Carotenoids, vitamin A, and their association with the metabolic syndrome: a systematic review and meta-analysis

Context: Modifiable factors that reduce the burden of the metabolic syndrome (MetS), particularly plant-derived biomarkers, have been a recent focus of rising interest. Objective: This systematic review and meta-analysis, which follows PRISMA guidelines, evaluates evidence from a period of 20 years that links vitamin A and carotenoids with the occurrence of MetS and following the PRISMA guidelines. Data Sources: PubMed and Cochrane databases (January 1997 through March 2017) were systematically assessed for studies, including case-control, cross-sectional, and cohort studies, that evaluated the associations of MetS with carotenoids and retinyl esters and retinol (vitamin A). Data Extraction: Key measures of associations were harmonized into odds ratios (ORs) and 95% confidence intervals (95% CI) of MetS per 1 standard deviation (SD) of exposure using forest plots and random effects models that pooled data points from 11 cross-sectional studies. Begg's funnel and harvest plots were constructed. Results: An inverse association between total carotenoids and MetS was found [ORpooled, 0.66; 95% CI, 0.56-0.78; 1 SD similar to 0.82 mu mol/L; n = 5 studies]. This association was the strongest for beta-carotene, followed by alpha-carotene and beta-crypotoxanthin. No association was detected between retinol and MetS (ORpooled, 1.00; 95% CI, 0.88-1.13; 1 SD similar to 2.14 mu mmol/L; n = 6 studies). Publication bias was absent, and harvest plots indicated consistency upon replication for beta-carotene and total carotenoid exposures. Conclusions: This review and meta-analysis suggests that, unlike retinol, total and individual carotenoids were inversely related to MetS.