Cataracts
What is it?
There are many disorders that affect the eyes. Some are localized problems while others are a sign of disease elsewhere in the body. Eye diseases are particularly common in elderly people. Cataracts are the most prevalent eye disease in the world and are a major cause of visual loss in developing and developed countries. Other common eye diseases include age-related macular degeneration, glaucoma, and diabetic retinopathy. There are a number of risk factors for these diseases; including age, high blood pressure, prolonged exposure to sunlight, and nutritional deficiencies.
A cataract is a cloudy or opaque area in the lens of the eye. As this cloudy area thickens, it prevents light from passing through the lens and focusing on the retina. Early changes may not affect vision but increasingly blurred vision, sensitivity to light and glare, increased shortsightedness or distorted images may develop. In the late stages, the lens becomes visibly opaque or white. Cataracts can occur in different places in the lens: nuclear cataracts occur in the center of the lens and are most commonly associated with aging. Other types include cortical cataracts and subcapsular cataracts, which are more commonly associated with diabetes. Although cataracts usually aren't painful, they may sometimes cause swelling in the lens and increased pressure in the eye.
Between the ages of 52 and 64, there is a 50 per cent chance of having a cataract. By age 75 most people have had a cataract and 50 per cent of people aged between 75 and 85 have lost some vision as a result. Cataract surgery is the number one therapeutic surgical procedure performed on Americans aged 65 and older.
Nutrition and the eye
Nutrient deficiencies can either directly affect eye function or increase the susceptibility to degenerative problems such as cataracts and macular degeneration.
Vitamin A
Vitamin A deficiency is the leading preventable cause of blindness in the world. Vitamin A plays a role in maintaining the cornea and enhancing night vision through a compound known as visual purple. One of the first symptoms of vitamin A deficiency is night blindness, and prolonged deficiency leads to xerophthalmia, a condition in which eyes become dry, ulcers appear on the cornea, and the eyelids become swollen and sticky. This condition eventually leads to blindness.
B vitamins
Several B vitamin deficiencies can lead to abnormal eye function. Riboflavin deficiency can cause aversion to bright light, dimness of vision and a burning and itching in the eyes. Low intake must occur for several months before these symptoms appear. A low level of riboflavin may also contribute to cataracts and macular degeneration. Low levels of vitamin B12 can also lead to poor vision.
Antioxidants
As oxidative damage is believed to play a role in the development of eye diseases, and in particular cataract and macular degeneration, many research studies have investigated the ability of antioxidant vitamins and trace minerals to prevent the onset or progression of the disorders. Basic research studies have shown that antioxidants can protect against the cumulative effects of oxidative stress in animal models of cataract and macular degeneration. Epidemiological evidence in humans suggests that people with comparatively higher intakes or blood concentrations of antioxidant vitamins are at a reduced risk of cataract and macular degeneration.
Causes of cataracts
Cataracts occur when certain proteins within the lens become damaged. Damage to the enzymes necessary to eliminate the abnormal proteins may also occur. This damage is probably caused by free radicals generated by repeated exposure to ultraviolet light and oxygen. Cataracts may form when the oxidative stress on the lens exceeds the capacity of the antioxidant systems to protect against it, which occurs increasingly with age. Other causes include exposure to X-rays or strong sunlight, inflammatory eye diseases, certain drugs (such as corticosteroids), or complications of other diseases such as diabetes. They're more common in older people but babies can be born with cataracts. Heredity may play a part in the susceptibility to cataracts.
Vitamins, minerals and cataracts
Carotenes
Researchers involved in the Nurses Health Study examined the link between cataract development and intake of various foods and antioxidant vitamins in over 50 000 women. The results of their studies showed that those with high beta carotene and vitamin A intakes were less likely to develop cataracts. Those whose diets contained spinach also seemed to have a lower risk. The researchers concluded that dietary carotenes, although not necessarily beta carotene, can decrease the risk of cataracts severe enough to require extraction.1
In a 1992 study, Finnish researchers compared the differences in beta carotene levels between patients admitted to eye wards for senile cataract and those without eye disorders. The results showed that those with low concentrations of beta carotene were 1.7 times as likely to suffer from cataract.2 As well as protecting against free radical damage, beta carotene may also act as a filter and protect against light-induced damage to the fiber portion of the eye lens.
Other carotenes may also exert protective effects. In a 1997 study, researchers at Arizona State University assessed the relationship between carotenoid pigments in the retina of the eye, including lutein and zeaxanthin, and the density of clouding in the lens. The study involved younger people (ages 24 to 36 years) and older people (aged 48 to 82 years). The results showed that lens density increased with age, and that the increase was related to lower macular pigment carotenes.3
Vitamin C
The vitamin C content of the eye is 20 times greater than that in the blood. Results from some studies, including the Beaver Dam Eye Study, suggest that people with high levels of vitamin C are at less risk of cataracts than those with low levels of vitamin C.4 Vitamin C causes more iron-binding protein, ferritin, to be produced which may reduce the oxidative damage done by iron. Vitamin C may act to protect enzymes within the lens that remove oxidation-damaged proteins.
Results from the Nurses Health Study previously mentioned showed that the risk of cataract was 45 per cent lower among women who used vitamin C supplements for ten or more years.1
Further results from this study reported in 1997 in the American Journal of Clinical Nutrition also suggests that vitamin C supplements taken for long periods can reduce the development of cataracts. Researchers from the US Department of Agriculture and Harvard School of Public Health examined the link between cataract development and vitamin C supplement use over a ten to 12-year period. The subjects were 247 Boston area nurses aged from 56 to 71. The researchers performed detailed eye examinations to determine the degree of opacity (clouding) of the eye lenses of the subjects. Results showed that use of vitamin C supplements for over ten years was associated with a 77 per cent lower prevalence of early lens opacities and an 83 per cent lower prevalence of moderate lens opacities.5
Vitamin E
Recent research suggests that cortical cataracts are more likely when plasma vitamin E concentrations are low. A 1996 Finnish study of over 400 men found an increased risk of cataracts in those with low vitamin E levels. The researchers evaluated the link between vitamin E levels and progression of eye lens opacities in 410 men with high cholesterol. The results showed that those with low vitamin E levels had almost four times the risk of lens opacities when compared with those in the highest intake group.6
Riboflavin
Riboflavin deficiency may be associated with the development of cataracts. Researchers involved in the New York State Lens Opacities Case-Control Study assessed the risk factors for various types of cataract among 1380 participants aged 40 to 79 years. They found an increased risk with low levels of several nutrients including riboflavin.7 Riboflavin is necessary for the activity of an enzyme that exerts protective effects on the eye.