There’s a long-debated theory that low serotonin levels can cause depression – but there’s much more to this story.

Depression is a mood condition that affects millions of adults in the United States.

While researchers know how to recognize depressive symptoms and even possible treatment options, depression is still not fully understood. One of the least known facts: What causes it.

For some time, it was believed that depression was the result of low levels of the neurotransmitter serotonin in the brain.

But as we learn more about how these neurotransmitters work in the brain, it’s becoming clear that depression can’t simply be boiled down to only one cause. Instead, it’s caused by a variety of factors.

Neurotransmitters are chemical signals in the brain that can travel through cells and encourage or discourage the body to react in a certain way.

More specifically, nerve cells (often called neurons) release neurotransmitters that travel and seek out receptor molecules to absorb the neurotransmitters, which trigger a reaction.

Neurotransmitters are responsible for many of our basic bodily functions, including heart rate, digestion, and breathing.

They’re also responsible for psychological functioning, such as regulating your emotions, appetite, memory, and your fight, flight, or freeze response.

Your brain is packed with neurotransmitters — to date, researchers have identified more than 100. The ones most talked about include:

  • Dopamine: Best known for its role in making you happy and helping you focus. Dopamine is released after or in anticipation of receiving a reward or encountering something that excites you.
  • Endorphins: They work as pain relievers in the body and can be released during stressful times. Endorphins are also released after a strenuous run or workout so that your body and mind feel good and encourage you to work out again.
  • Serotonin: One of its primary jobs is to regulate body functions like waking up and sleeping, blood clotting, feeling nauseated, digestion, and sexual desire. Serotonin is also believed to be responsible for stabilizing your mood.

Serotonin regulates a wide array of bodily functions and not only in your brain. It is also widely used throughout your nervous system. In your brain, though, it has its own specific purpose – to regulate emotions.

Your brain also manufactures its own serotonin because the neurotransmitter can’t be shared between your body and your brain. This means that if your brain has low serotonin levels, it can’t pull additional serotonin from another part of your body when it needs it.

This is because it can’t pass through the “blood-brain barrier” – the system that moves helpful things such as oxygen and carbon dioxide through the body and into the brain. As a result, your brain needs to create enough of its own serotonin to function properly.

Unusually high or low levels of serotonin can impact your mental and physical health.

High serotonin levels

“High serotonin levels could create something called ‘serotonin syndrome,’” says Marjorie Cooper-Smith, a licensed individual clinical social worker and psychotherapist in Washington, D.C.

Cooper-Smith explains that the symptoms of serotonin syndrome fall into three categories:

  • altered mental status, such as irritability, agitation, restlessness, and anxiety.
  • neuromuscular hyperactivity, which can include tremors, shivering, muscle rigidity, and muscle spasm
  • autonomic hyperactivity or rapid heartbeat, high blood pressure, sweating, and fever.

Unusually high levels of serotonin usually occur by accident. For example, it can happen if you mix two or more medications (like antidepressants) or mix certain herbal supplements.

Serotonin syndrome can also be caused when medications, cocaine or amphetamines, and/or supplements are mixed. For example, if you take an over-the-counter medication for a cough or cold after you’ve already taken an antidepressant.

When you mix different substances like this, it can cause your body to create serotonin much faster than it can be absorbed, resulting in a very high serotonin level.

If you think you might be experiencing the symptoms of serotonin syndrome, consider reaching out to your doctor. You’ll want to tell them every medication that you’ve recently taken so that they can accurately determine what might be causing the negative chemical interactions.

Low serotonin levels

Low serotonin levels can limit and impair your body’s ability to work properly or control your mood. This is called serotonin deficiency, and it can occur for several reasons.

“It seems dietary issues and childhood factors [can] impact serotonin levels,” says Cooper-Smith.

Because your body needs to create serotonin, which is largely composed of the essential amino acid tryptophan, your diet needs to include the elements that make up serotonin so that your body can pull out what it needs. If your diet doesn’t include the building blocks of serotonin, you risk a much lower level simply because your body can’t create it.

Researchers aren’t certain what causes low serotonin levels or why some people have lower levels than others. But it’s believed lower levels might be associated with:

  • increased anxiety
  • insomnia
  • impulsive behavior
  • irritable bowel syndrome
  • weight gain
  • social anxiety disorder
  • obsessive-compulsive disorder

The possible link between low serotonin levels and depression is a hotly debated topic.

For a long time, researchers believed in the serotonin hypothesis, which assumes that low serotonin levels in your brain could be an indicator or cause for depression.

However, there are few studies or evidence to prove that low serotonin levels directly lead to depression. In one review, it’s suggested that low serotonin levels could contribute to someone being likely to develop depression. However, other unknown or external factors, such as social support versus social isolation, also influence the onset of depression.

Selective serotonin reuptake inhibitors (SSRIs) are medications that work to slow serotonin absorption, resulting in more serotonin being present in the brain. And while it’s true that SSRIs can help improve symptoms of depression in some people, they don’t help every person living with depression.

This suggests that, while serotonin levels may contribute to depression, the true root cause of depression goes beyond low serotonin levels.

Researchers still are not sure what that is.

Because a diet lacking nutrients can lower serotonin levels, it appears that attention to diet can benefit serotonin production in your body. In particular, pairing tryptophan-rich foods with carbohydrates – such as a turkey and cheese sandwich, choosing salmon over rice, or even eating pretzels with peanut butter – appears to encourage serotonin creation.

Exercise, particularly aerobic exercise, is another great serotonin booster. When you exercise, your body uptakes certain amino acids for your muscles, which removes a lot of the competition that tryptophan faces on its way to your brain to get turned into serotonin.

“While the human body develops and makes serotonin naturally, there are ways in which one could increase its production,” explains Cooper-Smith. “For example, taking certain medications, ensuring certain dietary factors are in place, and engaging in exercise are ways one could increase serotonin levels.”

Though low serotonin levels can affect your body in some way, there’s no strong connection between it and depression.

Depression is a serious mental health condition, and if you think you have it, consider talking with someone you trust about your feelings. Many times you can find support simply by sharing how you feel.

If you have a family physician, consider talking with them. They can refer you to a mental health professional or specialist who can accurately diagnose depression and come up with a treatment plan that works for you.