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Researchers explore relationship between macular pigment and visuomotor function
Lutein and zeaxanthin may affect more aspects of vision than originally thought.
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Human macular pigment is composed of two dietary carotenoids, lutein and zeaxanthin, whose role it is to screen light and reduce photo-oxidation in the retina. Lack of macular pigment optical density has been linked to enhanced risk of age-related eye disease such as macular degeneration. Research is ongoing to discover the impact of lutein and zeaxanthin dietary supplements on prevention of retinal and lenticular diseases.
Carotenoids, including lutein and zeaxanthin, are also found in the frontal and occipital cortex of the human brain and have been implicated in the prevention of degenerative diseases such as Alzheimer’s. In fact, lutein and zeaxanthin levels in the retina are significantly correlated with levels in the cerebellum, with a trend for such a relationship in the occipital cortex and a significant relationship for zeaxanthin in the retina and the frontal cortex and pons. Because macular pigment optical density (MPOD) can be measured noninvasively, in situ it can be used as a biomarker of these carotenoids in brain tissue. As the cerebellum is the area of the brain that controls motor function, my colleagues and I had an interest in the connection between macular pigment and visuomotor behavior.
Three trials
We conducted randomized, double-masked, placebo-controlled trials of three groups of individuals: 50 young healthy adults between the ages of 18 and 30 years; 96 physically healthy adults with a mean age of 63.8 ± 7.3 years; and 52 elderly subjects with a mean age of 74.1 ± 5.6 years, of whom 23 met criteria for very mild or mild cognitive impairment.
In all three groups, MPOD was measured using customized heterochromatic flicker photometry. The 50 young healthy adults participated in a task that measured coincidence anticipation timing. Participants watched a light that moved at distinct speeds in a linear fashion and had to anticipate when it would arrive at a certain point. The results showed that MPOD was significantly related to the number of coincidence-anticipation errors when the light moved at a speed of 10 miles per hour, 15 miles per hour and 20 miles per hour (P < .01), but not at 5 miles per hour.
The second trial required the 96 healthy adults to take the Short Physical Performance Battery. This examines static balance, gait speed and getting in and out of a chair, and classifies an individual’s functional status and disability. In this group, MPOD was significantly related to improved balance ability (P < .05). Forty-five subjects in this group also completed the coincidence-anticipation time test, which was also significantly related to MPOD (P < .05).
The third trial measured cognitive function of the 52 elderly participants with the Repeatable Battery for the Assessment of Neuropsychological Status. In this trial, MPOD was related to visuospatial cognition in all participants (P = .03), with the relation between MPOD and visuospatial cognition stronger in impaired participants (P = .02). MPOD was only related to visual attention in impaired participants (P = .01).
In sum, our initial cross-sectional data show that MPOD is related to visuomotor ability in groups that vary significantly. The young adults in the first group were college students, a generally homogenous group that tends to be healthier than average. The other two study groups were taken from the community and so were representative of the general population of elderly. We had a fairly equal spread of men and women with a percentage of minorities that probably reflects the American statistical norm, yet all showed the correlation between MPOD and visuomotor ability.
Role of lutein, zeaxanthin
Lutein and zeaxanthin are obtained solely through dietary intake. Dietary intake of lutein and zeaxanthin for most Americans, however, is extremely low. Systemic stressors (eg, high body fat) can also lower the amounts available to nervous tissue. Taken together it is likely that most individuals, even in developed countries such as the United States, are significantly deficient. Given the diversity of health effects attributed to these carotenoids, this deficiency may have serious implications for long-term health and optimal functioning.
We tend to think of lutein and zeaxanthin as existing just within the eye itself, so the fact that they are in the brain as well opens up the possibility that they influence various aspects of visual performance. These carotenoids appear to be involved not just with the initial coding of light, but rather all aspects of vision, including motor functions and cognitive functions that have visual components. Vision is in the brain, not in the eye, per se, so lutein and zeaxanthin probably affect vision in a much more comprehensive sense than was previously thought.