Body mass index and daily physical activity in anorexia nervosa

The level of daily physical activity in 11 non-hospitalized women with anorexia (age: 21-48 yr, body mass index (BMI): 12.5-18.3 kg . m(-2)), compared with 13 normal-weight women (age: 20-35 yr, BMI 19.2-26.7 kg . m(-2)), was studied in relation to BMI. Daily physical activity over a 7-d period was determined from movement registration and by combining measurements of average daily metabolic rate (using the doubly labeled water method) and sleeping metabolic rate (measured in a respiration chamber). Group averages of daily physical activity were similar for subjects with anorexia and control subjects. However, women with anorexia had either a low or a high level of daily physical activity, whereas most control subjects had a moderate level of daily physical activity. In the women with anorexia, daily physical activity was significantly related to BMI (r = 0.84). Subjects with a BMI greater than or equal to 17 kg . m(-2) were equally or more active compared with control subjects, while subjects with a BMI < 17 kg . m(-2) were equally or less active compared with control subjects. The increased physical activity at BMI greater than or equal to 17 kg . m(-2) is considered to be facilitated by an improving physical capacity combined with the advantages of a low body mass during weight-bearing activities. At lower BMI, undereating and declining physical capacity may have caused the observed decrease in daily physical activity.