As thiamin status declines, the concentration of TPP in erythrocytes decreases at approximately the same rate as occurs in other tissues (Brin, 1964; McCormick and Greene, 1994). The TPP effect may be noted within 2 weeks after the initiation of a thiamin-restricted diet (Brin, 1962). Baines and Davies (1988) provided evidence that, compared with erythrocyte transketolase activity, erythrocyte TPP is more stable in frozen erythrocytes, easier to standardize, and less susceptible to factors that influence enzyme activity.

Because of the wide variety of signs and symptoms characteristic of thiamin deficiency, numerous other indicators of thiamin status have been reported. These include blood pyruvic acid values after exercise (Foltz et al., 1944); both pyruvic acid and lactic acid values after administration of glucose (Bueding et al., 1941; Williams et al., 1943); various indicators of work performance (e.g., maximum work test to exhaustion) (Wood et al., 1980); aerobic power, respiratory exchange ratio, and ventilatory equivalent (van der Beek et al., 1994); work output over time (Foltz et al., 1944); gross behavior changes (Williams et al., 1942); neurological changes (Wood et al., 1980); psychological changes (Wood et al., 1980); and quality of life (Wilkinson et al., 1997). None of these was judged to be a dependable criterion of thiamin status.

Data on the bioavailability of thiamin in humans are extremely limited. Levy and Hewitt (1971) reported that absorption of thiamin supplements taken with breakfast does not differ from that taken on an empty stomach. No adjustments for bioavailability were judged necessary for deriving the Estimated Average Requirement (EAR).

No studies were found that examined the effect of energy intake on the thiamin requirement. Some studies provided thiamin in