The primary aim of this NIA sponsored ancillary study of phase 2 CALERIE is to measure whether metabolic adaptation lasts up to 2 years in response to a 25% calorie restriction in normal-weight and slightly overweight individuals. Energy metabolism (24-hour sedentary and sleeping) will be measured in a respiratory chamber. Other measures include the activities of the sympathetic nervous and thyroid axes, oxidative stress in lipids, protein and DNA, organ size by MRI and echocardiography, and posture allocation (time spent engaging in different activities and the energy expended during these activities), and activity temperament (the innate predilection for movement). This set of studies will allow assessment of whether the previously observed metabolic adaptation in response to 25% caloric restriction is long lasting (up to two years), associated with reduced thyroid and sympathetic activities and reduced oxidative damage. The studies of organ sizes (liver, spleen, kidney, heart, adipose and muscle) by MRI and echo (heart) is essential to determine whether the metabolic adaptation is related to decreased organ sizes or improved organ energy metabolism efficiency. The above described measures will be performed at baseline and after 1 and 2 years of a 25% caloric restriction in 50 non-obese humans and in 25 sex-, BMI- and age- matched subjects who will not engage in caloric restriction.