Dietary vitamin and carotenoid intake and risk of age-related cataract

Author: Jiang H, Yin Y, Wu CR, Liu Y, Guo F, Li M, Ma L. 

Geographical coverage: The United States, Australia, Finland and Japan.

Sector: Vitamins, carotenoid

Sub-sector: Risk of age-related cataracts

Equity focus: Not reported

Study population: Patients with cataracts

Review type: Effectiveness review

Quantitative synthesis method: Meta-analysis

Qualitative synthesis method: Not applicable

Background

Globally, age-related cataract (ARC) is a leading cause of visual impairment and accounts for up to 35 % of all blindness. Although cataract surgery is effective, it imposes a substantial financial burden, with projected costs of more than USD 5.7 billion between 2010 and 2020. Delaying onset by a decade could potentially halve surgical demand. Oxidative damage to lens proteins and lipids is central to ARC pathogenesis, and dietary vitamins and carotenoids—potent antioxidants—may neutralise reactive oxygen species and stabilise lens-cell membranes. High lutein and zeaxanthin intake has been linked to lower risk of macular degeneration, suggesting possible benefit for cataract prevention. Evidence, however, remains inconsistent and the individual effects of specific vitamins and carotenoids require clarification.

Objective

To evaluate, through a meta-analysis of randomised controlled trials (RCTs) and cohort studies, the association between dietary vitamin and carotenoid intake and the risk of ARC.

Main findings

Twenty studies (eight RCTs, twelve cohorts) comprising more than 197 000 participants were included. RCTs were conducted in North America (n = 5), Finland (n = 2) and Australia (n = 1); cohort studies originated from the United States (n = 9), Australia (n = 2) and Japan (n = 1).

• Cohort evidence – protective associations:
  • Vitamin A: RR 0.81 (95 % CI 0.71 – 0.92; p = 0.959).
  • Vitamin C: RR 0.80 (95 % CI 0.72 – 0.88; p = 0.001).
  • Vitamin E: RR 0.90 (95 % CI 0.80 – 1.00; p = 0.244).
  • β-carotene: RR 0.90 (95 % CI 0.83 – 0.99; p = 0.023).
  • Lutein/zeaxanthin: RR 0.81 (95 % CI 0.75 – 0.89; p < 0.001).
    Dose–response analyses showed progressively lower ARC risk with higher intake.
• RCT evidence – null effects
  • Vitamin E supplementation: RR 0.97 (95 % CI 0.91 – 1.03; p = 0.937).
  • β-carotene supplementation: RR 0.99 (95 % CI 0.92 – 1.07; p = 0.975).
    Null findings were attributed to short follow-up, high supplement doses and healthy-participant bias.

Overall, long-term dietary intake of vitamins A, C and E and of carotenoids (β-carotene, lutein/zeaxanthin) is associated with reduced ARC risk, whereas short-term supplementation confers no clear benefit.

Methodology

Searches of MEDLINE, EMBASE, ISI Web of Science and the Cochrane Library to June 2018 identified RCTs and cohort studies examining dietary or supplemental vitamins/carotenoids and ARC. Reference lists were scanned and experts contacted for additional material. Two reviewers independently screened records, extracted data and assessed quality (Cochrane risk-of-bias tool for RCTs; Newcastle–Ottawa Scale for cohorts). Fixed- or random-effects meta-analyses were applied according to heterogeneity (Cochran Q, I²). Publication bias was explored with Egger’s and Begg’s tests and funnel plots; subgroup, meta-regression and sensitivity analyses investigated heterogeneity sources.

Applicability / external validity

Potential confounding, dietary-assessment error and variation in cataract classification limit certainty. Findings apply chiefly to populations in the United States, Australia, Finland and Japan; evidence from low- and middle-income countries is sparse. Long-term supplementation trials in diverse populations are warranted.

Geographic focus

No geographic limits were imposed; included studies were conducted in the United States, Australia, Finland and Japan.

Summary of quality assessment

Overall confidence in the review’s conclusions is high. Searches were comprehensive; inclusion criteria explicit; dual screening, data extraction and quality appraisal employed; study characteristics well documented; and appropriate meta-analyses performed with heterogeneity addressed. Limitations include absence of an excluded-studies list.