A protein called SR-B1, or scavenger receptor class B, type 1, is responsible for transporting nutrients to the eye, according to findings published in the Journal of Lipid Research.

Earlier work had identified SR-B1's involvement in the absorption of these nutrients in the intestine, so the researchers examined if the same transporter is also involved in the eye.

A large body of science supports the role of lutein and zeaxanthin against the development of age-related macular degeneration (AMD), the leading cause of legal blindness for people over 55 years of age in the Western world, according to AMD Alliance International.

"Our research to understand this mechanism might provide a greater appreciation for how one could intervene to possibly slow macular degeneration," said lead researcher Earl Harrison from Ohio State University.

Despite the fact that approximately 25 to 30 million people worldwide are affected by AMD, awareness of the condition is low, according to AMD Alliance International. And as the generation of Baby Boomers gets older, the Alliance expects incidence to be on the rise and triple by 2025.

AMD is a degenerative retinal disease that causes central vision loss and leaves only peripheral vision. Early detection is cited as a means of prevention so that treatment or rehabilitation can be undertaken early enough. However, links to diet have also been underscored.

The new study supports the potential of lutein and zeaxanthin by providing a valid mechanism, explaining how the compounds, known as xanthophylls, can be transported from the blood into the eyes.

They are commonly found in green, leafy vegetables, such as kale, spinach, broccoli, zucchini and peas, and in yellow or orange fruits and vegetables, such as carrots, papaya, squash and peaches.

Study details

The in vitro study examined the uptake of lutein, zeaxanthin and beta-carotene by human retinal pigment epithelial (RPE) cells.

Harrison and co-workers found that uptake of the xanthophylls was two-fold higher than beta-carotene uptake. Uptakes of beta-carotene, lutein, and zeaxanthin were 1.6, 2.5, and 3.2, respectively, after nine weeks, they said.

When they added compounds to inhibit SR-BI, the uptake of zeaxanthin and beta-carotene were significantly decreased by between 40 and 60 per cent. Inhibition of another transport protein, CD36, had no effect.

"It's fairly safe to say that if you inhibit this transporter, you inhibit the uptake of xanthophylls. So that certainly suggests that this transporter is involved in that process," said Harrison.

The researchers said that additional studies should investigate if these in vitro results can be repeated in vivo.

The researchers are affiliated with USDA-ARS, the Catholic University of Louvain (Belgium), and Ohio State University. The research was supported by grants from the National Institutes of Health.

Source: Journal of Lipid Research
August 2008, Volume 49, Pages 1715-1724
"Xanthophylls are preferentially taken up compared with β-carotene by retinal cells via a SRBI-dependent mechanism"
Authors: A. During, S. Doraiswamy, E.H. Harrison