A New Look at Excessive Exercising Among AN Patients

New attention on body temperature,
rather than a desire to burn calories

The patient with anorexia nervosa (AN) who is exercising excessively may not be trying to lose weight by burning off calories, but instead may be reacting to starvation-related hypothermia, according to researchers in The Netherlands. And, rather than anxiety or depression, the underlying cause may be ambient temperature (AT), according to the results of their recent study.

Animal models have shown that AT moderates activity among semi-starved rats and mice, and excessive activity among semi-starved rats has been associated with low levels of the fat-derived hormone, leptin (Biol Psychiatry 2005; 58:165), a pattern that has also been reported among patients with AN (J Clin Endocrinol Metab 2003; 88:5169; J Neural Transm 2007; 114:1233). In the first study designed to evaluate the effects of AT on patients with AN, Dr. Olaia Carrera and researchers at the University Medical Center, Utrecht, The Netherlands, studied hyperactivity among consecutive female patients with AN being seen at a specialized center for eating disorders (PLoS ONE 2012; 7:7, e41851). DSM-IV criteria were used to establish the diagnosis.

In their final study group of 37 patients aged 14 to 18 years of age (all but 5 were outpatients), the researchers measured physical activity using an accelerometer (Actiwatch, Cambridge Neurotechnology, Cambridge, UK), on 3 consecutive weekend days. The meter was strapped to the patient’s right ankle and worn from 9 pm on the first day to 9 pm on the fourth day, and was removed only during swimming or showering. The degree of underweight was calculated using body mass index (BMI, kg/m²) computed to Z-scores. The researchers also administered the Eating Disorders Inventory-2 (EDI-2) to all subjects, with attention to the total score and the scales for Drive for Thinness and Body Dissatisfaction. All patients also completed the State-Trait Inventory and the Children’s Depression Inventory.

The subjects were divided into two groups, “Warm” or “Cold,” according to the mean AT recorded during the 3-day test period. Mean daily outdoor AT was obtained from the Royal Netherlands Meteorological Institute for the weather station closest to each patient’s home. For inpatients, the AT was obtained from the weather station closest to the treatment center.

Temperature did have an effect

The main finding of the study was the significant effect of AT on physical activity in adolescent patients with AN. The Cold group was significantly more physically active than the Warm group, and the Warm group was more sedentary. The authors also were able to rule out depression or anxiety as underlying causes for hyperactivity. Accordingly, the increase in physical activity observed among AN patients during colder months may be an adaptive response to compensate for hypothermia derived from loss of body insulation due to body weight loss.

The authors also noted that hypothermia is common among patients with AN, and patients often complain of feeling cold and many warm themselves with hot drinks, sitting close to heaters, or wrapping up in blankets. Dr. Carrera also pointed out that subscales for the EDI-2 that measure drive for thinness and body dissatisfaction were not associated with any objective measure of physical activity. Their findings challenge the commonly held belief that physical activity is a weight-loss strategy deliberately used by patients with AN.