Methodological quality of the review: Low confidence

Author: Guan R, Zhang Q, Zhang R, Zhang X, Li L.

Region: United States of America (USA), and one each in Nigeria, Spain, Australia, Italy and Greece.

Sector: Age-related cataract

Sub-sector: Risk factor

Type of cataract: Age-related cataract

Equity focus: None specified

Quantitative synthesis method: meta-analysis

Qualitative synthesis method: Not applicable

Background:

Age-related cataract accounts for about half of the 32 million cases of blindness worldwide. It is important to identify the protective factors for age-related cataract and may help to enhance the quality of life for older people. Most fruits are rich in vitamin C, which can explain their protective role in cataract formation. In addition, lutein and zeaxanthin are highly concentrated in fruits and are the predominant carotenoids in the lens. These two substances could protect our eyes from photodamage in vitro and to be associated with reduced risk of cataract. As a result, many studies were conducted to assess the associations between dietary fruit intake and age-related cataract risk, however there is no consensus on the association between dietary fruit intake and age-related cataract.

Research objectives:

The aim of this study is to conduct a systematic review and meta-analysis to analyze the relationship between dietary fruit intake and age-related cataract risk.

Main findings:

Authors included a total of eight articles were included in this review comprising of 6,218 cataract cases and 111,093 participants were included in the meta-analysis. Three studies were conducted in the USA, and one each in Nigeria, Spain, Australia, Italy and Greece. Four studies were cohort in design, three were case-control and one cross-sectional. Based on the pooled analysis of the eight included studies included, authors suggested that the highest level of dietary fruit intake could significantly reduce the risk of age-related cataract (RR=0.80, 95% Confidence Interval (CI)=0.68-0.93, P=0.005; I2=47.5%, Pfor heterogeneity=0.064). Authors reported that the subgroup analysis indicated that the highest fruit level versus lowest level was significantly inverse associated with the risk of age-related cataract in European populations only [summary RR=0.60, 95% CI=0.45-0.81, I2=20.8%]. For the subgroup analysis by study design, the pooled results suggested that there is significant association between dietary fruit intake and age-related cataract risk both in case-control studies [summary RR=0.65, 95% CI=0.45-0.92, I2=20.8%] and cohort studies [summary RR=0.90, 95% CI=0.81-0.99, I2=20.8%].

Based on findings from this review, authors conclude that that highest level of dietary fruit intake could reduce risk of age-related cataract, particularly in European populations. Authors also note that more studies from countries outside Europe to investigate the association between dietary fruit intake and risk of age-related cataract.

Methodology:

Inclusion criteria for this review included: (1) human study, (2) case-control study or cohort study or randomized clinic trial or cross-sectional study, (3) the exposure was dietary fruit intake, (4) the outcomes was age-related cataract, (5) the relative risk (RR) with 95% confidence interval (CI) for highest fruit consumption versus lowest category with risk of age-related cataract was provided.

Authors conducted a database search of PubMed and Web of Knowledge for published studies. The following keywords and search terms were used: ‘fruit’ OR ‘dietary’ OR ‘diet’ AND ‘lens opacities’ OR ‘cataract’ in the English language. In addition, authors reviewed references of relevant articles.

To extract data of included studies two authors independently used a standardized form. Disagreements were resolved by discussion.

Authors conducted a meta-analysis using RR with 95% CI to assess the association between dietary fruit intake and risk of age-related cataract. Random-effects model was used to consider within-study and between-study variation. Authors examined heterogeneity by performing the Q and I2 statistics. In addition, authors conducted meta-regression and sensitivity analysis was performed to assess the potentially important covariates that might substantially impact on between-study heterogeneity. To assess presence of publication bias in the review, authors used Egger regression asymmetry and the Begg funnel plot.

Applicability/external validity:

Authors note that findings from this study are only applicable to European populations only and cannot be extended to populations elsewhere.

Geographic focus:

Authors reported that the highest fruit level versus lowest level was significantly inverse associated with the risk of age-related cataract in European populations, but not in USA or the other populations.

Quality assessment:

Overall, there is low confidence in the conclusions about the effects of this study. Although authors used appropriate methods to extract from included studies, they did not report methods used to screen studies for inclusion and did not report assessing the quality of studies, which impacts on the reliability of the findings from this review.