The effect of obesity on apparent volume of distribution of theophylline

An oral theophylline product was given to eight lean, normal subjects and eight obese subjects (four with asthma and four normals) in a single-dose study. The mean apparent volume of distribution (Vd) was 0.472 +/- 0.08 liters/kg for the lean group and 0.321 +/- 0.063 liters/kg for the obese group (P less than 0.001). When Vd wax expressed in liters, there was no difference between the two groups (lean:26.9 +/- 6.0 liters, obese: 28.5 +/- 6.3 liters). An inverse relationship existed between the ratio of total body weight to ideal body weight (TBW/IBW) and Vd expressed in liters/kg. One obese subject was studied before and after a weight loss of about 32 kg. The values of Vd in this subject were 32.1 and 34.9 liters, respectively. These data indicated that theophylline is not extensively distributed into adipose tissue and that loading-dose calculations should be based on ideal body weight. No difference in clearance of theophylline was seen between the two groups.