Use of [¹⁴C]-sodium bicarbonate/urea to measure physical activity induced increases in total energy expenditure in free-living healthy males

The aim of this study was to evaluate the utility of the [ ¹⁴C]-sodium bicarbonate/urea technique to detect physical activity-induced increases in total energy expenditure in free-living healthy men. Thirteen healthy males aged 34.1 ± 11.7 yrs with body mass index 24.1 ± 3.1 kg/m² were studied on three separate occasions, during which [¹⁴C]-bicarbonate was infused over 48-hours and urine was collected during the second 24-hours. On three separate occasions and in random order, subjects either remained sedentary, or performed a bout of physical activity on an electro-magnetically braked cycle ergometer sufficient to increase energy expenditure by 7% or 11% above predicted sedentary total energy expenditure. Urine samples were analyzed to evaluate the amount of [ ¹⁴C]-bicarbonate incorporated into urinary urea, thereby reflecting the amount of CO₂ produced per day, and upon conversion, the number of kilojoules of energy expended in 24-hours. All 13 subjects successfully completed the two physical activity treatments and there were no adverse events. As measured by the [¹⁴C]-urea assay, mean total energy expenditure values were not significantly different between sedentary activity (17902 ± 905 kJ/day), the physical activity treatment designed to increase TEE by 7% (17701 ± 594 kJ/day) and the physical activity treatment designed to increase TEE by 11% (18538 ± 485 kJ/day) (P=0.668). In conclusion, although the [¹⁴C]-sodium bicarbonate/urea technique was well tolerated and did not interfere with normal daily activities, it was not able to accurately measure physical activity-induced increases in EE in the range of 7-11% above predicted sedentary total energy expenditure.