In this study, we noted a rapid and significant increase in
zeaxanthin and lutein levels in the serum as well as a significant
increase in zeaxanthin density in maculae of rhesus monkeys after
dietary supplementation with a fraction of a lipid extract of
F.
lycii. Zeaxanthin contents in spleen and liver also
ap-peared to increase, although the increase was not
statistically significant. Our observation confirmed other
studies
16 26 that suggested that serum zeaxanthin could be
elevated by dietary supplementation. Our findings also suggest that
there was a selective uptake of zeaxanthin in the macular tissue in
these monkeys. Contrary to the studies in humans
27 and
M. fascicularis,
28 which indicate a higher
level of zeaxanthin than lutein in the central retina, we found that
there was more lutein than zeaxanthin in the maculae in rhesus monkeys
(
M. mulatta), suggesting interspecies differences in macular
zeaxanthin and lutein contents.
Basal serum zeaxanthin and lutein levels in our rhesus monkeys were 10
and 3 times lower than those reported in
M. fascicularis and
S. sciureus, respectively.
17 It was unlikely
that the low serum zeaxanthin and lutein levels in our rhesus monkeys
were due to a difference in dietary carotenoid contents because
analysis of monkey diet (including monkey chows and fruit supplement)
showed a daily intake of 2.1 mg of total zeaxanthin and lutein in our
vivarium, which was comparable to the monkey diet in the study by
Snodderly et al.
26 Slifka et al. reported the combined
zeaxanthin and lutein levels in the sera of different primates varied
from nondetectable level in the golden lion tamarin to an average of
1017 ng/ml in the sooty mangabey with most of the animals having a
level between 28 and 170 ng/ml.
18 The combined levels of
zeaxanthin and lutein in our animals ranged from 26 to 47 ng/ml (mean,
35) and remained in the lower end of the spectrum reported by Slifka et
al. Therefore, different primates may have varying ability to absorb
and metabolize various carotenoids, resulting in differences in serum
levels of these carotenoids.
Contrary to the study by Snodderly et al.
26 showing no
change in the concentration of serum lutein after supplementation of
zeaxanthin to squirrel monkeys, we noted a significant increase in
lutein levels in the serum after P1 supplementation. Because
F.
lycii has a very low content of lutein (less than 1%), it is an
unlikely source of serum lutein. It is possible that zeaxanthin can be
converted to lutein, but there is no evidence in the literature to
suggest that zeaxanthin is converted into lutein in serum or retina,
whereas the reverse has been proposed by Bone et al.
29 and
Khachik et al.
30 However, lutein and other carotenoids are
present in the monkey chows and daily fruits given to the animals. It
is possible that zeaxanthin or other components in the P1 extract may
enhance the absorption of lutein. It is also possible that there is a
metabolite of zeaxanthin coeluting with lutein that cannot be separated
by the present methodology.
Lutein, cryptoxanthin, and β-carotene are the most common carotenoids
found in fruits or vegetables, while zeaxanthin is present only in
minute quantities in most fruits and vegetables.
30 31 32 Therefore, dietary zeaxanthin intake is very low. In addition, feeding
corn and spinach, which are relatively rich in zeaxanthin, does not
alter serum zeaxanthin levels.
11 Recently, egg yolk was
suggested to be a good dietary source of zeaxanthin.
32 Plasma lutein and zeaxanthin levels were elevated by 39% and 128%,
respectively, after a dietary supplement of 1.3 egg yolks daily for 4.5
weeks.
33 However, the constant consumption of egg yolk
would increase the risk of cardiovascular diseases by increasing
low-density lipoprotein cholesterol. In our study, serum zeaxanthin
concentration increased by a factor of 2.5 with feeding of an extract
from 7.3 g
F. lycii (equivalent to 0.55 mg zeaxanthin
per kilogram body weight per day) for 6 weeks in two monkeys. The
dosage is relatively low compared with the study by Snodderly et
al.
26 in which 2.2 mg zeaxanthin was fed each day to
squirrel monkeys with body weights from 0.7 to 1.1 kg.
26 Therefore,
F. lycii is a good dietary source of zeaxanthin.
In a previous study, zeaxanthin was extracted from the dried fruits of
L. chinense (closely related to
L. barbarum) and
orally taken by three human subjects as dietary supplements. A rapid
elevation of serum zeaxanthin was noted.
16 In spite of the
low basal levels of zeaxanthin and lutein in our rhesus monkeys, we
also noted a rapid increase in zeaxanthin levels in the sera of two
monkeys, but not in monkey 4 which also had a low level of zeaxanthin
in liver and spleen. We deemed monkey 4 a nonresponder that showed
poor absorption of zeaxanthin. Nonresponders such as monkey 4 have
previously been reported.
11 34 We speculate that the
absence of sufficient carotenoid-carrying proteins may explain the low
levels of serum carotenoids irrespective of dietary intake. However,
the rapid increase in serum zeaxanthin levels in the responders, even
in those with low basal zeaxanthin levels, suggests that the serum
level of zeaxanthin may be easily modulated by supplementation and does
not necessarily depend on the basal level. However, the level of
macular zeaxanthin, but not lutein, of the nonresponding monkey, was
high after supplementation. Therefore, the uptake of zeaxanthin into
the retina may be highly specific.
Landrum et al.
10 and Bone et al.
35 reported a
gradual increase in macular pigment (MP) with a noninvasive
psychophysical method after dietary supplementation of zeaxanthin or
lutein in human subjects. It was assumed that the increase in MP
corresponded to the increase in serum zeaxanthin or lutein. Our HPLC
measurement showed that zeaxanthin levels in the maculae were elevated
by feeding a dietary supplement rich in zeaxanthin. This observation
supports the conclusion of Bone et al. The average zeaxanthin content
in the maculae of the P1-treated group is two times higher than those
of the normal and the vehicle-treated groups. Therefore, the uptake of
zeaxanthin from serum into the macula was highly effective.
The location of zeaxanthin and lutein in the retina is very unique. In
humans, zeaxanthin is highest in the center of the fovea, whereas
lutein is relatively abundant in the perifoveal region. Although the
level of lutein is higher than zeaxanthin in the central maculae in our
rhesus monkeys, the elevated level of zeaxanthin after P1
supplementation showed that the preferential absorption of zeaxanthin
also occurs in the central macula, independent of the relative
distribution of zeaxanthin and lutein. The absence of any carotenoids
in peripheral or equatorial regions in our study further suggests
specific uptake mechanisms at the center of the retina.
F. lycii, the red berry of
L. barbarum is an
important ingredient in Chinese herbal medicine for cataract, glaucoma,
and retinitis pigmentosa.
13 Whether
F. lycii is
beneficial in the treatment of these ocular diseases remains to be
studied. However, the abundant zeaxanthin in this berry and the ready
absorption of its zeaxanthin into serum and the macula of primates may
be beneficial in protecting the retina against free radicals and blue
light damages.
In summary, we noted significant levels of zeaxanthin and lutein in the
maculae of rhesus monkeys with a 1:2 ratio (zeaxanthin to lutein). We
also recorded elevated levels of zeaxanthin and lutein in the sera and
the maculae of rhesus monkeys after feeding an extract of F.
lycii. Therefore, F. lycii is a good source of
zeaxanthin to increase circulating and macular zeaxanthin.
The authors thank Josephine Ngai for technical assistance in
preparing the P1 extracts and HPLC analysis, Zhang Chun for surgical
removal of monkey tissues, Ngai Nga Man Laura for the mathematical
conversion of macular pigment in the 3-mm punches to that in 6-mm
punches, and Albert Cheung of the Center for Clinical Trials and
Epidemiologic Research, the Chinese University of Hong Kong, for
statistical analysis.