PURPOSE: To establish the effects of a diet combining intermittent fasting (IMF) with calorie restriction on energy expenditure and metabolic health, and to isolate the relative contributions of fasting and negative energy balance to any observed effects. METHODS: After a 4-week control phase, 36 lean adults (mean±SD; age = 42±11 y, BMI = 23.9±2.1 kg/m2) were randomised to one of three conditions for 20 days; 1) daily calorie restriction (75:75; 75% of habitual intake daily), 2) IMF with calorie restriction (0:150; alternating 24-h periods of fasting and feeding to 150% of habitual intake), 3) IMF without calorie restriction (0:200; alternating 24-h periods of fasting and feeding to 200% of habitual intake). In the IMF groups, transitions from feeding to fasting and vice versa occurred at 15:00 each day. In addition to free-living measures of energy intake (weighed record) and physical activity (combined heart rate/accelerometry), body composition (DEXA), metabolic rate and substrate oxidation (indirect calorimetry), fasted health markers and postprandial metabolic responses were measured at pre- and post-intervention. RESULTS: Energy intake was reduced in the two energy-restricted groups (75:75 = -2602±904 kj/d, 0:150 = -2105±1105 kj/d; p=0.24) and maintained by 0:200 (+63±1439 kj/d; p<0.01 v 75:75, p<0.01 v 0:150), a pattern mirrored by changes in body mass (75:75 = -1.9±1.0 kg, 0:150 = -1.6±1.1 kg, 0:200 = -0.5±1.1 kg; p=0.46 75:75 v 0:150, p=0.01 75:75 v 0:200, p=0.04 0:150 v 0:200). However, the decrease in fat mass with 75:75 (-1.8±0.8 kg) was greater than the decrease accompanying 0:150 (-0.8±0.9 kg, p=0.01 v 75:75), both of which differed from the stability seen following 0:200 (-0.1±0.7 kg, p<0.01 v 75:75, p=0.05 v 0:150). Furthermore, physical activity energy expenditure decreased following 0:150 when compared to 0:200 (0:150 = -410±707 kj/d, 0:200 = +247±594 kj/d; p=0.07) but was unaffected by 75:75 (+4±527 kj/d; p=0.24 v 0:150, p=0.31 v 0:200). Despite these differences, metabolic rate, substrate oxidation, fasting biochemistry and postprandial metabolism were all unaffected. CONCLUSIONS: In lean adults, restricting calories through a complete alternate-day approach to IMF attenuated reductions in fat mass and prompted declines in physical activity, whilst metabolic health was unaffected.