Long-term use of vitamin C reduced the risk of early lens opacities -- loss of transparency in a small area of the lens -- by 77%, and lowered the risk of moderate opacities by 83% in a group of women whose average age was about 62 years old, according to researchers from Tufts University and Harvard University School of Medicine in Boston, Massachusetts.
The researchers note that none of the women who took vitamin C supplements for 10 years or more developed moderate opacities in the core of the lens. At this point cataracts can be said to be causally related to oxidative stress.," Vitamin C may work against oxidative stress by preventing compounds called free radicals from causing molecular damage to lens tissue, particularly its proteins.
Of the dozen studies on the issue, this new study is unique because the reeachers have data on long-term supplementation, and it appears that one has to supplement (on vitamin C) for a long period of time to enjoy the benefit. How much vitamin C per day might be needed to gain its anti-cataract benefit? According to the authors, they found data suggesting that 250 milligrams per day y should saturate the eye tissue with vitamin C.
American Journal of Clinical Nutrition (1997;66:911-916)