Estimation of vitamin B1 excretion in 24‐hr urine by assay of first‐morning urine

Urinary B1 (vitamin B1) excretion is commonly determined in 24-hr urine specimens to obtain an estimate of nutritional status. The aim of our study was to investigate whether B1 in random urine specimens, corrected for the urine creatinine (Cr), can be substituted for B1 in 24-hr urines. Collection of such hour urines is often fraught with errors; an alternative method is described here. All urine specimens voided over 24 hr were collected from 32 healthy adults as were the first-morning urines from 30 healthy Japanese women. The B1 excretion was expressed as the ratio of B1 to Cr. Although the B1 excretion was expressed as the B1/Cr ratio, the B1 excretion varied with the urine volume and the time of urine collection. The B1/Cr ratio in random urine specimens not collected at a fixed time may mislead the evaluation of the nutritional status. We found that the B1/Cr ratio in the first-morning urine correlated significantly with the ratio in 24-hr urines (r=0.970, P<0.001) and also with the concentration of total B1 (B1 plus its phosphate esters) in whole blood (r=0.733, P<0.001). We conclude that the B1/Cr ratio in 24-hr urines could be estimated by measuring the ratio in the first-morning urine.