Fatigue and serotonin?

Dear Alice,

Is there a link between constant fatigue and serotonin levels?

— Budding Scientist

Dear Budding Scientist, 

Your hypothesis is correct — serotonin and fatigue do have a relationship to each other. Although scientists are still trying to understand this more, there are potential links between serotonin and chronic fatigue syndrome (CFS) specifically. While the exact causes of CFS are still unclear, researchers believe that levels of serotonin in the body may be a factor.  

To better understand this, it may be helpful to understand what serotonin does in the body. Serotonin is a neurotransmitter (a chemical involved in the transmission of nerve impulses between nerve cells) that is made from tryptophan, an amino acid found in food. When tryptophan enters the body, it’s broken down into an intermediary chemical called 5-hydroxytryptophan (5-HTP). 5-HTP is then transformed into 5-hydroxytryptomine, or serotonin. Serotonin is primarily found in three parts of the body — the brain, the lining of the digestive tract, and in blood platelets. In the brain, where it’s produced, serotonin's main effects include improving mood and giving you that "satisfied" feeling from food. It’s also thought to help promote sleep and relaxation.  

So where does the constant fatigue factor in? One way to understand the mechanisms that link serotonin and fatigue is to look at a condition called chronic fatigue syndrome (CFS). CFS is a disease that prevents the interaction of the endocrine, immune, and nervous systems — all of which play a part in the regulation of serotonin in the body. For example, some scientists demonstrated that inducing fatigue through exercise in rats led to a decrease in serotonin levels and an increase in serotonin transporters. In other words, there was not enough serotonin to bring to the brain to help regulate fatigue. Scientists believe that similar mechanisms take place in those who experience chronic fatigue. Without enough serotonin, the body can’t produce those “satisfied” feelings, perpetuating the sense of fatigue. 

Since serotonin and fatigue have some connection, one way to relieve fatigue could be to increase serotonin levels. For example, selective serotonin reuptake inhibitors (SSRIs) are drugs that are sometimes prescribed to increase levels of serotonin in those with depression. For those for whom a prescription doesn’t fit their needs, there may be some other ways to help reduce fatigue and increase serotonin:  

  • Getting adequate, regular, and consistent amounts of sleep each night. 
  • Eating a healthy, well-balanced diet and drink plenty of water throughout the day. 
  • Regularly being physically active. 
  • Reducing usage of alcohol, nicotine, and drugs. 
  • Getting more exposure to sunshine or natural light. 
  • Thinking positively — turns out, a good mood can lead to serotonin production, and vice versa! 

Feeling constantly fatigued may be a sign of other, more serious conditions, such as depression, anxiety, or CFS. If you’re concerned about yourself or a loved one, speaking with your health care provider or mental health professional may be helpful in finding a fatigue-busting and serotonin-boosting plan that works best for you. 

Last updated Feb 12, 2021
Originally published May 01, 1994

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