Urinary caffeine after coffee consumption and heat dehydration

This study evaluated the effect of heat-induced dehydration on urinary caff eine excretion after the consumption of a strong coffee solution. Following ingestion of coffee (caffeine 4.9 +/- 0.1 [SE] mg/kg, 3 - 4 cups), ten hea lthy males were intermittently exposed to heat in a sauna until they had lo st 2.9 % of lean mass. on a separate occasion, they consumed the same amoun t of coffee but remained quiet and euhydrated (control). Urine flow was red uced 7-fold in dehydration. At these low excretion rates (< 30 ml/h), caffe ine concentration was negatively correlated with flow. Peak urinary caffein e (C-max) was 7.6 <plus/minus> 0.4 (SE) mug/ml in dehydration and 7.1 +/- 0 .2 mug/ml in the control (p > 0.05). Compared with the control, dehydration delayed C-max by 1 hour, maintained higher saliva caffeine concentration ( 6.1 vs 5.2 mug/ml, p < 0.05) and a lower saliva paraxanthine/caffeine ratio (p < 0.001). The 24h-recovery of caffeine in urine was reduced (1.2 vs 2.8 % of dose, p < 0.001), however at least 2.6 % of dose were lost in sweat. These results suggest that the rise in circulating caffeine due to delayed metabolic clearance was partly opposed by a sizeable elimination in sweat. Therefore, heat dehydration did not lead to higher concentration of caffein e in urine after coffee ingestion.