How Serotonin Regulates Body Functions

Serotonin (sometimes called 5-HT because of its chemical name, 5-hydroxytryptamine) is a substance that occurs naturally in your body. As a neurotransmitter, serotonin carries signals along and between nerve cells (called neurons). It’s found mainly in your intestines but also in your central nervous system (CNS), which includes your brain and your blood platelets.

Serotonin appears to affect and/or regulate a number of body functions, including the following.

You could think of serotonin's effects in your brain as its “starring role” in your body. Widely known for playing a major part in regulating moods, serotonin has been called the body's natural "feel-good" chemical, because it's involved in your sense of well-being.

Serotonin plays a role in your bowel function as well as in reducing your appetite as you eat. In addition, your intestines produce more serotonin if you eat something that’s irritating or toxic to your digestive system. The extra serotonin helps move the affected food along so it’s expelled from your body more quickly.

The platelet cells in your blood release serotonin when you have any kind of tissue damage, such as a cut. This results in vasoconstriction—a narrowing of the tiny arteries, or arterioles, in your circulatory system—which slows your blood flow as part of the blood-clotting process.

Studies have shown that bone density and serotonin are linked—specifically, high circulating levels of serotonin the gut may be associated with osteoporosis. In fact, research suggests that antidepressants, specifically selective serotonin reuptake inhibitors (SSRIs) are associated with decreased bone mineral density and increased fracture risk. This isn't a reason to stop taking your SSRI, but rather, to have a conversation with your physician, especially if you have other risk factors, such as existing osteoporosis, a family history, or you smoke.

The increase in sexual desire that can accompany alcohol intoxication is believed to be due to low serotonin levels. On the other hand, decreased sexual desire can occur in people taking medications that produce higher-than-normal serotonin levels.

However, that’s only true when your serotonin level is within the normal range. What happens when it’s low? Perhaps the best-known condition believed to be associated with low serotonin levels is depression. Not surprisingly, increasing serotonin levels with medication has become a major part of depression treatment.

While there’s no one cause of low serotonin levels, it’s typically one of two reasons: your body doesn’t produce enough, or your body isn't using it efficiently, which can be due to having fewer or faulty serotonin receptors or breaking down and absorbing serotonin too quickly.

Vitamin deficiencies may also play a part. Low levels of certain nutrients essential for serotonin production, including vitamin B6, vitamin D, L-tryptophan, and omega-3 fatty acids, have also been linked to low levels of serotonin.

Depression is associated with chemical imbalances in the brain, including low levels of serotonin (among others). Increasing the amount of serotonin in the brain appears to help brain cells communicate, which has the effect of reducing depression symptoms and improving mood. This finding is the basis for many drugs used in the treatment of clinical depression and other mood disorders.

One group of serotonin-based medications called selective serotonin reuptake inhibitors (SSRIs) is used to reduce the symptoms of moderate to severe depression by increasing the level of serotonin in the brain. SSRIs are the most commonly prescribed antidepressants worldwide.

When your brain cells send signals, they release neurotransmitters like serotonin. To send the next signal, your cells must reabsorb and recycle the neurotransmitter they released in a process called reuptake. SSRIs help make more serotonin available in the brain by blocking that reuptake process.

Examples of SSRIs include:

• Prozac (fluoxetine)
• Paxil (paroxetine)
• Zoloft (sertraline)
• Celexa (citalopram)
• Luvox (fluvoxamine)
• Lexapro (escitalopram)
• Viibryd (vilazodone)

Another group of serotonin-based medications for treating depression are known as serotonin-norepinephrine reuptake inhibitors (SNRIs). They work similarly to SSRIs in that they block the reuptake of serotonin, but they also work on norepinephrine, another neurotransmitter implicated in mood. As such, they are sometimes referred to as “dual-acting antidepressants.”

Examples of SNRIs include:

• Effexor (venlafaxine)
• Cymbalta (duloxetine)
• Pristiq (desvenlafaxine)
• Savella (milnacipran)
• Fetzima (levomilnacipran)

Two older types of antidepressants, tricyclics (TCAs) and monoamine oxidase inhibitors (MAOIs), also affect serotonin levels. However, they are prescribed less often today because their side effects are more problematic than those of SSRIs and SNRIs.

TCAs appear to block the reabsorption of serotonin and epinephrine which effectively increases the amounts available in the brain.

Examples of TCAs include:

• Elavil (amitriptyline)
• Norpramin (desipramine)
• Asendin (amoxapine)
• Anafranil (clomipramine)
• Pamelor (nortriptyline)
• Tofranil (imipramine)
• Vivactil (protriptyline)
• Surmontil (trimipramine)
• Sinequan (doxepin)

MAOIs, on the other hand, block the effects of the monoamine oxidase enzyme which breaks down serotonin, epinephrine, and dopamine. By stopping the enzyme from breaking these neurotransmitters down, it effectively increases the amount available in the brain.

Examples of MAOIs include:

• Zelapar (selegiline)
• Marplan (isocarboxazid)
• Parnate (tranylcypromine)
• Nardil (phenelzine)
• Azilect (rasagiline)

These medications aren’t the only way to increase available serotonin. In fact, there are many ways to boost your levels naturally.

Many of the foods we eat naturally contain serotonin or other key nutrients, including tryptophan, vitamin B6, vitamin D, and omega-3 fatty acids, which the body needs to produce the neurotransmitter. A few good sources include:

• Bananas
• Turkey
• Eggs
• Oily, fatty, fish (salmon, tuna, mackerel) 
• Nuts and seeds (walnuts and flaxseed)
• Beans (chickpeas, kidney, pinto, black beans)
• Leafy greens (spinach or kale)
• Probiotic/fermented foods (kefir, yogurt, tofu)

Eating a high fiber diet rich in vegetables and fruit can also help. These foods will help keep your friendly gut bacteria healthy, which in turn, can help boost serotonin levels in the body.

Insufficient exposure to sunlight has also been associated with low levels of serotonin, which is why some people experience seasonal affective disorder (SAD) during the shorter, darker days of fall and winter. While light therapy is an option (particularly in areas with little to no sunlight), you can also spend 10 to 15 minutes outside in the sunlight each day. Doing so will work double-duty: You'll also get a boost of vitamin D, which plays a key role in serotonin production.

Regular exercise has been proven to boost serotonin in your brain, and some say it works just as well as serotonin-based medications. How much and which type works best? The Department of Health and Human Services recommends at least 150 minutes of moderate-intensity cardio exercise each week plus two days of strength training exercises for all adults. It's most important that you pick something you enjoy, so you're more apt to stick with it.

Massage therapy has increasingly been used in the treatment of depression. This is because massage has been found to promote the release of serotonin and decrease the stress hormone cortisol.

And you might not need a professional massage to reap the benefits. In a commonly cited study of pregnant women with depression published in the International Journal of Neuroscience, two 20-minute massage sessions given by their partners increased serotonin by 28% and dopamine by 31%.

Research has found that people with low serotonin also have deficiencies in some nutrients, so you might consider supplementation:

• Pure tryptophan
• SAMe (S-adenosyl-L-methionine)
• 5-HTP
• St. John's wort
• Probiotics

Always talk to your doctor before taking any medication, supplement, or herbal remedy to treat low serotonin. Some medications and supplements can cause your serotonin levels to get too high, which can result in serotonin syndrome, also known as serotonin toxicity. Symptoms of serotonin syndrome can range from unpleasant to life-threating.