Objective: The purpose of this study was to compare the energy cost of standardized physical activity (ECA) between patients with cystic ®brosis (CF) and healthy control subjects. Design: Cross-sectional study using patients with CF and volunteers from the community. Setting: University laboratory. Subjects: Fifteen patients (age 24.6 AE 4.6 y) recruited with consent from their treating physician and 16 healthy control subjects (age 25.3 AE 3.2) recruited via local advertisement. Interventions: Patients and controls walked on a computerised treadmill at 1.5 kmah for 60 min followed by a 60 min recovery period and, on a second occasion, cycled at 0.5 kp (kilopond), 30 rpm followed by a 60 min recovery. The ECA was measured via indirect calorimetry. Resting energy expenditure (REE), nutritional status, pulmonary function and genotype were determined. Results: The REE in patients was signi®cantly greater than the REE measured in controls (P 0.03) and was not related to the severity of lung disease or genotype. There was a signi®cant difference between groups when comparing the ECA for walking kg p FFM (P 0.001) and cycling kg p FFM (P 0.04). The ECA for each activity was adjusted (ECA adj ) for the contribution of REE (ECA kJ kg p FFM 120 min 71 7 REE kJ kg p FFM 120 min 71 ). ECA adj revealed a signi®cant difference between groups for the walking protocol (P 0.001) but no difference for the cycling protocol (P 0.45). This ®nding may be related to the fact that the work rate during walking was more highly regulated than during cycling. Conclusions: ECA in CF is increased and is likely to be explained by an additional energy-requiring component related to the exercise itself and not an increased REE.

These data suggest that the 13C breath test is a valid method that can be used to measure the energy cost of short duration physical activity in a field setting.