Carotenoids, a group of pigments found mainly in green leafy vegetables and colored fruits, are deposited selectively in different tissues. Lutein and zeaxanthin, found in kale and spinach, are deposited in the retina while lycopene, for example, found richly in tomatoes, is concentrated in the prostate.
Scientists have long suspected a link between lutein and zeaxanthin and improved eye function. In 1866, Schulze suggested that the yellow pigments of the macula led to improvements in human vision. These pigments were later found to be derived from dietary lutein and zeaxanthin.
In 1933, Walls and Judd suggested that these yellow intraocular pigments could improve visual performance by absorbing light scattered both within, by minimizing glare, and outside of the eye. This resulted, they suggested, from increasing visual range by absorbing blue light scattered in the atmosphere and by improving spatial vision through enhancing contrast and reducing chromatic blur.
The latest study concluded that: “It seems clear that MP (macular pigment) does influence visual performance through, at least, a few optical mechanisms. The most robust effects appear to be related to its actions as an optical filter.”
Macular pigment is thought to improve glare performance through absorption of forward scattered short-wave (blue) light. And there’s preliminary data to suggest that it increases visual range by absorbing short-wave scattered light in the atmosphere, according to the researchers.
The pigment also appears to enhance contrast by improving the visibility of colored edges through differential absorption across a color border.
Retina and lens
Lutein and zeaxanthin could also improve vision through biological means. There’s much evidence to suggest that the pigments protect the retina and lens (Hammond and Renzi) and prevent age-related eye diseases such as macular degeneration (Carpentier and others 2009) and cataracts.
But the ability of lutein and zeaxanthin to filter light depends on individual differences in the dietary intake of these carotenoids, said the researchers. One study (Curran-Celentano et al 2001) specifically linked low average levels of macular protein (a peak OD of 0.21) with low average intakes of lutein and zeaxanthin of about 1.1 mg/d - or the equivalent of several tablespoons of spinach.
So, argue the researchers, if amounts of macular pigment in the eye vary so significantly, any function these pigments might serve would vary equally significantly.
And it is likely that a healthier retina and lens, particularly in the elderly, is related to improved visual performance.
Source: Journal of Food Science
Title: The Influence of Dietary Lutein and Zeaxanthin on Visual Performance
Authors: J. Stringham, E. Bovier, J. Wong and B. Hammond.