Association of blood antioxidants and vitamins with risk of age-related cataract: a meta-analysis of observational studies

Methodological quality of the review: Medium confidence

Author: Cui YH, Jing CX, Pan HW.

Region: United States of America (USA), Europe and Asia.

Sector: Age-related cataract

Sub-sector: Blood antioxidants and vitamins.

Type of cataract: Age-related cataract

Equity focus: None specified

Review type: Effectiveness Review

Quantitative synthesis method: Meta-analysis

Qualitative synthesis methods: Not applicable

Background

Authors noted that both in vitro and animal experiments show that oxidative stress is involved in the development of cataracts and that antioxidants have shown to limit lens damage after ‘oxidative insult’. Therefore, it would be useful to investigate if antioxidants have a protective effect against cataracts. Many studies (randomized controlled trials and observational studies) have been carried out, but no meta-analysis on the association of blood antioxidants with risk of age-related cataracts had been conducted to date.

Research objectives

To determine whether an association exists between blood levels of antioxidants or vitamins and age-related cataracts in observational studies.

Main findings

Authors included 13 observational studies from the Finland, USA, United Kingdom (UK), Spain, India and Italy investigating the association between blood levels of antioxidants and risk of age-related cataract. All participants were 47 years of age and over. Four studies were prospective studies, one study was a case-control study and eight studies were cross-sectional studies.

Authors reported the following findings:

• Results showed that vitamin E (OR: 0.75; 95% CI: 0.58, 0.96), alpha-carotene (OR: 0.72; 95% CI: 0.59, 0.88), lutein (OR: 0.75; 95% CI: 0.65, 0.87), and zeaxanthin (OR: 0.70; 95% CI: 0.60, 0.82) were inversely associated with age-related cataract.
• Vitamins A (OR: 0.69; 95% CI: 0.58, 0.83) and C (OR: 0.67; 95% CI: 0.57, 0.78) were inversely associated with age-related cataract in Asian populations but not in Western population.
• Beta-carotene (OR: 0.90; 95% CI: 0.78, 1.05), lycopene (OR: 0.86; 95% CI: 0.65, 1.15), and beta-cryptoxanthin (OR: 0.83; 95% CI: 0.68, 1.02) had no significant association with risk of cataract.

Authors noted that these findings were of importance when further researching cataract prevention, and concluded that the meta-analysis supports the view that blood levels of certain antioxidants are ‘inversely associated with risk of age-related cataract’. However, the role of antioxidant or vitamin supplement intake in preventing cataract should be further investigated in interventional studies.

Methodology

The authors included observational studies that explored the association between blood antioxidants or vitamins and risk of age-related cataracts.

The authors conducted a systematic search of the literature on PubMed, EMBASE and Web of science (all-year timespan). No language restriction was applied. Authors also checked reference lists of included studies. Two authors independently conducted the study selection and data extraction. Authors noted that, as there is no universal scale available for the evaluation of observational studies, they created a modified scoring system which allowed a total score of 0-6 points. This was conducted by two reviewers independently.

Authors evaluated the association between blood levels of antioxidants and risk of age-related cataract by pooling the results from individual studies.

Applicability/external validity

The review does not discuss in full how generalizable the results are.

Geographic focus

Authors included studies from Finland, USA, UK, Spain, India and Italy. Authors noted that Vitamin A and C were inversely associated with age-related cataract in Asian populations but not in Western populations.

Quality assessment

The systematic review is based on searches in various databases including PUBMED, Web of Science and EMBASE. However, it is not sufficiently comprehensive that we can confident that relevant literature was not omitted. Relevant databases were not considered as part of the search strategy, reviewers did not contact study authors for additional references or information and date parameters used in each search was not clear. Nevertheless, authors used appropriate methods to reduce risk of bias in terms of study selection and data extraction of included studies.

In relation to data synthesis, the authors conducted meta-analysis, pooling data from the 13 included studies. Authors recognized the limitations of this data analysis, since study designs of the included studies varied between case-control, cross-sectional and cohort studies. Authors also noted that many observational studies that focused on the association between antioxidants intakes and cataracts reported imprecise/inconsistent results and were based on self-reported rather than objective measures. This may have impacted on the quality of the evidence as this may introduce recall and reporting bias.  As a result, authors noted that findings of the meta-analysis need to be met with caution. As such, this review was awarded medium confidence in the conclusions about the effects of this study.