L-tyrosine

What it is

L-tyrosine is an amino acid found in protein-rich foods such as meat, fish, dairy, eggs, nuts and soy. Your body also makes it from another amino acid, phenylalanine, so true deficiency is rare in people who eat enough protein.

It is a building block for dopamine, noradrenaline and adrenaline, and for thyroid hormones. That is why it is marketed for focus, mood and stress resilience.

It is sold in the UK as a food supplement. N-acetyl-L-tyrosine (NALT) is a common variant.

How it works

Brain levels of dopamine and noradrenaline can fall when you are stressed, sleep-deprived or mentally taxed. Taking extra tyrosine may help supply the raw material when those systems are under heavy demand. When demand is normal, extra tyrosine seems to add little.

What the evidence says

• Moderate Cognitive performance under acute stress (cold, sleep loss, multitasking)
  Several small studies suggest it helps preserve working memory or attention in demanding conditions; benefits in normal conditions are unclear.
• Weak Everyday focus and energy
  No convincing effect in well-rested people at ordinary workloads.
• Weak Mood or depression
  Trials have not shown a reliable benefit and it should not replace proper treatment.
• Limited Exercise performance
  Mixed results; some studies hint at less perceived fatigue but effects are small.

Evidence ratings are our plain-English summary of how much good human research exists, not a promise of results. How we rate evidence.

Dosage and timing

• Common supplement doses are 500-2,000 mg, taken once.
• Take on an empty stomach, because it competes with other amino acids for uptake into the brain.
• It is usually used occasionally before demanding situations rather than every day.
• There is no UK-specific upper limit; check current guidance and keep to label doses.

Forms and what to look for

• L-tyrosine is the standard form and is well studied.
• N-acetyl-L-tyrosine (NALT) is marketed as better absorbed, but evidence suggests it is converted poorly, so it may give less tyrosine per milligram.
• Check the label for milligrams of actual L-tyrosine, particularly in pre-workout or "nootropic" blends.

What to expect, and when

• Acute effects, if any, appear within an hour and last a few hours.
• Many people notice nothing obvious.
• Regular long-term use has little research behind it.

Side effects

• Nausea, heartburn or stomach upset.
• Headache.
• Jitteriness, restlessness or trouble sleeping, especially late in the day.
• Raised heart rate or blood pressure in some people.

Interactions

• Levothyroxine and other thyroid medicines: tyrosine may affect absorption or thyroid hormone levels; separate by several hours and ask a clinician.
• MAOI antidepressants: combining with tyrosine may cause a dangerous blood pressure spike. Avoid.
• L-DOPA (Parkinson's medicines): tyrosine can compete for absorption.
• Stimulants and caffeine: effects can stack, increasing jitters.

Who should avoid it

• Anyone taking MAOIs.
• People with an overactive thyroid (hyperthyroidism) or Graves' disease.
• People with high blood pressure or heart rhythm problems, without medical advice.
• People with phenylketonuria (PKU) should follow their specialist's dietary advice.
• Pregnancy and breastfeeding: insufficient data for supplemental doses.

Risks and safety

• Supplement doses are far above normal dietary intake; long-term safety is not well studied.
• Not on the WADA prohibited list.
• Pre-workout blends may combine tyrosine with stimulants, which raises the overall risk.

Buying well: quality and testing

• Choose products stating L-tyrosine and the mg per serving.
• Third-party tested brands (Informed Sport, NSF) are preferable if you compete in sport.
• Be cautious with proprietary blends.

Bottom line

Tyrosine looks most useful as an occasional aid in demanding, stressful situations, not as a daily focus booster. If you eat enough protein you probably already get plenty. Anyone on thyroid medicines or MAOIs should check with a pharmacist before trying it.