Association between vegetable consumption and the risk of age-related cataract: a meta-analysis

Methodological quality of the review: Medium confidence

Author: Huang G, Wu L, Qiu L, Lai1 J, Huang Z, Liao L

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

Sector: Cataract

Sub-sector: Risk of cataract

Type of cataract: Age-related cataract

Equity focus: None specified

Quantitative synthesis method: Meta-analysis

Qualitative synthesis method: Not applicable

Background:

Recent studies had reported that antioxidant vitamins such as vitamin A, vitamin C and vitamin E may play an important role in cataract prevention. These antioxidants cannot be synthesized by the body and must be obtained from dietary sources, mainly from vegetables. However, no meta-analysis was performed to assess this association. In addition, quantification of the association between the consumption of vegetables and risk of age-related cataract is still conflicting.

Research objectives:

Assess the cataract risk for the highest versus lowest categories of vegetables consumption.

Main findings:

A total of nine articles involving 6,464 cataract cases and 112,447 participants were included in the meta-analysis. Out of the nine studies, 5 were cohort studies, 3 were case-control studies and 1 was a cross-sectional study. Four studies were conducted in the USA, and one each was conducted in Australia, Greece, Italy, Nigeria and Spain.

Pooled results suggested that highest vegetable consumption level compared with lowest level was inverse with the risk of age-related cataract [summary relative risk (RR) = 0.723, 95% Confidence Interval (CI) = 0.594-0.879, I2 = 72.8%]. The associations were also significant in USA [summary RR = 0.872, 95% CI = 0.791-0.960] and Europe [summary RR = 0.507, 95% CI = 0.416-0.619], but not in the other population. No publication bias was found by the authors. Based on these findings, authors note that higher vegetables consumption might be inversely associated with risk of cataract.

Methodology:

For inclusion, studies had to fulfill the following criteria: (1) have a prospective or case-control or cross-sectional design; (2) the exposure of interest was vegetables; (3) the outcome of interest was age-related cataract; (4) the subtype cataract was as an independent study when the total cataract was not observed; and (5) the relative risk (RR) with 95% confidence interval (CI) for vegetables consumption with risk of age-related cataract was provided.

Studies were identified by a literature search of PubMed and Web of Knowledge through April 2015 and by hand-searching the reference lists of the computer retrieved articles. Two investigators searched articles and reviewed of all retrieved studies, and extracted data of included studies independently. Disagreements between the two investigators were resolved by consensus with a third reviewer.

The pooled measure was calculated as the inverse variance-weighted mean of the natural logarithm of multivariate adjusted RR with 95% CI for vegetable consumption and age-related cataract risk. A random-effects model was used to combine study-specific RR (95% CI), which considers both within-study and between-study variation. The Q test and I2 of Higgins & Thompson were used to assess heterogeneity. Meta-regression with restricted maximum likelihood estimation was performed to assess the potentially important covariate exerting substantial impact on between-study heterogeneity.

Applicability/external validity:

Authors note that higher vegetables consumption might be inversely associated with risk of cataract.

Geographic focus:

Authors included one study conducted in Nigeria and the remaining studies were conducted in Europe and the USA. However, authors emphasize that the association between vegetable consumption and the decreased risk of cataract was only observed in USA and in Europe only.

Quality assessment:

Medium confidence was attributed in the conclusions about the effects of this study. Authors used appropriate methods to screen studies for inclusion and extract data of included studies. However, authors did not conduct a thorough search to ensure that unpublished studies were included in the review. In addition, authors did not assess the methodological quality of studies included in the review which may impact on the reliability of the results.