BACKGROUND: Short-term physical inactivity affects energy balance and is 
considered conducive to weigh gain. Long-term effects are unknown.
OBJECTIVE: The objective of the study was to use a bed-rest model to determine 
the long-term effects of physical inactivity on energy balance regulation and 
test the effect of exercise training on energy balance adjustment to physical 
inactivity.
DESIGN: Sixteen lean women were divided into two groups (n = 8 each): a control 
group subjected to a strict 60-d bed rest and an exercise group subjected to a 
combined aerobic/resistive exercise training concomitantly to bed rest. Body 
composition, spontaneous energy intake, hunger, total energy expenditure (TEE), 
and fasting gut hormones were measured.
RESULTS: Based on bed-rest-induced body composition changes, the control group 
were in slight negative energy balance (-0.4 +/- 0.4 MJ/d; P = 0.01 vs. zero), 
essentially due to muscle atrophy (P < 0.001 vs. zero). The stable fat mass (P = 
0.19 vs. zero), and the matching between spontaneous energy intake and TEE 
indicated, however, a stable energy balance. Hunger and gut hormones remained 
unchanged during the bed rest. In the exercise group, TEE was 24% higher than in 
the control group (P = 0.004). Unexpectedly, desire to consume food (P = 0.025) 
decreased and spontaneous energy intake (P = NS) was not stimulated, promoting a 
negative energy balance (-1.1 +/- 0.5 MJ/d, P = 0.0003 vs. zero).
CONCLUSIONS: Energy balance is regulated during 2 months of physical inactivity, 
contrasting with short-term experiments. Conversely, exercise-induced energy 
expenditure in bed-resting subjects who have no spontaneous physical activity 
did not induce hunger and promoted a negative energy balance, suggesting a 
potential role of nonexercise physical activities in energy balance regulation.