The xanthophyll carotenoids lutein and zeaxanthin are concentrated in the macula lutea of the retina. It is thought that the two plant pigments may play a role in preventing the development of age-related macular degeneration (AMD). Exposure to damaging light plays a role in retinal deterioration and AMD. The antioxidant capabilities of lutein and zeaxanthin, combined with their ability to filter short-wavelength blue light, may serve to protect the retina from photooxidative damage and photoreceptor cell death. In the first study, chicks’ were fed diets supplemented with lutein and zeaxanthin and exposed to damaging light; functionality of the retinas was assessed with electroretinography. The second study attempted to create lutein and zeaxanthin-deficient chicks to investigate carotenoid deficiency and the impact on retinal functionality.
In the first study, 12 newly hatched White Leghorn chicks were fed a control diet or a lutein and zeaxanthin supplemented diet for 32 days. The supplemented diet was prepared by supplementing the control diet with 30g/g of lutein and 15g/g of zeaxanthin. All chicks were raised in cyclic light. At 32 days of age, 6 were exposed to 10 hours of intermittent light. Baseline and post-treatment electroretinography (ERG) data were collected on all groups of chicks, 32 days and 36 days, respectively. Biochemical analysis of lutein and zeaxanthin in plasma, retina, and other tissues was assessed by HPLC at Day 40.
In the second study, lutein and zeaxanthin-deficient chicks were to be produced from White Leghorn laying hens fed a deficient diet. Hens were placed on a deficient diet and after 28-30 days mated with a rooster. The subsequent fertilized eggs were collected. The eggs were stored in an incubator for hatching. Biochemical analysis was used to confirm that egg yolks were deficient in lutein and zeaxanthin. Hatched chicks were to be raised on deficient diets for 32 days and exposed to the same methods employed in the first study.
In the first study, plasma and tissue levels of chicks significantly increased their contents of lutein with the supplemented diet; the same result was found for zeaxanthin except for the plasma and skin. Interestingly, total zeaxanthin levels were greater in the retina than the total lutein levels in both groups of chicks (6.40 g/g versus 3.62 g/g). In the retina, zeaxanthin also was found to exist in the cis form. There were significantly greater levels of cis in supplemented diet groups independent of light exposure. For both the supplemented and control diet chicks, there were significant differences within baseline and post-treatment ERG responses in non-light exposed and light exposed chicks (F 9.1, p 0.007,=0.05). No significant difference was found for the maximum amplitude or sensitivity (Kd) ERG responses for lutein supplemented or control chicks between treatment groups.
In the second study, the lutein and zeaxanthin content of yolks from eggs laid by laying hens fed a carotenoid-deficient diet declined from Day 2 to Day 36 (11.19 g/g and 18.09 g/g to 0.19 g/g and 0.23 g/g, respectively). However, the laying hens suffered dramatic weight loss, decreased egg laying and hatchability, and illness during the course of the experiment. Even with new housing facilities, a new rooster, and increased animal care, the hens did not produce carotenoid-deficient progeny and thus the proposed light exposure experiments could not be accomplished.
In the first study, plasma levels and tissues of chicks fed supplemented diets significantly increased in lutein and zeaxanthin concentrations; the retina actually had higher levels of zeaxanthin even though the diet had greater lutein levels. This suggests the retina has a preference for zeaxanthin. The cis isomer of zeaxanthin, which is isomerized from the trans form by bright light, was only present in the retina. However, significantly greater cis was present in the retinas of supplemented diet chicks a result of higher zeaxanthin levels in the diet and not light exposure. In addition, there were no significant differences between control and supplemented diet chicks as a result of light damage in retinal photoreceptor sensitivity and maximum amplitude response. These results indicate that damage to the retinas of both supplemented and control diet chicks did not occur as a result of light damage treatment.
In the second study, assessment of retinal function in carotenoid-deficient chicks could not be accomplished due to lack of production of deficient progeny. Lutein and zeaxanthin concentrations in egg yolks dramatically decreased with subsequent deterioration of the maternal hens’ healths. The conclusions drawn from our efforts were that producing carotenoid-deficient chicks from a maternal hen is difficult and perhaps physiologically impossible. Modifying the experimental design for the deficient hen and investigating other methods to produce carotenoid-deficient progeny are necessary for further studies.
In conclusion, these two studies were the first attempts at investigating retinal function, dietary intervention, and light damage within the chick model. With greater power and different experimental approaches more promising results that can be generalized for the human eye disease, AMD, may be uncovered.