Tryptophan: function, sources and significance for the body | Hardworkout Magazine

Key takeaways

- Tryptophan is an essential amino acid, which means the body cannot produce it and must obtain it from the diet.

- Most tryptophan is not used to make neurotransmitters, but for protein synthesis and energy-related processes such as the production of NAD⁺.

- How much tryptophan reaches the brain depends on its ratio to other amino acids in the blood, because they share the same transport mechanism across the blood-brain barrier.

- The conversion of tryptophan to serotonin runs via 5-HTP and depends on enzyme activity and nutrients including B vitamins, vitamin D, iron and magnesium.

- Serotonin is the precursor to melatonin, so serotonin levels in the brain influence how much melatonin can be produced.

Tryptophan is an amino acid that most people consume daily, but few know what it actually does in the body. It is involved in several processes that are important for both physical and mental function.

What tryptophan is actually used for is influenced by more than just intake through diet. To understand this, one must take a closer look at how tryptophan is metabolized and regulated.

What is tryptophan?

Tryptophan is an essential amino acid, which means that the body cannot produce it on its own and therefore must obtain it through the diet.

It is involved in protein synthesis and is necessary for the building and maintenance of muscles and other tissues.

In addition, tryptophan acts as a precursor to neurotransmitters such as serotonin and melatonin. This makes it relevant for the regulation of, among other things, sleep and several processes in the brain.

At the same time, most of the tryptophan is used for other purposes in the body, including the production of NAD⁺, which is important for the cells' energy production.

Research shows that a large portion of tryptophan is broken down in processes other than the formation of neurotransmitters and is involved in reactions that are important for the energy metabolism in the cells.

Which foods contain tryptophan?

Tryptophan is found in protein-rich foods such as meat, fish, eggs, and dairy products, but also in plant-based sources like legumes, nuts, and seeds.

The total intake will therefore largely follow how much protein you consume throughout the day.

How does tryptophan get into the brain?

For tryptophan to be used for the production of neurotransmitters in the brain, it must be transported from the blood and across the blood-brain barrier.

This transport occurs via a mechanism shared with several other amino acids. This means that tryptophan competes with these for transport into the brain. How much gets through therefore depends on the proportion of tryptophan compared to other amino acids in the blood.

A high protein intake increases the levels of several amino acids simultaneously. This increases competition and can reduce the proportion of tryptophan that is transported into the brain.

Carbohydrates indirectly affect this balance through insulin. When insulin is released, several of the competing amino acids are taken up by muscle cells. Tryptophan is more bound to transport proteins in the blood and is therefore taken up by muscle cells to a lesser extent than several of the other amino acids.

[Studies](https://www.frontiersin.org/journals/nutrition/articles/10.3389/fnut.2023.1140054/full) show that this is not just about how much tryptophan you consume, but also about the ratio to other amino acids in the blood, which affects how much is actually transported into the brain.

What decides whether tryptophan becomes protein or a neurotransmitter?

Even though tryptophan is available in the body, it is not guaranteed how it will be used further. The body distributes amino acids according to need, and tryptophan is involved in several different processes simultaneously.

A large portion is used for basic functions such as protein synthesis and energy-related processes in the cells. At the same time, a smaller portion may be used for the production of signaling substances in the brain.

How much goes to the various processes is influenced, among other things, by energy availability, nutritional status, and the body's needs at any given time. This means that supply alone does not determine the outcome, but rather how the body prioritizes usage.

How is tryptophan converted into serotonin?

A portion of the tryptophan that is transported into the brain is used for the production of serotonin, but this does not happen directly.

First, tryptophan is converted to 5-HTP (5-hydroxytryptophan) with the help of the enzyme tryptophan hydroxylase. Then, 5-HTP is further converted to serotonin in a new enzyme-driven step.

Both of these steps depend on enzyme activity and the availability of nutrients involved in the process. This means that even if tryptophan is present, it is not guaranteed how much is actually converted to serotonin.

According to recent [research](https://www.sciencedirect.com/science/article/pii/S2161831325001620), this process is influenced by B vitamins, vitamin D, iron, and magnesium, which are part of the enzymes that control the conversion from tryptophan to serotonin.


What does serotonin actually do in the body?

Serotonin is often referred to as a mood signaling substance, but its function is much more extensive.

It is involved in regulating attention, impulse control, sleep, appetite, and how the brain responds to stress.

In the brain, serotonin affects how signals between nerve cells are regulated. It matters for how smoothly you can maintain focus, how you handle impulses, and how you react when exposed to stress. It also influences how strong the reactions are and how quickly they calm down.

At the same time, a large portion of the body's serotonin is found in the gastrointestinal system. Here, it helps regulate bowel movements and affects both digestion and satiety.

Serotonin is also involved in other processes in the body, including the regulation of blood vessels and in mechanisms involved in wound healing.

Serotonin is also the precursor to melatonin, the hormone that regulates the circadian rhythm.

How does tryptophan affect melatonin and the circadian rhythm?

Melatonin is formed from serotonin through a further transformation in the brain, where serotonin is first converted to N-acetylserotonin and then to melatonin.

This process mainly occurs in the pineal gland and is regulated by the body's internal circadian rhythm.

The production of melatonin is closely linked to light and darkness. Signals from the retina are sent to areas in the brain that function as the body's “clock,” and these control when the pineal gland should produce melatonin. Light inhibits production, while darkness initiates it.

Melatonin is significant for when you feel tired in the evening and when you wake up in the morning. It helps keep the circadian rhythm stable, so that sleep occurs at approximately the same time from day to day.

Since melatonin is formed from serotonin, the levels of serotonin in the brain will affect how much can be further produced into melatonin. Tryptophan thus indirectly plays a role in regulating the circadian rhythm.


Sources:

A Comprehensive Review of Nutritional Influences on the Serotonergic System – [Advances in Nutrition/ Science Direct](https://www.sciencedirect.com/science/article/pii/S2161831325001620)

Tryptophan metabolism in health and disease – [Cell Metabolism/ Science Direct](https://www.sciencedirect.com/science/article/pii/S1550413123002140)

Tryptophan intake, not always the more the better – [Frontiers in Nutrition](https://www.frontiersin.org/journals/nutrition/articles/10.3389/fnut.2023.1140054/full)

Melatonin – [Neuroscience/ Science Direct](https://www.sciencedirect.com/topics/neuroscience/melatonin)