With the long-term bright light exposure and oxidative damage, the presence of oxidized metabolites in the photoreceptor outer segment of retinal pigment epithelium (RPE) might lead to the formation of drusen and pigment disturbances in the macula, which eventually results in AMD development and vision loss.
29,30 Thus, antioxidants are hypothesized to be beneficial for improving visual function and delaying the progression of AMD.
31 As the majority ingredients of MP, L, and Z are uniquely concentrated at the macula, which might have a possible specific function in the maintenance human eye health.
32 In concordance with previous studies, which reported the protective effects of these carotenoids on visual performance of AMD patients, our result revealed a significant improvement of VA with the supplementation for AMD patients. Meanwhile, in late AMD, the dysfunction or loss of macular photoreceptors could not undergo processes of biological renewal.
4,9 Our finding, that a greater improvement in VA was found for early AMD patients receiving xanthophyll carotenoids supplements, provided strong support for this hypothesis. Besides, VA is only related to an ability to resolve details of maximum contrast. Therefore, other tests of visual function are important to detect subtle alteration of the macula; and measurements of CS can provide the fundamental description of spatial vision and prediction of poor quality vision, particularly for patients with macular disease.
33,34 In our study, patients supplemented with these xanthophyll carotenoids had significantly increased CS at all spatial frequencies as comparing with those who received placebo. Furthermore, our dose-response analysis indicated that participants treated with higher doses supplement had more improvements in VA and CS at low and middle frequency when comparing with those who received lower doses. L and Z could offer protection to reduce the lipofuscin accumulation and enhance lysosomal stability and viability, which preserves macular health and leads to improve functional abnormalities of the central retina in animal studies.
35,36 Bian et al.
37 indicated that incubation of the N-retinylidene-N-retinylethanolamine (A2E)-containing RPE with L or Z could significantly attenuate the photooxidation-induced inactivation of the proteasome. Results from animal experiments showed that 6-month supplementation with xanthophyll carotenoids in quails could markedly decrease the level of photoreceptor apoptosis in response to light injury.
38 The potential mechanism of action of these carotenoids was also thought to involve in the reduction of chromatic aberration by absorbing the blue light.
39 As refraction of different wavelengths are focused on different extents, multiple overlapping images arise, which potentially degrades a retinal image.
40 Absorption of poor-focused short wavelengths by the MP would not only decrease the effects of chromatic aberration, but also improve visual resolution.
41 Because CS could be served as a more accurate and reliable indicator to assess visual capacity comprehensively, the positive effects of xanthophyll carotenoids supplementation on CS improvement might lead to a clinically important benefit for the preservation of visual function for AMD patients. Although xanthophyll carotenoids had been generally recognized as safe (GRAS), their intake was far below recommended level, which was associated with the prevention of age-related eye diseases and tended to decrease in Western countries.
42 Therefore, increasing intake of xanthophyll carotenoids from food sources or supplement should be guaranteed, especially for the AMD patients.