NEW YORK (Reuters Health) – New evidence contradicts the commonly held belief that voluntary physical activity reduces symptoms of anxiety and depression, according to an analysis of data from the Netherlands Twin Register. Rather, genetic factors that influence exercise behavior appear to affect symptoms of anxiety and depression as well, investigators report in the August Archives of General Psychiatry.

Evidence from multiple longitudinal studies and randomized trials “makes it tempting to interpret the association at the population level as reflecting a causal effect of exercise on the symptoms of anxiety and depression,” Dr. Marleen H. M. De Moor and colleagues note.

However, “exercise behavior and symptoms of anxiety and depression are heritable traits,” they maintain, which may explain some of the observed associations.

To test the causal effects of exercise on mood symptoms, the research team at VU University Amsterdam conducted a population-based longitudinal study that included 5952 twins, 1357 additional siblings, and 1249 parents. Subjects periodically completed questionnaires about exercise behavior and symptoms of anxiety and depression between 1991 and 2002.

The authors observed that in monozygotic twin pairs, exercise behavior in one twin was found to be predictive of anxiety and depressive symptoms in the co-twin. Moreover, one twin who exercised more was not less depressed than the co-twin who exercised less.

The same was not true in dizygotic twin pairs or sibling pairs, the report indicates.

Furthermore, regression analyses showed that changes in physical activity did not predict changes in anxious and depressive symptoms over time.

“None of these tests supported the causal hypothesis,” Dr. De Moor’s group states. Instead, “there is a common genetic vulnerability to lack of regular exercise and risk for anxiety and depression in the population.”

Nonetheless, they caution, “it is crucial to make a distinction between the effects of prescribed and externally monitored exercise in selected subgroups and the effects of voluntary leisure-time exercise at the population level.”

In other words, the researchers conclude, “the antidepressant effects of exercise may only occur if the exercise is monitored and part of a therapeutic program.”

Arch Gen Psychiatry 2008;65:897-905.