and age, BMI, sex, and smoking status were analyzed by
multiple linear regressions. Statistical significant level was set as P
values less than 0.05.
RESULTS
Measurement of Macular Pigment Optical Density
A total of 281 healthy subjects aged from 17 to 85 years
participated in the study. Ninety-six were male and 185 were
female. Characteristics of the study subjects are given in Table
1. None of the study subjects reported the use of lutein/
zeaxanthin supplementation. A comparison of MPOD levels
between right and left eyes at 0.258, 0.58, 1.08, and 1.758
eccentricities is shown in Figure 1. Substantial interocular
agreement was obtained at 0.258, 0.58, 1.08 and 1.758,
eccentricities (r ¼ 0.81, 0.82, 0.74, and 0.69, respectively).
The average MPOD level of right and left eyes of each study
subject was thus calculated and used for subsequent analysis.
Table 2 shows the MPOD measurement among study
subjects. The measurement was defined as successful if the
subject could complete the task at test point of 0.258, 0.58,
1.08, or 1.758, respectively. The rate of successful measurement
for MPOD was 79% (222/281) at 0.258, 96.8% (272/281) at 0.58,
MPOD was measured psychophysically in both eyes using heterochromatic flicker photometry (HFP) in a dark room.22 The examiner
familiarized the subject with the apparatus and showed an instructional
video that was designed to provide a uniform introduction to the task
(www.macularmetrics.com). Four foveal test configurations at 0.258,
0.58, 1.08, and 1.758 eccentricity points were measured. The former
two are solid disks and the latter two are annuli. The parafoveal
reference test target is a solid disk set at 78 eccentricity. Flicker
frequency of the two test wavelengths (460 nm and 530 nm) usually
began with 14 Hz for the foveal and 12 Hz for the parafoveal
measurement. The frequency would be decreased if the subject could
not recognize sensation of flickering or increased if the subject could
not find a no-flicker zone.
The subjects were asked to affix their gaze on the exact center of
the test target and perceive the flickering. For the parafoveal test, the
fixation target is a small red light-emitting diode located to the left of
the background field. The task for the participant was to identify
whether the target is flickering or not. The examiner adjusted the knob
from the flicker zone to the no-flicker zone, clockwise first and then
counterclockwise to obtain two values indicating the border of the
flicker and no-flicker zones (minimum and maximum intensity of blue
light). The exact mathematical average of the two values, which stands
for the middle of the no-flicker zone, was calculated. Measurement at
each eccentricity point was repeated five times, and the average was
used to calculate MPOD.
FIGURE 1. Comparison of macular pigment optical density levels in
subjects with successful measurements (mean 6 95% confidence
interval) between right and left eyes at 0.258 (n ¼ 222), 0.58 (n ¼ 272),
1.08 (n ¼ 281), and 1.758 (n ¼ 235) eccentricity points.
2108
Yu et al.
IOVS, April 2012, Vol. 53, No. 4
TABLE 2. Macular Pigment Optical Density Measured by Heterochromatic Flicker Photometry among Study
Subjects
Degree of Eccentricity
Subjects
0.258
0.58
1.08
1.758
Age, years
30
31–40
41–50
51–60
60
Total
0.61
0.63
0.51
0.52
0.55
0.56
6
6
6
6
6
6
0.20
0.17
0.18
0.16
0.20
0.19
(43)
(45)
(41)
(53)
(40)
(222)
0.51
0.53
0.44
0.47
0.50
0.49
6
6
6
6
6
6
0.19
0.17
0.16
0.15
0.20
0.18
(54)
(48)
(45)
(61)
(64)
(272)
0.38
0.39
0.30
0.33
0.37
0.36
6
6
6
6
6
6
0.14
0.14
0.13
0.13
0.18
0.15
(54)
(48)
(46)
(64)
(69)
(281)
0.19
0.20
0.15
0.19
0.23
0.19
6
6
6
6
6
6
0.10
0.12
0.09
0.10
0.15
0.12
(44)
(45)
(43)
(56)
(47)
(235)
Sex
Male
Female
0.59 6 0.20 (75)
0.55 6 0.18 (147)
0.52 6 0.20 (92)
0.47 6 0.17 (180)
0.37 6 0.17 (96)
0.35 6 0.14 (185)
0.19 6 0.13 (80)
0.19 6 0.11 (155)
0.57 6 0.19 (96)
0.56 6 0.19 (99)
0.56 6 0.19 (27)
0.49 6 0.17 (119)
0.49 6 0.19 (116)
0.47 6 0.18 (37)
0.36 6 0.13 (121)
0.35 6 0.16 (123)
0.35 6 0.16 (37)
0.18 6 0.10 (102)
0.20 6 0.13 (106)
0.22 6 0.13 (27)
0.56 6 0.19 (181)
0.57 6 0.18 (41)
0.48 6 0.18 (219)
0.51 6 0.19 (53)
0.35 6 0.15 (226)
0.36 6 0.17 (55)
0.19 6 0.12 (191)
0.20 6 0.13 (44)
BMI, kg/m2
23
23–27
‡ 27
Smoking status
Nonsmokers
Smokers
Data are expressed as mean 6 standard deviation. The number of subjects with successful measurement is
given in parentheses.
100% (281/281) at 1.08, and 83.6% (235/281) at 1.758
eccentricities. Mean MPOD level and standard deviation from
study subjects of successful measurement was 0.56 6 0.19 at
0.258, 0.49 6 0.18 at 0.58, 0.36 6 0.15 at 1.08, and 0.19 6 0.12
at 1.758.
As shown in Figure 2, a significant age-related decline in
MPOD levels was observed at 0.258 eccentricity point (r ¼
0.165, P ¼ 0.014). However, there was no significant
association between MPOD and aging at 0.58 (r ¼ 0.025, P
¼ 0.68), 1.08 (r ¼ 0.053, P ¼ 0.38), or 1.758 (r ¼ 0.094, P ¼
0.15) eccentricities.
A comparison of MPOD levels between males and females is
shown in Figure 3. Females tended to have relatively lower
MPOD level than males at 0.258, 0.58, and 1.08 eccentricities.
This sex difference was statistically significant at 0.58 (P ¼
0.025) but was not observed at 0.258 (P ¼ 0.21) and 1.08 (P ¼
0.16). No correlation was observed between smoking status
and MPOD (P ¼ 0.68 at 0.258, 0.21 at 0.58, 0.68 at 1.08, and 0.67
at 1.758) or between BMI and MPOD (P ¼ 0.84 at 0.258, 0.71 at
0.58, 0.79 at 1.08, and 0.19 at 1.758) among study subjects.
The results from a multiple linear regression model are
given in Table 3. Age was found to be the only factor showing
statistically significant inverse correlation with MPOD, but only
at 0.258 eccentricity point (multiple R2 ¼ 0.063, P ¼ 0.003).
The effect of sex on MPOD became insignificant in the model
of multivariate analyses.
FIGURE 2. The relationship between macular pigment optical density
at 0.258 eccentricity point and aging (r ¼ 0.165, P ¼ 0.014) in 222
subjects with successful measurement.
FIGURE 3. Comparison of macular pigment optical density levels
(mean 6 95% confidence interval) between males (96 subjects) and
females (185 subjects) at 0.258, 0.58, 1.08, and 1.758 eccentricity points.
Measurement of Macular Pigment
IOVS, April 2012, Vol. 53, No. 4
2109
TABLE 3. Multivariate Analyses for the Association of Macular Pigment Optical Density Measured at 0.258 with
Variable Factors
Variables
Coefficient
Standard Error
P Value
Age, years
BMI, kg/m2
Sex (reference¼male)
Smoking status (reference¼nonsmoker)
Light exposure
0.003
0.003
0.024
0.017
0.008
0.001
0.005
0.034
0.042
0.009
0.003*
0.52
0.48
0.68
0.40
* Multiple R2: 0.063.
DISCUSSION
This study determined the effects of age, sex, BMI, and
smoking status on MPOD in a relatively large sample of the
Chinese population. We found that the MPOD levels decrease
with aging in the center of the retina (at the 0.258 eccentricity
point). Because MPOD plays an important role in blocking
harmful blue light and in quenching reactive oxygen species,
this age-related decline in MPOD may partially explain the agerelated increase in the risk for macular degeneration. It should
be noted, however, that only 6.3% of the decline in MPOD in
the central fovea could be attributed to aging in the multiple
linear regression model (multiple R2 ¼ 0.063). Thus, the
hypothesized relationship between MPOD and the risk of AMD
may be relatively weak.
The age-related decline in MPOD was also reported in
several previous studies20,24–30; however, there were studies
that did not detect this age-related difference in MPOD.31–35
The discrepancy in the age–MPOD relationship between
different studies may be related to differences in sample size,
subject selection, or methods of measurement. In a comparative study that used different methods to investigate the
possible effect of age on MPOD levels, a small but statistically
significant age-related decline of MPOD was detected by the
method of HFP, but this age-related difference in MPOD was
not observed when the fundus reflectance spectroscopy or
scanning laser ophthalmoscope was used.33 Among all of the
available methods for noninvasive measurement of MPOD,
including HFP, autofluorescence,36 Raman spectroscopy,37
fundus reflectance spectroscopy,38 and motion photometry,39
the HFP method has been the most commonly used in research
studies.20,21,26,40–44 The advantages of the HFP method include
the relatively inexpensive instrument,22,45,46 easy training of
research staff to operate the machine,22,46 and no requirement
for pupil dilation.22,45,46 In addition, HFP remains the only
validated technique uniquely unaffected by changes in the
ocular media with aging,22,45,46 thus robust to patients with
mild age-related cataract. It should be noted, however, that this
method of HFP, as well as other techniques except Raman
spectroscopy, uses peripheral points of the retina as a
reference. If there were an age-related diffusion of the macular
pigment from macula to the periphery,32 the use of HFP
technique may result in an age-related decrease of MPOD
because of the increase of MPOD at the parafoveal reference
point.33 Future studies using complementary methods of
TABLE 4. Comparison of Macular Pigment Optical Density (MPOD) Measured by Heterochromatic Flicker Photometry in Various Published Studies
Study
Population
Author (year)
Study
Subjects
Sample
Size
Age Range, y
(mean 6 SD)
69–86
(79.1 6 3.2)
50–79
(66)
17–92
(41.5 6 19.7)
(29.6 6 13.1)
18–50
48–82
(68.7 6 9.5)
21–81
(51 6 18)
20–60
16–85
(39.1)
18–23
17–85
Iannaccone et al. (2007)43
U.S.
Healthy
222
Snodderly et al. (2004)46
U.S.
CAREDS
54
Hammond et al. (2000)26
U.S.
Healthy
217
Hammond et al. (2002)47
U.S.
Healthy
680
Ciulla et al. (2001)41
Ciulla et al. (2001)40
U.S.
U.S.
Healthy
Cataract
280
24
Beatty et al. (2001)24
European
Healthy
46
Nolan et al. (2007)30
Lam et al. (2005)20
Irish
Chinese (Hong Kong)
Healthy
Healthy
828
92
Tang et al. (2004)21
Present study
Chinese (Hong Kong)
Chinese (Mainland)
Healthy
Healthy
67
272
* Caucasian.
† African American.
‡ Subjects with BMI > 29 kg/m2.
§ Subjects with BMI < 29 kg/m2.
jj Patients with cataract before cataract exaction.
¶ Patients with cataract after cataract exaction.
CAREDS, Carotenoids in Age-Related Eye Disease Study.
Foveal
Eccentricity
Parafoveal
Reference
MPOD (mean 6 SD
or range)
0.58
78
0.58
78
0.37 6 0.19*
0.22 6 0.23†
0.42 6 0.22
0.58
48
0.22 6 0.13
0.58
48
0.58
0.58
48
48
0.4758
68
0.18
0.22
0.21
0.21
0.18
0.29
0.58
0.58
5.58
78
0.30 6 0.17
0.30 0.60
0.58
0.58
48
78
0.48 6 0.23
0.49 6 0.18
6
6
6
6
6
6
0.12‡
0.14§
0.13
0.13jj
0.12¶
0.16
2110
Yu et al.
MPOD measurement are warranted to verify the age-related
decline in MPOD in this Chinese population.
It has been reported that body fat25,47 and smoking
status26,30 affect the level of macular pigment. However, the
present study did not detect an association between MPOD
and BMI or smoking status. The association between MPOD
levels and sex has been reported in several previous studies,
with females having relatively lower levels of MPOD than
males.20,26,48 A similar trend of sex-related difference in MPOD
was observed in this study. However, the impact of sex on
MPOD became insignificant in the model of multivariate
analyses. Given the fact that several studies that did not find
this sex-associated difference in MPOD,41,43 the authors’ data
indicate that the sex-related difference in MPOD is marginal.
The level of MPOD and its distribution in retina may be
affected by factors such as genetic background, demographics,
or lifestyle characteristics.49 It has been reported, for example,
that females tend to have broader distribution of macular
pigment than males.50 Females therefore are more likely to
have higher residual pigment density at the parafoveal
reference point, which could lead to a relatively lower MPOD
measurement. Moreover, it is thought that females tend to
have higher percentage of body fat and adipose tissue than
males, which may compete with the retina for uptake of lutein
and zeaxanthin.25 If the relatively higher body fat in females
contributes to the association between sex and MPOD in the
previous publications, the lack of association between BMI
and MPOD may partially explain the weak sex–MPOD
association in this study. It is plausible that the higher dietary
intake of lutein and zeaxanthin in Chinese population19 may
saturate the adipose tissue with these carotenoids and
attenuate the competition with the retina for lutein and
zeaxanthin, thus negating the effects of BMI and body fat on
MPOD.
The level of macular pigments varies among different
populations. MPOD in blacks, for example, has been reported
to be 41% lower than that in whites,43 although another study
did not find such a difference.51 It should be noted, however,
that comparison of MPOD levels among studies are difficult or
even impossible because different studies may employ
different methodologies or study protocols. MPOD measured
by HFP at the same eccentricity point with a 48 reference
point, for example, would be expected to be lower than that
with a 78 reference point because of the relatively higher
residual pigment at 48 reference point, which could lead to
underestimation of MPOD levels.21,26 MPOD levels measured
by HFP at a 0.58 eccentricity point are reported to be 0.18 to
0.22 log units in the U.S. population with a 48 parafoveal
reference point 26,40,41,47 and 0.29 0.30 log units in Europeans
with 5.58 or 68 parafoveal reference point.24,30 Using the same
psychophysical method of HFP and 78 parafoveal reference
point, the MPOD levels measured in the present study were in
agreement with the data obtained from a study in Hong Kong
Chinese.20 However, the MPOD levels reported in the present
study were higher as compared with studies in other
populations that used the similar parameters (Table
4).24,26,30,40,41,43,46,47 The relatively higher measurement of
MPOD levels reported in the Chinese population in the present
study and in Hong Kong Chinese20 is in agreement with the
reports that the Chinese population consumes more lutein and
zeaxanthin19 and may have relatively higher serum concentration of carotenoids.52 The lack of dietary information, however,
is a limitation of this current study.
In summary, the authors reported measurement of MPOD
levels using a method of HFP in a relatively large sample of
healthy Chinese population and observed a significant agerelated decline in MPOD levels at the fovea. Approximately
6.3% of the decline in MPOD in the central fovea could be
IOVS, April 2012, Vol. 53, No. 4
attributed to the age. No significant association of MPOD levels
was found with BMI and smoking status. This age-related
decline in MPOD may contribute to the increased risk of AMD
with aging. The Chinese population has relatively higher
dietary intake of lutein and zeaxanthin19 and higher MPOD,
which may partially explain the relatively lower prevalence of
AMD in the Chinese population.14,15 Future studies to verify
the age-related decline in MPOD and to elucidate the
underlying mechanisms are warranted.
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