Measuring energy costs of leisure activity in adolescents using a CO2 breath test.

To determine whether a 13C-bicarbonate, isotope dilution technique could be used to estimate relative changes in energy expenditure of leisure activities of short duration, we studied eight adolescents who performed the following activities: watching television (120 min); playing a stringed instrument (60 min plus 60 min of sitting); and walking plus rest during two approximately isocaloric sessions (slow walk at 40% of peak VO2 for 43 min plus 77 min of sitting; fast walk at 73% of peak VO2 for 22 min plus 98 min of sitting). The rate of appearance of CO2 (RaCO2) was determined from the ratio of the oral dose of 13C-bicarbonate and the isotopic enrichment of breath CO2. The net rates of excretion of CO2 (VCO2) and oxygen consumption were measured. VCO2 and RaCO2, were correlated (r = 0.93; P < 0.05). To adjust for the systematic difference in CO2 production between methods, determinations were expressed as a fraction of that during television viewing. For RaCO2, the ratios for instrument playing, walking at 40% peak VCO2, and walking at 73% peak VO2 were respectively 133 +/- 20%, 186 +/- 38%, and 206 +/- 34%; for VCO2, the respective ratios were 129 +/- 19, 210 +/- 50, and 232 +/- 39 (P > 0.05 for methods and interaction, two-way ANOVA). RaCO2 may be a useful method for detecting relative differences in energy expenditure associated with leisure activities of brief duration.