Serum and retinal responses to three different doses of macular carotenoids over 12 weeks of supplementation 

This 12 week double-blind, placebo-controlled trial from 2016 was a pilot study to understand the retinal impacts of supplementation with macular carotenoids lutein (L), zeaxanthin (Z), and mesozeaxanthin (MZ.) Twenty-eight healthy subjects, aged 18-25 yo participated and were assigned to one of four daily supplementation groups: placebo (safflower oil; n = 5), 7.44 mg total macular carotenoid (n =7), 13.13 mg total macular carotenoid (n=8), and 27.03 (n=8) mg total macular carotenoid. Authors concluded that even at a relatively low dose (7.44 mg) retinal carotenoids can produce meaningful responses in the serum and retina over 12 weeks and can still process relatively high amounts (27.03 mg) of retinal carotenoids.  

References:

Stringham JM, Stringham NT. Serum and retinal responses to three different doses of macular carotenoids over 12 weeks of supplementation. Exp Eye Res. 2016;151:1-8. doi:10.1016/j.exer.2016.07.005 https://pubmed.ncbi.nlm.nih.gov/27426932/