Q & A: Hyperactivity Among Anorexia Nervosa Patients

Q: I’m curious to know why some patients with anorexia nervosa (AN) develop marked behavioral hyperactivity, as in excessive motor movements and compulsive exercise, whereas others are much less inclined to be so physically active. (S.T., Seattle)

A: The phenomenon you describe is well known clinically, and in various series hyperactivity has been reported in 31% to 80% of patients with AN. However, although some recent findings offer grounds for interesting speculations, little is known about the specific mechanisms that differentiate hyperactive from non-hyperactive patients with AN.

In patients with AN, levels of hyperactivity as well as inner and outer restlessness have been correlated with hypoleptinemia. These associations are stronger than simple associations between low BMI and hyperactivity (Biol Psychiatry 2005; Dec 22; [Epub ahead of print]). But clearly, these observations say nothing about cause and effect; they simply describe temporal associations.

However, along these lines, in animal studies of semi-starvation-induced anorexia, hyperactivity increases. And, when these animals are given leptin, their semi-starvation-induced hyperactivity decreases (Mol Psychiatry 2000; 5(5):476). Leptin has also been noted to decrease anxiety-related movements in leptin-deficient ob/ob mice. To my knowledge, no observations have yet been published on the potential effects of leptin on hyperactivity in human AN.

So, it’s possible that hypoleptinemia may contribute to hyperactivity in patients with anorexia nervosa. But we don’t know for sure, and many other processes are undoubtedly involved as well. Coincidentally, recent research based on the hypotheses that hyperactivity in AN might be related to upregulated arginine vasopressin or enhanced pituitary sensitivity to arginine vasopressin failed to show evidence for these associations (J Psychiatr Res 2006 Jan 30; [Epub ahead of print]).