THE INCREASING use of total fasting as an effective method of producing weight loss in the very obese has led to a review of the status of vitamin nutrition in such individuals. Although it has been suggested by some authors 1,2 that vitamin deficiency does not occur during a total fast and others 3,4 have recommended the routine use of vitamin supplements, no supporting data have been offered. Such data as are available were obtained during semistarvation regimens,5-7 were usually secured during observations of normal nonobese humans and were based upon urinary vitamin excretion. Since, in current use, fasting as a therapeutic procedure ordinarily involves no caloric intake for periods of a week or more in markedly overweight patients, we have undertaken this study of thiamine changes in patients so treated using erythrocyte transketolase activity as an improved technique for functional evaluation of thiamine metabolism. The observations indicate