Tryptophan

In this Article (Index)

• Tryptophan
• Functions
• Dietary Sources: Foods rich in Tryptophan
• Recommended Daily Allowance
• Health Effects and Properties of Tryptophan
• Risks and Side Effects

Tryptophan

Tryptophan is an essential amino acid, which means that your body can't produce it, you must get it from your diet.

It is found in the smallest concentration of all 20 amino acids in the human body, but it plays a critical role in many essential metabolic functions. (6)

Tryptophan was discovered in 1901 in the milk protein casein. It was named after the enzyme trypsin used to isolate it from casein. Trypsin in turn comes from the Greek "tripsis" = rubbing, or friction, because this enzyme was discovered after rubbing the pancreas with glycerin. The suffix "-phan" comes from the Greek word "phainein" = to bring to light, to show. In other words, the enzyme trypsin allowed the discovery of this amino acid.

Functions

It takes part in the synthesis of the hormone melatonin, which regulates sleep-wake cycles, and immunity, and serotonin helps regulate appetite, sleep, mood, and pain. It is linked to dopamine and modulates prolactin, growth hormone, and cortisol. It is used by the body to produce niacin (vitamin B3). (6),(1)

Tryptophan is used by the body in two main metabolic pathways: (2)

1. The kynurenine pathway. By which it forms nicotinic acid, a precursor to nicotinamide (Vitamin B6) and adenine dinucleotide (NAD) coenzymes.
2. The serotonin pathway. That produces a neurotransmitter called serotonin, and the hormone melatonin.

We will look into both pathways in detail in the following sections.

L or D-Tryptophan?

Organic molecules can have an identical composition (carbon, hydrogen, oxygen, etc) but have a different shape, like your left and right hand are identical, but symmetric. This symmetry alters their properties. Almost all amino acids found in animals and plants are known as L-type because if placed in suspension in water, they bend polarized light towards the left (the D-type bends it to the right). However, when produced by man-made technologies, usually equal quantities of L and D-type amino acids are produced.

Dietary Sources: Foods rich in Tryptophan

Recommended Daily Allowance

Diet is the only source of tryptophan as your body cannot synthesize it.
The recommended daily allowance suggested by the World Health Organization for adults is 3.5 to 6.0 mg/kg of body weight per day. For an average person, this means between 250 and 425 mg/day. As the average dietary intake in America is between 900 and 1,000 mg per day, on average there is no deficiency of tryptophan in the U.S. (6)

Tryptophan, Protein, Carbohydrates and Alcohol

What you eat affects how your body absorbs tryptophan. Its availability can be improved by eating it with carbohydrates, and it worsens if you ingest it with protein or alcohol.

Carbohydrates decrease the concentration of other amino acids that compete with tryptophan. Protein has a higher proportion of competing amino acids compared to the low concentration of tryptophan. Even tiny amounts of protein (4%) in a carbohydrate-rich meal can lower the availability of tryptophan.

Alcohol decreases the ratio of tryptophan vs. other amino acids by around 10% in 30 minutes and 20 to 25% about 11/2 hours after drinking.

The following table shows the amount of L-tryptophan in some common foods, expressed in milligrams (mg). The last column shows the "ratio" between tryptophan and competing amino acids. Absorption of tryptophan by the brain depends on the relative amounts of other amino acids trying to cross the blood-brain barrier. More competing amino acids mean a lower chance for tryptophan to enter the brain.

The competing amino acids are Isoleucine, Leucine, Phenylalanine, Tyrosine, and Valine.

Content of Tryptophan in Different Foods

A higher "ratio" value means a better absorption of tryptophan by the brain. (6)

Health Effects and Properties of Tryptophan

• Serotonin
• Melatonin, tryptophan, and sleep
• Niacin Synthesis
• Other Functions
• Cancer Treatment

Serotonin

Serotonin is a chemical produced by your nerve cells using tryptophan.

Only 3% of dietary tryptophan is used for serotonin synthesis throughout the body, and one-third of it is used for serotonin synthesis in the brain where it plays an important role in regulating mood, sleep, sexual desire, memory, and social behavior.
However, roughly 95% of serotonin is found inside the gastrointestinal tract where it modulates bodily functions such as appetite, digestion, inflammation, and immunity.

Serotonin levels regulate mood, and low levels of it are linked to anxiety and depression. This suggests that an increase of tryptophan levels in the blood could improve its uptake in the brain and boost serotonin synthesis.

A randomized study (4) using 25 healthy adults analyzed the effects on anxiety, mood, and depression after consuming both a high-tryptophan diet with more than 10 mg/kg of body weight per day, and a low-tryptophan diet (less than 5 mg/kg; per day). It disclosed that "the participants’ mood indicated significantly more positive affect scores after consuming a high tryptophan diet as compared to a low tryptophan diet... Also, consuming more dietary tryptophan resulted in less depressive symptoms and decreased anxiety."

Melatonin, tryptophan, and sleep

Melatonin is a hormone produced by the brain's pineal gland using serotonin. It is what makes you sleepy when it gets dark. (6)

Found in plants, animals, and even bacteria, where it originated billions of years ago. Its original function was as an antioxidant, and it evolved to cover many roles in the immune, digestive, and reproductive systems. It is also a factor in Winter Blues or Seasonal Affective Disorder (SAD).

A trial followed 35 subjects aged 55 to 75 years, for 3 weeks who were split into two groups: the control group received 22.5 mg of tryptophan per dose, in enriched cereals for breakfast and dinner, and the treatment group received 60 mg per dose. The results showed that "the higher dose in tryptophan increased sleep efficiency, actual sleep time, immobile time, and decreased total nocturnal activity, sleep fragmentation index, and sleep latency." (5)

Niacin Synthesis

The liver uses tryptophan to manufacture niacin (vitamin B3). Roughly 60 mg of tryptophan generates 1 mg of niacin.

Vitamin B3 plays a role in energy metabolism and DNA production. Since the daily allowance of niacin is 14 mg/day for women and 16 mg/day for men, and the average intake of niacin through food in America is much higher (41mg/day for men, and 28 mg/day for women). (6)

So, people eating a balanced diet would not require additional tryptophan for the synthesis of niacin. However, people who do not consume enough iron, riboflavin (vitamin B2), and pyridoxine (vitamin B6) convert less tryptophan to niacin because the chemistry in this process requires those nutrients to function properly. (1)

Other Functions

Tryptophan is linked to several neurotransmitters produced in the brain whose levels increase after an intake of tryptophan like dopamine, β-endorphins, and norepinephrine or noradrenaline. These play a role in memory, mood, emotions, motivation, attention, pleasure and reward, and the body's fight or flight response. It may also be involved with hormones like cortisol, prolactin, and growth hormone.

Cancer

Tryptophan levels modulate immunity. An increase in tryptophan levels activates the immune system's T cells. But tryptophan is broken down into simpler molecules or catabolites in a process known as catabolism, by an enzyme called indoleamine-2,3-dioxygenase (IDO) and these catabolites tend to deactivate and suppress the T cells. Certain cancer cells (gastric, renal, and colon) produce large amounts of IDO and blunt the immune reaction provoked by a higher tryptophan intake. However, a study shows that tryptophan used in combination with IDO inhibitors would be expected even more powerful strategy to fight against cancer. Further studies are needed to validate this hypothesis. (3)

Risks and Side Effects

Very high doses (200 mg per kg of body weight per day) can have nasty side effects such as dizziness, tremors, and nausea.
Serotonin syndrome, a rare condition can occur and it has serious symptoms like delirium or coma.

The risk of serotonin syndrome increases with the use of certain medications such as Prozac or Zoloft (selective serotonin reuptake inhibitor medications) combined with high doses of tryptophan. (4)

Tryptophan supplementation has adverse interactions with monoamine oxidase inhibitors (MAO inhibitors) used as antidepressants wich can cause transient serotonin syndrome.