Comparison of the Effects of Vitamins and/or Mineral Supplementation on Glomerular and Tubular Dysfunction in Type 2 Diabetes

Response to Rossing et al.

We thank Rossing et al. (1) for their interest in our article (2) and agree that one measurement of albumin-to-creatinine ratio in morning spot urine at baseline and after 3 months is a potential limitation to our study. This limitation is described in detail in our discussion section. Although we did not obtain multiple spot urine samples or a 24-h urine collection to assess microalbuminuria, the random microalbumin-to-creatinine ratio has high reported sensitivity and specificity compared with 24-h urine microalbumin testing (3). The ratio of protein or albumin to creatinine in an untimed urine specimen has replaced protein excretion in a 24-h collection as the preferred method for measuring proteinuria. Using a ratio corrects for variations in urinary protein concentration due to hydration and is far more convenient than timed urine collections. The ratio of protein or albumin to creatinine in an untimed urine sample is an accurate estimate of the protein or albumin excretion rate (4). Also, it should be noted that after 3 months of supplementation, significant decreases in our vitamins group and minerals and vitamins group for albumin-to-creatinine ratio were observed from baseline and when compared with the placebo group. There were no significant changes in the other two groups. If the results were related to chance, microalbuminuria would have decreased in the other two groups.

We acknowledge Rossing et al. for drawing our attention to an error in Table 1. The diastolic blood pressure in the minerals and vitamins group should be amended to 84 ± 11 mmHg. Blood pressure was recorded semiautomatically using a Dinamap recorder (Critilzon, Tampa, FL). Before the study, the calculated sample size was 18 patients in each group, having 80% power to detect the postulated differences in HDL cholesterol, blood pressure, and microalbuminuria with an α error of 5%. But, after the supplementation, the calculated power for albumin-to-creatinine ratio was 0.7.

In conclusion, we do not agree with the concerns of Rossing et al. that our study was inadequately powered for detecting realistic changes in urinary albumin/protein excretion rate, that insufficient methods for valid characterization of urinary albumin/protein excretion rate were applied, and that the effect of minerals and vitamins on urinary albumin/protein excretion rate was inconsistent, which would suggest a type 1 error or a chance finding. In view of this, we believe our conclusions are valid.