Milk is immensely rich in nutrients, including proteins, lipids, vitamins, minerals, and, of course, the ingredient that is responsible for its cell-regenerating abilities: lactic acid.
Today, lactic acid is used in aesthetics to deliver a host of benefits to the skin and correct signs of aging, 

What is Lactic Acid?
L-lactic acid is a keratolytic alpha hydroxy acid that is also known as ‘milk acid.’ It is gentler than glycolic acid and provides exfoliation without provoking irritation. It also softens the skin, increases desquamation, stimulates cell regeneration, improves the skin’s texture, and has natural brightening abilities.
Lactic acid is also chiral in that it contains two optimal isomers. Essentially it has two parts: a left-handed part and a right-handed part. This is why lactic acid is often accompanied by an ‘L.’ This letter denotes the chirally-correct molecule of the acid is being used. The ‘L’ form is absorbed more easily by the skin and increases the overall performance.
The body naturally produces lactic acid during normal metabolism and exercise. When the demand for energy spikes in the body, such as during strenuous or power workouts, glucose is broken down and oxidized to pyruvate, which then stimulates the production of lactate. This process is beneficial because it helps ensure energy production is maintained.1
Even during rest, lactate continues to be produced as a result of metabolism in red blood cells that lack mitochondria. Some research has even shown lactate to play an important role in early-stage development for brain metabolism and as an energy source for the brain in the metabolism of neurons.2
L-lactic acid is an important alpha hydroxy acid that may be used in peels, facials, and homecare regimens to help correct the signs of aging, hyperpigmentation, some acne and acne scarring, uneven tone and texture, and more. While many alpha hydroxy acids are derived from fruit, lactic acid is most-commonly derived from fermented milk or sugar and is considered a non-toxic, active, and organic substance.

Common Uses for Lactic Acid
Although L-lactic acid may be used to help correct numerous skin issues, it is particularly beneficial in the treatment of aging. It also works well for sensitive skin and skin of color, which may be more prone to dryness, post-inflammatory hyperpigmentation, and discoloration. Acid selection will, of course, always vary by client and should be chosen based on their specific skin type and goals.
In treating aging skin, L-lactic acid really shines. It stimulates cell turnover and collagen production to firm sagging skin, sheds pigmented cells, brightens the overall tone, and delivers hydration beyond the surface. Because of its exfoliating capabilities, this acid allows for more efficient penetration than other pro-youth ingredients.
It also increases dermal and epidermal thickness, thus revealing more firmness and less fine lines and wrinkles. While lifting epidermal cells, lactic acid stimulates the synthesis of collagen and hyaluronic acid at the cellular level, resulting in smoother and younger-looking skin.
In the treatment of acne, L-lactic acid helps to loosen follicle impactions, reduce corneocyte cohesion, and thicken the corneum layer. It also helps stimulate cell turnover and tissue regeneration while delivering important hydration and brightening benefits. For hyperpigmentation and other sun-induced discolorations, L-lactic acid is beneficial for many skin types and provides exfoliation and skin-brightening support. It is also gentler than glycolic acid and typically does not induce skin irritation.
When used in peel formulas, L-lactic acid will typically come in concentrations ranging from 10 to 75 percent. In most professional treatment scenarios, 10 to 25 percent will be most beneficial for providing the exfoliation, hydration, and regeneration that is sought out without introducing unnecessary risks and the potential for complications. Twenty percent, for instance, is an ideal choice for a second cleanse or rinse because it will lift away surface residue without stripping excess fats and lipids. The efficacy of an alpha hydroxy acids is not solely based on the percentage used in the formula, however. In fact, the pH and base formula are the key determining factors in the effectiveness of an alpha hydroxy acid. The lower the pH, the more intense the acid; a good acid pH range to stay within is 2.5 to 3, particularly for peel treatments.3

Getting the Most Out of Lactic Acid
Skin care professionals can get the most out of lactic acid by teaming it with other synergistic ingredients, working with quality formulations, and applying it to the right skin types. When these actions are taken, L-lactic acid will assist in producing optimal results for clients.
Other ingredients that work well with L-lactic include glycolic acid; humectants; physical exfoliators, like jojoba beads; melanin suppressants; and other enzymes, such as bromelain. L-lactic acid works to pave the way for these other nourishing and corrective ingredients to penetrate and go to work. For instance, when teamed with a humectant like hyaluronic acid, hydration is intensified. With enzymes like bromelain and papain, cell digestion and detoxification increases and the overall effectiveness is enhanced. When used in treatments with melanin suppressants, the brightening effects are also increased. Lower-percentage L-lactic acids may also be combined with physical exfoliators, like jojoba beads, to provide a deeper cleanse prior to a peel or enzyme application.
The combination of glycolic acid and L-lactic acid is extremely beneficial in inducing cell turnover and hydration, particularly in rougher or thicker skin conditions. Glycolic acid, which is also an alpha hydroxy acid, removes dead cells, smooths skin texture, and reduces the signs of wrinkles. Since it has a small molecular size, it is able to penetrate cellular walls quicker than L-lactic acid and actively stimulate collagen production and cell turnover. With the hydrating, skin-softening, and exfoliating properties of L-lactic acid, the two acids will strip away dead cells and promote the growth of new ones, without being too harsh, as well as restore hydration and pH balance.

Contradications and Neutralization
Since lactic acid is such a versatile acid, it works well with numerous other ingredients and layers with specific peels that provide exfoliation and healing properties. Do keep in mind, however, the combinations will greatly depend on the client’s skin, what it can tolerate, and what the desired outcome is. Do not blend ingredients if there is any uncertainty about the outcome as it may produce undesired complications.
It is also important to note that while lactic acid does work well for most all skin types, alpha hydroxy acid-sensitive skin and rosacea-affected skin typically responds better with other acids, so always use caution. It is not recommended to use alpha hydroxy acids when working with clients who are pregnant or lactating; on certain acne medications, such as Acutane; are planning to have exposure to the sun in the coming days; or are currently recovering from a sunburn.
As with any acid, having a clear picture of the client’s skin before performing any kind of corrective treatment or prescribing homecare is important. During treatment, be sure to ask the clients if they feel any sensitivity and closely monitor their skin. Several rinses with cool water are generally enough to neutralize and remove the acid. A neutralizing solution is important in the treatment room when working with alpha hydroxy acids over 10 percent and a pH below 3.
Finally, educate the clients about the importance of avoiding sun exposure and applying a daily SPF to prevent deeper damage to the skin. This is always important, but when clients are using any sort of acid, it is imperative.

When properly used, L-lactic acid is an invaluable tool in the treatment room and at home to correct the visual signs of aging, effectively treat acne, and brighten and even the skin tone. Prescribing clients to take a daily milk bath is not necessary, but a daily dose of lactic acid will provide many benefits for their skin.

Rhonda Allison, a pioneer in the skin care industry, is the founder and CEO of Rhonda Allison Cosmeceuticals and RA for Men. Allison is also an author and an internationally-known speaker with more than 30 years of aesthetic experience. rhondaallison.com and ramethod.com

Refrences
1 McArdle, Katch & Katch (2010). Exercise Physiology: Energy, Nutrition, and Human Performance. Wolters Kluwer/Lippincott Williams & Wilkins Health.
2 Zilberter,Y., Zilberter, T., & Bregestovski, P. (2010). Neuronal activity in vitro and the in vivo reality: the role of energy homeostasis. Trends in Pharmacological Sciences, 31(9), 394–401.
3 Allison, R. (2000). The Art of Skin Peels: A Step by Step Peel Guide for Professionals. 13

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