Previous studies have suggested that elevated resting energy expenditure contributes to weight loss in patients with Parkinson's disease (PD). Body weight is, however, ultimately determined by variation in daily energy expenditure and not just resting energy expenditure. Therefore, we examined the hypothesis that PD patients are characterized by elevated daily energy expenditure. Sixteen patients with levodopa responsive PD and 46 healthy elderly controls were characterized for daily energy expenditure and its components (resting and physical activity energy expenditure) using a combination of the doubly labeled water technique (over 10 days) and resting indirect calorimetry. Fat-free mass and fat mass were measured by dual energy x-ray absorptiometry. Results showed that fat mass and fat-free mass did not differ between groups. Daily energy expenditure was 15% lower (2214 +/- 460 vs. 2590 +/- 497 kcal/d; p < 0.01) in PD patients compared to controls. This was primarily due to lower physical activity energy expenditure (339 +/- 366 vs. 769 +/- 412 kcal/d; P < 0.01) in PD patients as resting energy expenditure was not different between groups (1655 +/- 283 vs. 1561 +/- 219 kcal/d). These results show that daily energy expenditure is lower in PD patients compared to healthy elderly, primarily due to reduced physical activity energy expenditure. These results argue against the hypothesis that an abnormally elevated daily energy expenditure contributes to weight loss in PD.