Depression is one of the leading causes of disability worldwide. More than 300 million people suffer from this mental illness, according to the World Health Organization.
Despite the usual depression, scientists still have much to learn.
Among other things, it is known that depression is not a single disease, but a variety of feelings and behaviors that may have different causes.
"Depressive disorders can be different in different people, and what is known today is that depression affects not just the brain but the entire organism," said Natalie Rasgon, a professor of psychiatry and behavioral science at Stanford University.
But a new study in which Rasgon is a senior author finds evidence of a potential biomarker for major depression that could lead to better treatment for this sometimes debilitating disease.
A study by a panel of researchers from the US and Sweden shows that a specific, naturally occurring chemical called acetyl-L-carnitine or LAC is lower in the blood of people suffering from depression. The research was published this week in Proceedings of the National Academy of Sciences.
"Earlier animal studies convincingly demonstrated the role of LAC in depression models, and this study is the first to confirm the results of animal studies in people with depression," said Rasgon ABC News.
Levels of the chemical were more severe among people with severe depression
The researchers measured LAK levels in the blood of 116 participants and found that those with depression had significantly lower levels of the chemical than healthy individuals. Two groups of people in two different hospitals were examined, with the same results.
LAK levels were particularly low in people whose depression was more severe, who had previously been sick or who were traumatized during childhood. The lowest levels were in women whose depression did not improve with medication or therapy called "therapy-resistant depression," where childhood trauma or neglect had occurred.
The human body naturally produces LAC from a nutrient called carnitine, but researchers found that carnitine levels were the same in people with and without depression. This suggests that different levels of LAC are due to depression rather than diet.
What do we know about this chemical?
LAC performs a number of important tasks, including regulating how the brain and nervous system consume energy. It can also interact with the DNA to alter how certain genes are expressed.
Previous studies have shown that LAC effectively and rapidly improves the symptoms of depression in mice.
A review of several studies published in the Journal of Psychosomatic Medicine in 2017 showed that acetyl-L-carnitine can be better in treatment of depression than placebo and as effective as traditional antidepressants, but with less side effects. Review authors noted that the studies were very small and that larger studies are needed to confirm benefit.
There is an LAC supplement that was used for dementia, but a 2003 study by Cochrane showed that it is unlikely to be useful for this purpose. Some studies suggest a benefit for a type of nerve pain called neuropathy.
However, it is important to note that this study has not tested whether LAC can be used to treat depression.
"We are at the beginning of this discovery and can not recommend purchasing this supplement in the GNC business," Rasgon warned.
How Further Research Could Lead to Possible Treatments
LAC has the potential to be a biomarker, something that is measurable in the body and reflects the presence of a disease. As such, it could potentially be used to screen and diagnose severe or treatment-resistant depression and to allow for earlier and more aggressive treatment.
The researchers hope that future studies could shed light on whether physical activity, dietary habits, and sleep can affect LAC levels.
This study also found that people with antidepressants still had lower levels of LAC in their blood, suggesting that taking chemical supplements is helpful or even necessary to fully benefit from a drug.
This research, along with previous studies, suggests that correcting a lack of LAC could be a step towards more targeted treatment for depression, especially for those who had a childhood trauma or had depression at a young age.
"We are excited about these results and are working to expand them to better understand the role of LAC in patients being treated for depression," Rasgon said. "It's one of the pieces of a very big puzzle that makes depressive disorders a disease."
While this study has important implications for understanding depression, Rasgon warned that larger studies were needed to confirm the findings. "There are many questions to answer – who will ultimately benefit from taking this supplement, what is the right dose, what is the appropriate duration of use."
The study was a collaboration between researchers from Stanford, Rockefeller University, Duke University, Weill Cornell Medical College, the Icahn School of Medicine at Mount Sinai, and the Karolinska Institute in Sweden.
Michael MacIntyre is a psychiatric resident and works with the ABC News Medical Unit.