Vitamin D supplementation significantly improved fasting blood glucose, postprandial blood glucose, and quantitative insulin sensitivity check index in diabetes and prediabetes with baseline 25(OH)D3<30 ng/ml. Higher percentages regressing from prediabetes to normal glucose status and lower percentage progressing from prediabetes to diabetes were found in the supplementation group. The positive effects of vitamin D supplementation on body mass index, waist, HDL-C, LDL-C, and CRP were also demonstrated. Here, CRP is linked to diabetes mellitus.