Chemistry Reference
In-Depth Information
Photophobia is not limited to changing levels of brightness but can even be chronic as in migraine
headaches, for example. A variety of clinical conditions such as RP and AMD can also cause
photophobia. Stringham et al. (2003) investigated its dependency on wavelength and found that
photophobia was predominantly induced by light of shorter wavelengths (blue light). Wenzel et al.
(2006) have measured MPOD and its relationship to photophobia light threshold and reported that
the measured thresholds are inversely correlated with MPOD. In addition to photophobia, bright
light can induce the sensation of glare. Sensitivity to glare is often exacerbated by increasing age
and by diseases of the lens that result in increased light scattering within the eye. Glare sensitiv-
ity may be assessed by measuring contrast sensitivity in the presence of a nearby glare source, for
example, a pair of halogen lamps that simulate the headlights of an oncoming car. In 36 healthy
non-supplemented subjects, MPOD was measured by HFP and sensitivity to glare was measured by
assessing their photostress recovery time, the time span until vision returns after the subjects had
been “blinded” by a bright glare light. It was found that photostress recovery time was signii cantly
shorter for subjects with higher MPOD levels (Stringham and Hammond 2007).
These correlational data were later extended by supplementing 40 healthy subjects with a mixture
of 10 mg lutein and 2 mg zeaxanthin for 6 months and again measuring photostress recovery time.
Supplementation increased MPOD levels on average by 35% and along with this MPOD increase
photostress recovery time was signii cantly (
p
= 0.01) reduced (Stringham and Hammond 2008).
Although the study was not placebo-controlled or randomized, together with the results of the cor-
relational study mentioned above, its data strongly support an inverse relationship of MPOD and
photostress recovery time. It is possible that increasing the level of MP would diminish the amount
of scattered blue light reaching the photoreceptors, and this might also result in lowered sensitivity
to glare (Hammond et al. 2001). However, light scatter within the eye has been demonstrated to be
independent of wavelength (Whittaker et al. 1993). Thus, the scattered longer wavelengths would
not be removed. This may be the reason why supplementation with lutein, zeaxanthin, or a combina-
tion of both carotenoids was consistently shown to reduce intraocular light scatter in healthy eyes,
but not at a level of statistical signii cance (Kvansakul et al. 2006).
13.8.4 P
OSSIBLE
A
CTIONS
OF
L
UTEIN
AND
Z
EAXANTHIN
B
EYOND
THE
R
ETINA
The retina had been named an “approachable part of the brain” (Dowling 1987) and indeed emerg-
ing data suggest that lutein and zeaxanthin supplementation can have effects on the brain and on
cognitive performance. Generally, this appears plausible because of the natural occurrence of lutein
and zeaxanthin throughout the nervous system, particularly in locations relevant for cognitive and
visual processing (Craft et al. 2004). In this context, Johnson et al. (2008b) have recently supple-
mented 11 elderly subjects with 12 mg lutein/day for 4 months and reported statistically signii cant
improvements in verbal l uency and memory scores along with marked increases of MPOD. In
an epidemiological study, Renzi et al. have investigated the relationship of MPOD and cognitive
function in 118 older adults. MPOD turned out to be the strongest and most consistent correlate of
cognitive function across all tested indices, although in absolute terms the xanthophylls accounted
for only small but signii cant proportions of variance (Renzi et al. 2008b).
More specii c to the events after electrical signals have been generated in the retina are observa-
tions that critical l icker fusion thresholds, a classical measure of central processing speed relevant
to the dynamic functioning of the visual system, are directly proportional (
p
< 0.001) to MPOD, as
i rst reported by Hammond and Wooten (2005) and coni rmed by Renzi et al. (2008a) in a larger
population.
13.8.5 X
ANTHOPHYLLS
AND
THE
D
EVELOPING
E
YE
Two recent articles (Hammond and Frick 2007, Zimmer and Hammond 2007) review and discuss
the potential importance that lutein and zeaxanthin have for the developing retina. Indeed, the
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