Importantly, the supplements also appeared to normalise distribution of retinal pigment epithelium (RPE) cells, disrupted by a lack of these nutrients in the diet.
The study, sponsored by DSM Nutritional Products, underlines the role that such supplements can have on eye health. RPE cells are responsible for the proper functioning of photoreceptors, key to vision.
Researchers led by Dr Max Snodderly from the Schepens Eye Research Institute and the department of Ophthalmology at Harvard Medical School, tested the supplements on rhesus monkeys that had been fed a carotenoid-free diet from birth until seven to 16 years.
Zeaxanthin and lutein normally occur in high concentration in the macula lutea, the center of the retina, where visual acuity is sharpest. But the monkeys used in the trial had no detectable carotenoids either in plasma or the retina, where the normal macular yellow pigment was totally absent.
This life-long absence of carotenoids in the retina also led to an abnormal spatial distribution of RPE cells. These cells are responsible for the proper functioning of photoreceptor cells and when not working properly, photoreceptor function is severely impaired, finally leading to the death of photoreceptors and consequently to loss of vision.
Writing in the September issue of Investigative Ophthalmology and Visual Science (45:3234-3243), Snodderly's team show that lutein and zeaxanthin play a crucial role in maintaining the distribution of RPE cells.
One group of monkeys was fed a pure lutein supplement, another zeazanthin only and a third group had no dietary changes. During supplementation, serum levels of the carotenoids increased rapidly over the first four weeks and from 16 weeks onward maintained similar levels, both several times higher than in the monkeys whose diets had not changed.
Macular pigment optical density increased to a relatively steady level by 24 to 32 weeks in both lutein and zeazanthin groups.
The authors write that "both macular pigments [lutein and zeaxanthin] ... appear to be important for the development and/or maintenance of a normal density profile of RPE cells in the central retina".
Lutein and zeaxanthin are the primary components of macular pigment and have been associated with a reduced risk of age-related macular degeneration (AMD), the leading cause of vision loss for people over the age of 50 in the Western world, affecting approximately 25-30 million people.