A new study found little evidence linking low serotonin levels to clinical depression, despite the two being associated in the past.

A serious mental illness impacting a person’s quality of life and well-being, clinical depression is one of the most common mental health conditions today. It currently affects about 280 million people worldwide, according to data from the World Health Organization (WHO).

Depression causes people to experience various symptoms affecting their emotions, mood, and ability to go about their daily lives.

In the past, experts theorized that serotonin levels were inextricably linked to this condition since serotonin is a neurotransmitter that impacts mood and behavior. As such, it’s thought that selective serotonin reuptake inhibitors (SSRIs) could be effective in treating depressive symptoms.

But new systematic umbrella review disputed this long-held notion. Published in Molecular Psychiatry, the study found little evidence supporting the idea of low serotonin levels affecting a person’s risk of depression.

Examining data from 17 systematic reviews, meta-analyses, and large database studies (not including animal studies or studies on depression sub-types), the researchers found weak evidence that low levels of tryptophan might affect people with family histories of depression.

Tryptophan lowers a person’s serotonin levels. However, most data suggested that low serotonin level was not associated with depression. Contrary to that, the researchers found evidence that long-term antidepressant use might lead to lower serotonin levels in the body.

“The main message of the paper is that scientific evidence accumulated over several decades does not support the theory that depression is caused by a deficiency of serotonin. Since serotonin is the main brain chemical thought to be involved in depression and the one that has been most thoroughly researched in modern times, this means the idea that depression is due to a chemical imbalance is not scientifically established,” said study author Prof. Joanna Moncrieff from the University College London.

According to other experts, the new research might affect how people view the use of antidepressants for clinical depression. Rather than a quick “fix,” people may regard antidepressants more as a part of a treatment plan for the condition.