Association of keratoconus with serum levels of 25-hydroxyvitamin D and antioxidant trace elements: A systematic review and meta-analysis

The aim of this systematic review and meta-analysis was to summarize and compare the available evidence on the level of vitamin D and antioxidant trace elements between the keratoconus (KC) patients and healthy controls. Seven case-control studies with 830 subjects were found eligible with a systematic search using PubMed, SCOPUS, Web of Science, and EMBASE till November 21, 2021. Data were synthesized with a DerSimonian and Laird random-effects method of meta-analysis. The mean serum vitamin D level was significantly lower in the patients with KC [standardized mean difference (SMD):-0.71; P < 0.001] as compared with the control group. The mean serum vitamin D level decreased more in the progressive patients (SMD:-0.80; P = 0.016) than in the stable patients (SMD:-0.66; P < 0.001) when compared with the control group. The mean serum zinc level was found significantly lower in the patients with KC compared with the control group (SMD:-1.98; P = 0.005). Pooled analysis based on the two studies showed significantly lower mean selenium levels in the KC patients (SMD:-0.34; P = 0.003). Regular evaluation of serum vitamin D, zinc, and selenium levels among the patients with KC at disease onset and future follow-ups could be promising in predicting the progressive disease and disease severity.