Alpha Lipoic Acid

Alpha lipoic acid — listed on ingredient labels under its INCI name thioctic acid — is a small sulphur-containing antioxidant built around an eight-carbon fatty acid backbone related to caprylic acid, capped with a five-membered ring holding two sulphur atoms. The body makes tiny amounts of it naturally, where it acts as a cofactor for enzymes involved in energy production, and traces occur in foods such as spinach, broccoli and offal. The material used in skincare is produced synthetically.

What makes it unusual among cosmetic antioxidants is its solubility. Most antioxidants are at home in either water or oil; alpha lipoic acid is amphipathic, meaning it can act in both the watery and the fatty parts of skin. In the bottle it is actually awkward to work with — it dissolves poorly in plain water, so formulators usually carry it in a glycol, alcohol or an emulsion before blending it into a cream or serum.

In finished products it appears as a leave-on active, typically somewhere between about 1% and 5%, and is more often paired with other antioxidants than used alone. In raw form it is a pale yellow powder with a faintly sulphurous odour, which is why alpha lipoic acid formulas sometimes carry a yellow cast and a slight smell — normal, not a fault. It is also notably unstable in light and heat, so well-made versions arrive in opaque, air-restricted packaging rather than clear jars.

Once applied, alpha lipoic acid's dual solubility lets it distribute through both the watery spaces between skin cells and the lipid-rich layers of the surface barrier. Laboratory work using a 5% solution on animal skin found it crosses the outermost layer readily, with levels in the deeper layers peaking within roughly two hours.

Skin can convert alpha lipoic acid into its reduced form, dihydrolipoic acid, and this reduced form carries out much of the antioxidant activity. Both forms neutralise free radicals — the unstable molecules generated by UV exposure, pollution and everyday metabolism that contribute to the visible signs of ageing. In laboratory conditions dihydrolipoic acid can also help regenerate other antioxidants that have already been spent, including vitamins C and E and the skin's own glutathione — though most of that work sits outside topical use. It is why alpha lipoic acid is usually described as supporting a whole antioxidant network rather than working in isolation.

Honesty matters here: direct evidence for what topical alpha lipoic acid does on human skin is thinner than the enthusiasm around it suggests. In the best-known controlled facial study, a 5% cream outperformed its placebo, but the placebo side improved substantially too — a reminder that consistent moisturising accounts for a good deal of what people notice.

Alpha lipoic acid is a leave-on active, best used in a serum or moisturiser rather than anything you rinse away.

• Evening is the sensible slot. The molecule degrades quickly in sunlight, so night use keeps it away from UV while it sits on skin. Keep the lid closed and store away from windows and radiators. If you prefer it in the morning, finish with a broad-spectrum SPF.
• Start low. Around 1% is a comfortable introduction. The 5% level used in research is the better-studied one, but it is far more likely to tingle, sting or feel warm, particularly around the nose and eyes.
• Apply to clean, dry skin before heavier creams and oils, and follow the frequency on your product's packaging — twice daily was used in the facial research, though once daily suits most people.
• Give it three months. The controlled study ran twelve weeks; changes in the look of texture and fine lines are gradual, not overnight.
• Don't stack it with retinoids or strong exfoliating acids on the same night while you build tolerance.