Nutrition and Ophthalmology: Cataracts

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Nutrition and Ophthalmology: Cataracts

Eighteen percent of Americans aged 52 through 64 and 45% of those between 65 and 74 have cataracts.( 1) The Framingham Eye Study found that nine out of ten people older than 75 have cataracts.( 2) The major treatment offered by allopathic medicine is surgery. Cataract surgery and related expenses consume the largest single piece of the Medicare budget (estimated in 1990 to far exceed three billion dollars per year.)( 3) Virtually all responsible physicians agree that prevention would be a better alternative.

Cataracts and oxidative damage

Cataracts have been linked to smoking, diabetes, use of steroids, and excess exposure to sunlight and radiation. The leading risk factor is simply aging. Most of these factors create or are associated with oxidative damage. Oxidative damage to proteins in the lens in turn has been linked to cataract formation in both animals( 4) and humans.( 5)

When antioxidants (like vitamins C and E) are fed to animals, their risk of forming cataracts diminishes.( 6, 7) People with low serum levels of antioxidants or those who eat limited amounts of antioxidant-rich fruits and vegetables are at high risk for developing cataracts.( 8, 9)

Vitamin C

The major antioxidants in the lens are vitamin C( 10) and glutathione (GSH).( 11) When cataracts form, lens concentrations of GSH decrease. Vitamin C is needed to activate vitamin E( 12) which, in turn is needed to activate GSH. Levels of vitamin C in the lens are manyfold higher than other tissues or in serum. It is reasonable to speculate that this high concentration exists to protect the lens from oxidative damage.

Ocular levels of vitamin C decrease with age in animals and probably in people as well.( 13) Supplementing vitamin C increases ocular vitamin C levels.( 14) High levels of vitamin C are linked to a low cataract risk.( 15, 16) Vitamin C supplementation has reduced cataract formation in animals.( 17)

In a 1989 study, Canadian researchers reported that healthy controls were more likely to be taking vitamin C (and E) supplements than were cataract patients.( 18) In 1991, these researchers studied 40 case control pairs. Those who supplemented 300 mg of vitamin C per day or more had a 70% decreased risk of forming cataracts compared with those not supplementing C.( 19) In 1992, the Nurses Health Study reported that women who used vitamin C supplements for at least ten years had a 45% decreased cataract risk.( 20) Both studies found these associations with supplements as opposed to just food sources. Even the one study not linking vitamin C supplements to prevention, found people who took multivitamins (containing vitamin C) had a reduced risk.( 21) Though negative data exist,( 22) the protective effect of multivitamins against cataract formation has been independently confirmed.( 23)

Vitamin E

Vitamin E protects against the formation of animal cataracts.( 24) This may be so because vitamin E has a role in maintaining levels of GSH.( 25) The Canadian study linking vitamin C supplementation to prevention also found a protective link with vitamin E supplements (400 IU per day).( 26) Of 76 case control pairs, those supplementing vitamin E had a 56% decreased risk of cataract formation. The Canadian trial reported a dose response relationship, thus strengthening the idea that vitamin E was responsible for protection and not merely a marker.( 27) Other workers have also found that high vitamin E intake correlates with low cataract risk.( 28)

Beta-Carotene

The Nurses Health Study reported that people in the top quintile of dietary á-carotene intake had a 27% lower risk of forming cataracts than those in the bottom quintile.( 29) While á-carotene supplements were not studied in this report, people supplementing preformed vitamin A for over ten years had a 77% decreased risk of cataract formation. Theoretically, if antioxidant activity is key, then á-carotene should be more effective as it is a more potent antioxidant than retinol. On the other hand, lens concentrations of á-carotene have been reported to be undetectable (though low levels of retinol and retinyl palmitate do exist in the lens).( 30) These data suggest that dietary á-carotene may only be a marker for an as yet undiscovered variable.

Riboflavin and Niacin

GSH reductase protects glutathione. Decreased GSH reductase has been reported in human lenses containing cataracts.( 31) Riboflavin is needed to activate this enzyme; thus, indirectly, riboflavin leads to antioxidant activity.

In India, riboflavin deficiency has been reported in most cataract patients, but only in a minority of people without cataracts.( 32) Years ago, a link between riboflavin deficiency and cataracts was also reported at the University of Alabama.( 33) In China, people over 65 given 3 mg of riboflavin daily with 40 mg of niacin showed a significant decrease in the risk of forming nuclear cataracts compared with the placebo group.( 34) Niacin (as NADH) is also needed to maintain GSH levels.

An increased risk of posterior subscapular cataracts was reported in the B vitamin-supplemented group in China, but this was probably due to chance. Oddly, no one in the placebo group formed this uncommon cataract. Therefore, even the very low incidence in the B vitamin group was statistically higher than zero.

Note the very low dose of riboflavin used in the Chinese trial. The Textbook of Natural Medicine has suggested that "...more than 10 mg per day of riboflavin should not be prescribed for cataract patients...." because it might create free radicals in the lens, a idea supported by experimental research.( 35) Nutrition Reviews agreed that supplementing excess riboflavin "would confer no added benefit and could be considered potentially harmful."( 36) Until more is known, supplemental riboflavin appears important, but the daily dose should be low.

Selenium

Selenium activates GSH peroxidase, which helps GSH act as an antioxidant in the lens. While lens levels of selenium appear normal, the amount of selenium in other portions of the eye (aqueous humor) have been reported to be low in cataract patients.( 37) This has caused some observers to speculate that cataract formation might be linked with a lack of selenium.( 38) However, preliminary animal research has yet to show that selenium helps protect against cataracts.( 39) The suggestion that selenium actually exacerbates cataracts in some rats( 40) has not been reported in humans. Nevertheless, there is a lack of evidence supporting selenium in the prevention of cataracts.

Other Antioxidant Possibilities

In the Nurses Health Study, eating spinach correlated better with cataract prevention than consuming any other food surveyed. Spinach is very high in the carotenoids lutein and zeaxanthin. Increasing evidence has linked these antioxidants to prevention of macular degeneration, the leading cause of irreversible blindness in the elderly.( 41, 42) The Nurses Health Study finding suggests the possibility that future research may reveal lutein and zeaxanthin are also protective against cataracts. While lutein and zeaxanthin exist in the human lens, to date high concentrations have not correlated with protection,( 43) leaving the relationship between these carotenoids and cataract formation in doubt.

The antioxidant enzymes catalase and superoxide dismutase (SOD) exist in the lens of the eye.( 44) Both are zinc (Zn) dependent. SOD also requires copper (Cu) and manganese (Mn). Whether supplementing Zn, Cu, and or Mn would result in cataract prevention has yet to be adequately explored. In fact higher levels of Zn and Cu have been found in lenses which contain cataracts,( 45) though the meaning of this remains unknown.

Vaccinium myrtillus (Bilberry)

Researchers studying ocular health first became interested in Vaccinium myrtillus (bilberry) when extracts from the plant improved both night vision and visual adaptation to glare in healthy people.( 46, 47) Bilberry extract has also improved vision in other clinical trials.( 48) In a preliminary study, bilberry in combination with vitamin E halted the progression of senile cataracts in a group of 50 patients.( 49) Bilberry contains anthocyanosides -- potent antioxidants which might be responsible for this outcome.

Diabetes increases the risk of cataract formation. Diabetic cataracts can result from sorbitol accumulation in the lens. Bioflavonoids are known to reduce the sorbitol concentrations in experimental studies.( 50) There is reason, therefore, to expect that bioflavonoids such as those found in bilberry might inhibit diabetic cataract formation.( 51) Bilberry extract is used to treat diabetic retinopathies( 52) which are also linked to sorbitol accumulation.

Other Factors

Sunglasses protect against UV light, which is known to damage the lens. Maintaining ideal body weight may also have some importance; by sharply increasing the incidence of diabetes,( 53, 54) obesity may indirectly increase the risk of forming cataracts.( 55)

A Final Word About Supplements

GSH is an important antioxidant in the lens. Some data support the theory that supplementation of N-acetyl-cysteine (NAC) will increase GSH levels; therefore it has been suggested that NAC supplementation should be used to reduce cataract risk.( 56) No clinical data are available which address this hypothesis.

To date, only one modern intervention trial has been published using antioxidants to prevent cataracts.( 57) In this report, low doses of many supplements were given in different combinations to the nutritionally deficient Chinese population mentioned above. It is questionable whether the results of this trial tell us much about the effects of larger doses of antioxidants when taken by Americans. Nonetheless, the major findings of the study were:

- multivitamins significantly reduced the risk of nuclear cataracts in the elderly (RR = 0.57, 95% confidence interval of 0.36-0.90)

- riboflavin and niacin supplementation reduced the risk of nuclear cataracts (RR = 0.59, 95% confidence interval of 0.45-0.79)

- Protective effects were mostly seen in the elderly. Other types of cataracts were not reduced in this trial.

Epidemiologic and case control studies show that supplements usually (though not invariably) associate with a reduction in cataract risk.( 58) Moreover, a clear mechanism exists explaining how antioxidants should protect the lens. Nevertheless, the philosophy of conventional medicine dictates that until more intervention trials are published, we should wait, age, and lose vision in the process. This inaction is considered "prudent" because it's possible supplements will ultimately not be found to protect. I believe a more reasonable strategy is to take those antioxidants which currently appear most protective against cataract formation. Until more is known, supplementing vitamin C (300-600mg per day), vitamin E (400 IU per day), vitamin B2 (3 mg per day), vitamin B3 (40 mg per day) and standardized extract of bilberry (240-480mg per day) is both safe and arguably prudent.

References
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(57) Sperduto RD, Hu TS, Milton RC, et al. The Linxian cataract studies. Arch Ophthalmol 1993; 111:1246-53.

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By Steve Austin

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