Arginine

What it is

Arginine is a proteinogenic amino acid, one of the building blocks from which keratin is also made. The body produces it in part itself and it occurs in food. In cosmetics it has two roles that are often filled at the same time in a product: it is a basic substance and is therefore used to adjust pH, and it counts as a moisture-binding conditioning ingredient.

With this record it is particularly worth keeping safety and efficacy apart. That the amino acid is well tolerated is one statement. Whether it reaches the hair fibre in a relevant quantity and does something structural there is quite another — and considerably less well established.

Why

• “Made by the body” is not a safety argument in itself. Quantity, manner of use and target group remain decisive — the same rule that applies to alanine.
• The classification “very low risk” rests on external use, the usually small quantities and the absence of indications of a relevant irritant or sensitising potential in this use. It is a statement about tolerability, not about effect.
• “No indications of a risk” is not the same as “evidence of safety”. That is why confidence is “medium” and evidence “C” here rather than better: the classification rests on the substance class and long use, not on a substance-specific evaluation.
• The efficacy claim is the weaker side. An effect on the internal hair structure presupposes that the substance reaches the fibre in sufficient quantity and stays there. No evidence for that is available here; the short fields claim more than this assessment supports.
• Often pH adjustment is the actual reason for using it. That is not a devaluation of the substance but a different and simpler explanation of why it appears in the list.

When the assessment differs

• Broken or badly irritated scalp: every substance behaves differently across a disrupted skin barrier. No particular expectation applies here, but the classification is for intact skin.
• Markedly higher concentrations, for instance in intensive treatments: not assessed here, because the classification assumes the usual small quantities.
• Leave-on compared with rinse-off products: longer contact, but no known turning point for the tolerability of this substance.
• Oral intake, for instance as a supplement: an entirely different context with its own yardstick. Statements from it cannot be transferred to external use, and just as little the other way round.
• Children: not assessed here.

Open questions

• Concentration in the actual product: not available.
• Evidence of efficacy: not evaluated. Whether and to what extent arginine reaches the hair fibre has not been examined here; the statement above describes the open question, not a result.
• Regulatory status: not checked; statements on use and maximum levels are not quoted second-hand.

Related substances

Applies to: Amino acids and peptides

• Acetyl Tetrapeptide-3 — Peptide (hair density, efficacy evidence limited)
• Alanine — Amino acid, moisture-binding

Further substances from the same group for which a long-form assessment already exists.