Many diabetics have low blood levels of zinc, and those with damage to the retina have even lower levels. A study in the European Journal of Clinical Nutrition found that zinc may guard against oxidation damage to the retina.
Subjects took 30 mg's of zinc per day for three months, and their blood levels of zinc went up, and the markers showing oxidation went down.
In the general population, people don't get enough zinc. You should take around 15 mgs per day.