Calorimetric measurements made on rats during repeated periods of weight gain and weight loss

1. Rats in respiration chambers were grown for 32 days and their weight reduced by 27-40% on three occasions and realimentated. After the first cycle rats lost body weight more rapidly and regained that weight more quickly than previously. 2. The initial growth phase took 32 days compared with only 8 days in the final period. Each gram of weight gain was associated with 13-14 kJ of dietary net energy during the last two periods of realimentation compared with 19-20 kJ during the first period. 3. Calorimetric measurements showed that although maintenance energy requirement increased during the periods of growth, mean net availability of metabolizable energy was 0.91 compared to 0.60 during weight loss. 4. During the final period much of the weight gain was in the form of lean (67%) but the majority of energy retained was as fat (67-70%).