**6. Conclusions**

Lutein, synthesized in plants but not in mammals, is absorbed and highly accumulated in the macula. The uneven distribution of lutein is thought to afford a distinct function in the retina. Up to now, numerous epidemiological studies have demonstrated that higher levels of lutein in diet and plasma are correlated with lower risk of AMD, especially the late stage of AMD. Randomized and controlled clinical trials such as AREDS2 have reported that supplementation of lutein alone or with other nutrients leads to the increase of MPOD, improvement of visual functions, and decreased risk of progression to advanced AMD, especially the wet AMD. Laboratory experimental data also indicate that lutein can protect impaired retina by filtering blue light, attenuating oxidative stress and inflammation, and enhancing neuroprotection. However, the optimal dose of lutein, the best ratio of lutein and other antioxidants, therapeutic effects at different stages of AMD, adverse effects with even longer intake of lutein supplements in high dose, and the relationship between MPOD and AMD at different phases need to be further investigated in future studies. Although there are several studies assessing the effects of lutein on DR in clinical trials and laboratory experiments, further evaluations to fully understand its protective role in DR are necessary.
