Depression and fatigue reinforce each other in a vicious cycle, say researchers in the journal Psychosomatic Medicine.
Depression at the beginning of a year-long study raised the likelihood of fatigue by four times, while fatigue at the start more than doubled the chances of depression, says lead author Petros Skapinakis of the University of Ioannina School of Medicine in Greece.
Doctors have long been aware of the association between depression and unexplained fatigue, but were never sure which symptom came first. To find the answer, Skapinakis and colleagues looked at patient data from the World Health Organization study of psychological problems in general health care.
"Fatigue and psychiatric disorder are not the same," Skapinakis says. "It is evident from the literature that fatigue and depression have different risk factors."
But they may also overlap, he says. Fatigued persons might feel psychologically distressed over their condition, and decreased energy is one symptom of depression. Levels of physical activity could explain the interaction between fatigue and depression.
"Physical activity is known to have a protective effect on depression," Skapinakis says. "It has also been suggested that physical deconditioning might be an important factor in the development of unexplained fatigue."
Since treatment for both chronic fatigue and depression includes exercise, future research might look at whether physical activity could explain the interrelation of the two conditions.