MEASURING ENERGY COSTS OF LEISURE ACTIVITY IN ADOLESCENTS USING A CO2BREATH TEST

To determine whether a C-13-bicarbonate, isotope dilution technique co uld be used to estimate relative changes in energy expenditure of leis ure activities of short duration, we studied eight adolescents who per formed the following activities: watching television (120 min); playin g a stringed instrument (60 min plus 60 min of sitting); and walking p lus 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 p eak 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 C-13-bic arbonate and the isotopic enrichment of breath CO2. The net rates of e xcretion of CO2 (VCO2) and oxygen consumption were measured, VCO2 and RaCO2 were correlated (r = 0.93; P < 0.05). To adjust for the systemat ic difference in CO2 production between meth ods, determinations were expressed as a fraction of that during television viewing. For RaCO2, the ratios for instrument playing, walking at 40% peak VCO2, and walki ng 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 en ergy expenditure associated with leisure activities of brief duration.