Nutrient glossary
What is phenylalanine?
Phenylalanine is the raw material for tyrosine, dopamine, noradrenaline and thyroid hormone.
Reviewed 20 July 2026. General information, not medical advice.
The short version
Phenylalanine converts to tyrosine, which then becomes dopamine, noradrenaline and adrenaline, the catecholamines of drive and stress response, plus thyroid hormone and skin melanin. The requirement figure covers phenylalanine and tyrosine together.
It is abundant in soy, cheese, meat, fish, eggs, nuts and seeds. People with the genetic condition PKU cannot break it down and manage intake medically, which is why aspartame carries a phenylalanine warning.
Daily reference
1.9 g a day
The adult requirement is about 25 mg per kg of body weight a day for phenylalanine plus tyrosine combined.
This is the fixed adult reference used across these pages. In the app, your amount adjusts to your body and way of eating.
Best food sources of phenylalanine
| Food | Per serving | Per 100g |
|---|---|---|
| Chicken Breast | 2.06 g | 1.21 g |
| Sirloin Steak | 1.74 g | 1.16 g |
| Ribeye Steak | 1.71 g | 1.14 g |
| Duck Breast | 1.61 g | 1.15 g |
| Roast Chicken | 1.61 g | 1.15 g |
| Tuna Steak | 1.42 g | 1.09 g |
| Pork Chop | 1.4 g | 1.08 g |
| Turkey Breast | 1.4 g | 1.17 g |
Common questions
- Why do diet drinks warn about phenylalanine?
- The sweetener aspartame breaks down to phenylalanine, which people with PKU must strictly limit. For everyone else the amounts are trivial.
- Does phenylalanine improve mood or focus?
- It is the precursor of dopamine and noradrenaline, but supplement trials show little effect in people eating enough protein.