Beta-carotene for cataract prevention in smokers

Beta-carotene - a naturally occurring antioxidant found in fruits and vegetables - may reduce cataract risk in male smokers, so suggests a 12 year US supplementation trial of age-related cataract in men.

The randomised, double-blind, trial was part of a 'Physicians' Health Study I' led by W. Christen​ designed to evaluate low-dose aspirin and beta-carotene in the primary prevention of cardiovascular disease and cancer.

Approximately 22,000 male physicians aged 40 to 84 years were randomly assigned either beta-carotene (50 mg on alternate days) or placebo for 12 years. In each group, 11 per cent of participants were current smokers.

While analysis did not reveal any difference for non-smokers.the study revealed a 26 per cent reduction in the risk of cataract for those in the group receiving beta-carotene who also smoked. Baseline information included height, weight, history of cigarette smoking, history of alcohol use, blood pressure level, cholesterol levels, history of diabetes mellitus, and history of multivitamin use and personal cataract history.

A total of 20,968 physicians did not report cataract at baseline and so were included in the analyses.

The main outcome measures were defined as age-related cataract and extraction of age-related cataract, defined as an incident, age-related lens opacity, based on the self-report confirmed by medical record review.

During the 12 years a total of 2015 cataracts and 1177 cataract extractions were confirmed. The results showed a slight decrease of risks with increasing duration of treatment, but the trend was not statistically significant. Overall, the researchers reported no significant benefit or harm of beta-carotene supplementation on risk of cataract, with 998 cases in the beta-carotene group compared to 1017 in the placebo group.

Similarly, the researchers did not discover any effect of beta-carotene supplementation on risk of cataract extraction - 584 cases in the beta-carotene group versus 593 in the placebo group.

On the other hand, in subgroup analysis, the researchers discovered that the effect of beta-carotene supplementation on cataract appeared to be modified by smoking status. The trial data suggest that among current smokers, men assigned to the beta-carotene group had a statistically significant 26 per cent reduced risk of cataract compared with those in the placebo group - 108 cases compared to 133 cases.

Similar results were observed for cataract extraction. Among current smokers, men assigned to beta carotene had a 27 per cent reduced risk of extraction compared with men assigned to placebo - 68 cases versus 86 cases.

For both cataract and cataract extraction, the reduction in risk among current smokers assigned to beta carotene was apparent during the early years of treatment and follow-up (years 1-5)). Yet there was no significant trend toward greater benefit with an increasing duration of treatment for either cataract end point.

'Cigarette smoking is a well-established risk factor for cataract', noted the scientists. As such, to explore the relative effects of baseline smoking status and beta-carotene treatment assignment on risk of cataract, models were fit with current non-smokers assigned to placebo as the reference group. For current smokers assigned to placebo, there was a statistically significant 65 per cent increased risk of cataract compared with the reference group. According to the researchers this excess risk associated with being a current smoker was markedly attenuated by assignment to beta-carotene treatment.

Cataract is a leading cause of visual impairment in the United States. As such, there is considerable interest in determining whether vitamins with antioxidant properties lower risks of cataract development and progression.

"Laboratory studies support this possibility by showing that supplementation with antioxidant vitamins and minerals prevents or delays cataract development in vitro and in animal models,"​ wrote the researchers. As reported​ by Nutraingredients in February 2002, previous studies in humans claim to provide further support by showing that persons with higher intakes of fruits and vegetables, or higher plasma levels of various antioxidant nutrients including vitamin C and E, tend to have lower risks of cataract.

However, despite the positive results obtained in this trial, the researchers warn that these subgroup findings must be interpreted with caution as they cannot be certain as to the reason for the registered effect. As a result they consider further randomised trials will be necessary.

Full details of this study can be obtained in the March edition of the Archives of Opthalmology​.