MTHFR and type 2 diabetes mellitus: Highly credible evidence was discovered for single nucleotide polymorphisms (SNPs) rs1024611 at MCP-1 gene and SNP rs3025039 at VEGF gene with diabetic retinopathy (DR) in type 2 diabetes; SNP rs2268388 at ACACB gene, insertion/deletion (Ins/Del) variant at ACE gene, SNP rs1801133 at MTHFR gene, and SNP rs7903146 at TCF7L2 gene with diabetic kidney disease (DKD) in type 2 diabetes; and SNP rs4880 at SOD2 gene with diabetic peripheral neuropathy (DPN) in type 1 diabetes.