Background: The so-called macular carotenoids (MC) lutein (L), zeaxanthin (Z), and meso-zeaxanthin (MZ) comprise the diet-derived macular pigment (MP). The purpose of this study was to determine effects of MC supplementation on the optical density of MP (MPOD), repeated-exposure photostress recovery (PSR), and disability glare (DG) thresholds.
Methods: This was a double-blind, placebo-controlled trial. Fifty-nine young (mean age = 21.7), healthy volunteers participated in this study. Subjects supplemented their daily diet with either 10 mg L + 2 mg total Z (1 mg Z + 1 mg MZ; n = 24), 20 mg L + 4 mg total Z (2 mg Z + 2 mg MZ; n = 25), or placebo (n = 10) for 12 months. The primary outcome was a composite measure of visual performance in glare, defined by change in DG and PSR. Secondary outcomes included MPOD and visual fatigue. The primary endpoint for outcomes was 12 months. MPOD was assessed with customized heterochromatic flicker photometry. PSR times for an 8 cycle /degree, 15 % contrast Gabor patch target were determined after each of five successive exposures to intense LED lights. DG threshold was defined as the intensity of a ring of lights through which subjects were able to maintain visibility of the aforementioned target. Measures of all parameters were conducted at baseline, 6 months, and 12 months. Repeated-measures ANOVA, and Pearson product-moment correlations were used to determine statistically significant correlations, and changes within and between groups.
Results: MPOD for subjects in both supplementation groups increased significantly versus placebo at both 6- and 12-month visits (p < 0.001 for all). Additionally, PSR times and DG thresholds improved significantly from baseline compared to placebo at 6- and 12-month visits (p < 0.001 for all). At baseline, MPOD was significantly related to both DG thresholds (r = 0.444; p = 0.0021) and PSR times (r = -0.56; p < 0.001). As a function of MPOD, the repeated-exposure PSR curves became more asymptotic, as opposed to linear. The change in subjects' DG thresholds were significantly related to changes in PSR times across the study period (r = -0.534; p < 0.001).
Conclusions: Increases in MPOD lead to significant improvements in PSR times and DG thresholds. The asymptotic shape of the repeated-exposure PSR curves suggests that increases in MPOD produce more consistent steady-state visual performance in bright light conditions. The mechanism for this effect may involve both the optical filtering and biochemical (antioxidant) properties of MP.
Trial registration: ISRCTN trial registration number: ISRCTN54990825. Data reported in this manuscript represent secondary outcome measures from the registered trial.
Keywords: Disability glare; Lutein; Macular pigment; Mesozeaxanthin; Photostress recovery; Visual cycle; Visual performance; Zeaxanthin.