From Celeste Robb-Nicholson, M.D., editor in chief,Harvard Women’s Health Watch:

“A cataract is a vision-clouding area in the lens of the eye. About half of all people ages 65 to 74, and 70% of those ages 75 and over, develop cataracts, which are the leading cause of blindness worldwide. Fortunately, surgery is safe and effective; in places like the United States, where it’s one of the most common operations performed, it leads to improved vision in about 90% of cases.

“Cataracts aren’t made of calcium but rather of clumps of protein. Normally, the lens of the eye works like a camera lens to focus light on the retina at the back of the eye (see illustration) and to adjust the eye’s focus, which allows us to see things clearly at varying distances. The cells of the lens are composed of water and protein arranged in a way that keeps the lens clear. For reasons that aren’t fully understood, the protein molecules may clump together and start to cloud the lens. This is the beginning of a cataract. The effect has been likened to cooking an egg white.

“You may not notice anything at first, but cataracts typically progress, becoming denser or clouding more of the lens and blurring vision. Eventually, vision may be so severely affected that surgery is needed to remove the lens and replace it with an artificial one. Cataracts usually form in both eyes but may not progress at the same rate or affect vision equally.

“We know how to treat cataracts, but we don’t know much about why they develop. Aging is obviously a factor — possibly because of changes in the chemical composition of the lens or possibly because of normal wear and tear. Most people develop some lens opacity, or clouding, by the age of 60. Other risk factors include injury to the eye, previous eye surgery, diabetes, use of corticosteroid drugs, and having a family member with cataracts. Many studies have implicated smoking and drinking as well. And a study suggests that hormone therapy may increase the risk. Cataracts also seem to be more common in people who’ve had long-term exposure to sunlight.

“We don’t know if avoiding or treating these risk factors will prevent a cataract from forming. But it can only do you good to refrain from smoking, moderate your alcohol consumption, and protect your eyes from sunlight with hats and sunglasses.

“Evidence regarding the role of diet in cataract prevention is mixed. Some experts believe that antioxidant vitamins might help prevent cataracts by getting rid of molecules called free radicals, which may trigger or fuel protein clumping. But despite several studies, there’s no convincing evidence yet that vitamin supplements prevent or slow cataract growth. In a 2008Archives of Ophthalmology study, researchers found that women ages 50 to 79 whose diets were rich in lutein and zeaxanthin had fewer cataracts. These phytochemicals are abundant in dark green leafy vegetables such as spinach, kale, Swiss chard, watercress, and dandelion greens. But these vegetables contain many other healthy substances, so it’s not clear whether lutein and zeaxanthin are responsible for the eye benefits.

“In any case, you should have your eyes examined every two years (annually after age 60). And even if you can’t do much to prevent or slow the growth of cataracts, you can reduce their impact on your life in various ways, such as adjusting your eyeglasses, getting anti-glare sunglasses or magnifying lenses, or just using brighter lights at home and work.”