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Amino acid

L-Phenylalanine

Phenylalanine is an essential amino acid that comes from food and is the starting material for tyrosine.

CognitionStress & Mood

What it is studied for

The strongest rated effects first. Bars show the size of the effect, the letter grades the evidence, and each rating lists the studies behind it.

Mental EnergyCognitionSmallCNo linked study
StressStress & MoodSmallCNo linked study
Mood SupportStress & MoodSmallCNo linked study
Brain FogCognitionSmallCNo linked study
Focus And AttentionCognitionSmallCNo linked study
MemoryCognitionNo effectFNo linked study
Every rating with its studiesGrouped by health goal, with the publications behind each rating

What is phenylalanine?

Phenylalanine is an essential dietary amino acid and a starting material for tyrosine. 1

Why people take it

Essential Health18.9%
Energy18.6%
Fitness17.2%
Immune Support11%
Cognition6.6%
Weight Management5.1%

SuppCo community snapshot, 2026-09-28. Reasons for use are not evidence that it works.

Questions people ask

Does an essential amino acid automatically improve mood?

Its nutritional role alone does not establish a treatment effect. The small historical depression study tested DL-phenylalanine and lacked placebo. 2

Why do products mention phenylalanine?

Aspartame contains phenylalanine, which people with PKU cannot process normally. 3

Can someone with PKU use the general safety assessment?

No. VKM explicitly excludes people with PKU from its reassuring conclusions for L-phenylalanine. 4

Sources

  3. Phenylketonuria MedlinePlus Genetics