Young people with depression exhibit abnormal cellular energy patterns in both the brain and blood, characterized by overactive cells at rest, but unable to respond appropriately to physical stress. This biological manifestation of fatigue suggests that early medical intervention may help restore cellular balance before long-term physical damage occurs. The study was published in the journal translational psychiatry.
Fatigue is a common and disabling symptom of major depressive disorder. Despite the prevalence of this severe fatigue, its biological origins remain largely a mystery. Medical professionals need a clearer picture of how depression changes the body at a microscopic level to develop better treatments.
To understand this phenomenon, researchers turned to adenosine triphosphate, commonly known as ATP. ATP serves as the main energy currency for all living cells. It provides the fuel needed for basic biological functions, from muscle contraction to the transmission of nerve signals in the brain.
The human brain requires enormous amounts of energy just to maintain baseline operation. Even when a person is at rest, the brain uses most of the body’s glucose and oxygen to keep cells alive and communicating. It relies heavily on mitochondria, small power plants within cells that are responsible for producing ATP.
When this cellular energy production is reduced, the brain is unable to function optimally. When brain cells are unable to maintain the necessary energy levels, symptoms such as deep fatigue, sluggishness, and cognitive decline can occur. This energy deficit theory has been around for years, but it has always been technically difficult to study it in living human brains.
Katherine R. Cullen, a psychiatrist at the University of Minnesota, led a team of researchers to investigate these energy dynamics in human patients. They thought that young people experiencing the early stages of depression might show measurable defects in the way their cells produce and use ATP. The researchers wanted to see if these cellular changes appeared simultaneously in the central nervous system and peripheral circulatory system.
To achieve this, the research team decided to look not only at neurons in the brain, but also at immune cells circulating in the blood. Blood cells are much easier to collect and study than brain tissue. If researchers can prove that blood cells and brain cells have the same energy problems, a simple blood test could one day help diagnose and monitor depression.
The research team recruited a group of volunteers between the ages of 18 and 24. This group included individuals diagnosed with major depressive disorder and a control group of healthy participants. All participants underwent a thorough clinical evaluation to determine the severity of depression and fatigue.
To measure brain energy, the researchers used an incredibly powerful 7 Tesla magnetic resonance imaging scanner. Most hospital scanners operate at much lower magnetic strengths and only show the physical structures of the brain. This specialized 7 Tesla machine allows scientists to safely examine the chemical composition of brain tissue in living people.
This advanced scanning technique allowed the team to specifically look at the visual cortex, an area at the back of the brain. The visual cortex was chosen because it is located close to the skull, making it easier for the scanner to detect distinct chemical signals. The scanner not only took still photos of the brain’s anatomy, but also tracked the real-time chemical reactions involved in ATP production.
By tracking these chemical exchanges, the researchers were able to precisely calculate the rate at which brain cells manufacture new energy molecules. On the same day as the brain scans, the team also took blood samples from the participants. They isolated specific immune cells from the blood and measured their baseline ATP levels.
The researchers then applied chemicals to these immune cells in a laboratory setting to intentionally stress their mitochondria. These chemicals act like a treadmill test for your cells, forcing them to burn off energy as quickly as possible. This mimics high energy demand conditions and reveals the maximum capacity of the mobile power plant.
This result was contrary to the research team’s initial expectations. In the visual cortex, depressed young adults actually produced ATP at a higher rate than healthy controls. Blood samples reflected this hyperactivity, showing elevated levels of ATP in immune cells during rest in the depressed participants.
Increased rates of ATP production in the brain and elevated blood ATP levels both correlated with patients’ self-reported severity of fatigue. Participants who felt the most fatigue had the highest baseline cellular energy activity levels. Susanna Tye, an associate professor at the University of Queensland who collaborated on the study, pointed out the uniqueness of this dual discovery.
“This suggests that symptoms of depression may be rooted in fundamental changes in how the brain and blood cells use energy,” Tai said. She added that fatigue is notoriously difficult to treat and these biological insights could ultimately lead to more direct medical interventions. It often takes years for patients to find the appropriate treatment for their disease.
Blood cell stress testing revealed another layer of cellular dysfunction. When the researchers used chemical stressors to force immune cells to work harder, the healthy participants’ cells readily increased their oxygen consumption. They seamlessly generated more energy to deal with the simulated threat.
In contrast, cells from depressed participants showed a blunted response. There was insufficient spare capacity to meet the increasing demand and the maximum output limit was quickly reached. University of Queensland researcher Roger Varela explained that the cells of depressed patients appear to be overactive just to maintain normal resting function.
“This suggests that cells may be working too hard in the early stages of the disease, which could lead to long-term problems,” Barrera said. Constant over-exertion causes cells to lose their reserve reservoir to cope with additional stress. Since they are already running at full speed, they have nothing to give when energy demand increases.
“This was surprising because people with depression would be expected to have lower energy production within their cells,” Varela said. Instead, the data point to a biological compensation mechanism. The body senses an energy crisis and increases production at rest to compensate, but when pushed further, mitochondria eventually reach a plateau.
This reduced ability to cope with higher energy demands may be the root cause of the depressed mood and decreased motivation seen in patients. Varela hopes that highlighting the physical reality of the disease will change public perceptions and reduce the stigma surrounding mental illness. “This shows that multiple changes are occurring in the body, including the brain and blood, and that depression affects energy at a cellular level,” he says.
Although these observations provide a new perspective on depression, this study has several limitations. The sample size was very small, with brain imaging data available for only 18 participants. The researchers note that these results need to be replicated in a larger group of people to confirm the pattern.
Many participants with depression were also taking psychiatric medications and had other symptoms, such as anxiety. These variables make it difficult to completely separate the specific effects of depression from the potential effects of drugs and other mental health disorders. When the researchers adjusted the mathematical model to account for age and gender, some of the differences between the groups were not statistically significant.
Future studies will need to follow patients over longer periods of time to see if this cellular overwork ultimately leads to a complete collapse of energy production later in life. Some medical experts suspect that chronic overactivity of mitochondria may contribute to age-related neurodegeneration in patients. Understanding this timeline may help doctors intervene early enough to preserve cellular function.
Ultimately, recognizing depression as a systemic metabolic disease could open up entirely new avenues for drug development. Rather than focusing solely on brain chemicals like serotonin, future drugs may directly target mitochondria. By helping cells manage energy more efficiently, doctors may finally be able to alleviate the severe fatigue that plagues many young people with depression.
The study, “ATP Bioenergetics and Fatigue in Youth with and without Major Depression,” was authored by Kathryn R. Cullen, Susannah J. Tye, Bonnie Climes Dugan, Hannes M. Wiesner, Roger B. Varela, Brooke Moras, Lin Zhang, Wei Chen, and Xiaohong Zhu.
