Increased energy expenditure (EE) has been reported in patients with obstructive sleep apnea (OSA). It has been postulated that CPAP treatment may induce weight reduction in obese patients, even if it has not been confirmed by recent studies.

In the present study we investigated the effect of OSA on EE and the effect of CPAP on body weight and physical activity.

One hundred and seven obese OSA patients and 25 healthy obese volunteers, as control group, matched for age, sex and BMI, were enrolled. The following evaluation was performed only in OSA patients after 6 months of CPAP treatment.

Baseline total EE was similar in the two groups, but OSA patients showed higher EE during the night, while control group during daytime. In patients, EE correlates with OSA severity, degree of daytime sleepiness or obesity. At follow-up evaluation, BMI and total EE were unchanged. However, daytime EE increased (1066 ± 131.5 vs 1104 ± 133 Kcal/die, p < 0.001, respectively) and night EE decreased (694 ± 69 vs 595 ± 73, p < 0.0001, respectively).

Statistically significant changes in BMI, sleepiness scale, blood gases and EE were found only in patients who were compliant to CPAP.

Obese OSA patients showed an increased night EE that was normalized only in those patients compliant to CPAP. Body weight and daily physical activity did not change in statistically significant way.