The level of fecal Clostridium cluster XI correlated positively with the fat intake (r = 0.301, p = 0.021), protein intake (r = 0.362, p = 0.005), and the HDL-C level (r = 0.277, p = 0.034) in the T2DM patients, and in the control subjects, the Clostridium cluster XI level correlated positively with the insulin level (r = 0.408, p = 0.001), HOMA-IR value (r = 0.413, p = 0.001), and the FBG level (r = 0.270, p = 0.038), and it correlated negatively with the FFA level (r = −0.281, p = 0.031). The gene discussed is INS; the disease is type 2 diabetes mellitus.