But the study, published in the current issue of Psychosomatic Medicine, suggests that depression and fatigue may act as independent risk factors for each other.
Depression and Fatigue Linked
In the study, researchers used information on more than 3,200 people who participated in the World Health Organization's study of psychological problems and were monitored for one year.
Researchers found people who were depressed at the start of the study were more than four times as likely to have an episode of unexplained fatigue during that year.
In addition, those participants who were suffering from fatigue at the start of the study were nearly three times as likely to develop depression during the study.
Researchers say fatigue and the psychological disorder of depression are not the same, and the two have different risk factors. However, they may have some overlap.
"One can possibly understand how a fatigued person can start feeling psychologically distressed because of his or her condition, but the opposite is more difficult to explain," writes researcher Pertos Skapinakis, MD, of the University of Ioannina in Greece, and colleagues.
Researchers say examining the role of physical activity may be an important factor in helping to explain the association between the two conditions in the future.
"Physical activity is known to have a protective effect on depression," writes Skapinakis. "It has also been suggested that physical deconditioning might be an important factor in the development of unexplained fatigue."