Elevated blood concentration of homocysteine (Hcy) has been suggested as a modifiable, independent risk factor for coronary artery disease, stroke, and deep vein thrombosis. Prevalence of hyperhomocysteinemia and folic acid deficiency in China are significantly higher than those in Europe and USA. The investigators' preliminary research demonstrated that blood concentration of Hcy was negatively correlated to estimated glomerular filtration rate (eGFR), a key index of kidney function. However, the question as to whether Hcy-lowering therapy with folic acid can reduce the risk of chronic kidney disease(CKD) remains to be answered.