Age-related macular degeneration and cataracts are the leading causes of vision loss in the U.S. Macular degeneration impacts 1.7 million Americans and is characterized by a distortion or obstruction in the center of one’s vision. There is no cure, although anti-angiogenic drugs, laser therapy, experimental surgery and antioxidant supplements can slow progression.
Cataracts are characterized by blurred vision, and an estimated 1.5 million cataract surgeries are performed annually in the U.S.1 However, research is uncertain whether antioxidants can slow the progression of cataracts.
To generate energy, cells remove electrons from sugars (i.e. burn the sugars) and add these electrons to oxygen. This forms highly reactive compound – unstable and electrically charged in such a way as to combine quickly with other elements. When oxygen finally combines with hydrogen, it forms a stable compound - water.
A molecule with an unpaired electron must either acquire an additional electron from some other molecule or get rid of the odd one. As oxygen combines, recombines and electrons are exchanged, other unstable molecules are generated, containing unpaired electrons.
These normal metabolic processes generate oxygen "fragments" or free radicals, including the hydroxyl radical and singlet oxygen. These free radicals are unstable, volatile, and eventually damage DNA proteins, carbohydrates and lipids.
Oxidative damage is a prominent feature of cataracts and with increasing age the lens becomes more susceptible to oxidation. For example, ultraviolet light (i.e., sunlight) generates free radicals in the eye’s lens. And among diabetics, excess sugar in the eye’s lens is converted into sorbitol – an odorless crystalline alcohol. This causes osmotic changes and oxidative stress. When these events occur, the lens’ cells form light-scattering globules. These milky balls produce opaque cataracts.
In general, the oxidation process can be prevented or ameliorated by antioxidants – vitamins A, vitamin C, vitamin E, selenium. The cartenoids, such as beta carotene and lycopene, are known to inactivate electronically charged compounds, such as singlet oxygen.
In 2001, the Age-Related Eye Disease Study (a 7-year interventional study) reported that high levels of antioxidants and zinc significantly reduced the risk of advanced macular degeneration and vision loss – but they had no significant effect on cataracts.2 Similarly, the more recent Antioxidants in Prevention of Cataract Study, an interventional trial conducted in India, found no effect of antioxidants, beta carotene and vitamins E and C, on cataract progression.3
While these interventional studies did not guarantee positive findings, many epidemiological studies have shown a positive correlation between higher dietary antioxidant intake and decreased cataract formation. As recent as October 2011, an epidemiological study reported that among 5,638 adults aged 60 years and older, vitamin C was inversely associated with cataracts.4
Although interventional studies do not suggest that cataracts may be reversed, epidemiological studies do suggest that the oxidation process causing cataracts may be prevented or ameliorated by increased intakes of the antioxidants. Researchers are now suggesting the larger doses of antioxidants be evaluated in future interventional studies.