August 8, 2008 — New research shows that in some individuals exercise does not have a positive effect in mental health outcomes such as depression or anxiety. Nevertheless, researchers say its beneficial effect on physical health is not in dispute, and patients should be encouraged to stay physically active.
According to investigators at the University of Amsterdam, in the Netherlands, the study's findings suggest an individual's psychological response to exercise may, at least in part, be predicted by genetics.
A large, longitudinal population-based study in identical twins round the twin who exercised more did not display fewer anxious or depressive symptoms than the twin who exercised less.
Further, longitudinal analyses showed that increase in exercise participation did not predict decreases in anxious and depressive symptoms.
"The findings do not mean that exercise has no effect on anxiety and depression, but we think that this effect may not be the same for everyone; some people may respond positively toward exercise, while in others it may not improve their mood much," first author Marleen .M. De Moor, from the University of Amsterdam, told Medscape Psychiatry.
The study is published in the August issue of the Archives of General Psychiatry.
Epidemiological studies have consistently reported that lack of regular exercise is associated with depressive symptoms and anxiety. Some randomized controlled clinical investigations have suggested that exercise training causes a depression-lowering effect that is comparable to antidepressant use.
While it may be tempting to interpret these findings as supporting the conventional wisdom that exercise decreases depression and anxiety symptoms, the researchers note that results from experimental studies cannot always be extrapolated to the general population.
According to the study, genetic factors may play a role, since they account for about 50% to 60% of variations in exercise behavior and about 40% to 50% of variations in symptoms of anxiety and depression.
Taking this potential genetic confounding into account, the investigators examined a large population to see whether changes in voluntary exercise led to changes in symptoms of anxiety and depression.
They analyzed data from an ongoing study in adult twins who voluntarily registered with the Netherlands Twin Register.
This included information on individuals aged 18 to 50 years from 1991 to 2002. The participants included 5952 twins, 1357 siblings, and 1249 parents. Data from self-reported replies to questionnaires about exercise behavior, including the type, frequency, and duration, and symptoms of anxiety and depression were available at baseline and 2, 4, 7, 9, and 11 years later.
The researchers focused specifically on leisure-time exercise. Non–leisure-time activities such as walking or biking to work, which are very common in the Netherlands, were not counted as exercise. "We were really interested in physical activity that people choose themselves," said De Moor.
Anxious and depressive symptoms were measured using continuous scales for depression, anxiety, somatic anxiety, and neuroticism.
Exercise Still Beneficial
Cross-sectional and longitudinal tests were conducted to determine whether there was a causal relationship between exercise and symptoms of anxiety and depression.
The analysis showed that exercise behavior was associated with decreased anxious and depressive symptoms.
However, rather than an effect of the exercise itself, the researchers note the reduction in these conditions is more likely due to common genetic factors.
For instance, although in the case of identical twins where, when 1 was physically active and 1 was sedentary, the researchers saw no difference in anxiety and depression, this was not the case with fraternal twins.
Longitudinal data showed that individuals who increased their exercise levels during the 11-year follow-up period did not become less anxious or depressed.
Over the same time period, participants who stopped exercising did not become more anxious and depressed.
The findings do not detract from beneficial effects of regular exercise on cardiovascular health, but they suggest that cross-sectional studies showing exercise and mental health correlations should be interpreted cautiously, the researchers note.
According to Ms. De Moor, it is important to replicate these findings and obtain more insight into when exercise does and does not work to improve mental health outcomes and to determine the types of exercise that might be most beneficial.
"Exercise may help [mental health], but exercise may not help everyone, and certain types of exercise may be better than others," she said.