A few years ago, the popular "Got Milk?" ad campaign encouraged us to drink up to maintain bone mass and to stay energized. But the idea that "milk does a body good" also applies to the skin. Milk is a good source of lactic acid, a natural alpha-hydroxy acid (AHA) known for its skin-care benefits. Used in lotions, gels and bath tablets, it gently exfoliates the top layer of skin cells to reveal softer, younger-looking skin.

"Part of the appeal of lactic acid is that it creates very little inflammation as it assists in new cell growth," says Penny Kendall-Reed, a naturopathic doctor at the Urban Wellness Centre in Toronto. Since lactic acid already exists in our cells, protecting against injury and UV damage, the body recognizes and accepts it more readily than it would a harsher synthetic or fruit-derived AHA, according to Kendall-Reed. "It’s like a milder version of a chemical peel," she says.

A better whey

Using milk to treat the skin is not new. Cleopatra supposedly maintained her skin with daily milk baths. But you don’t have to be a spoiled royal to get the benefits. And immersing yourself in a tub of homogenized milk will have no noticeable effect on the skin, according to Dr. Sheetal Sapra, a dermatologist at the Institute of Cosmetic and Laser Surgery in Oakville, Ont. The benefits only occur when the milk has gone sour and the lactose has been converted into lactic acid.

Say when

So how do you choose the product that is best for you? Kendall-Reed suggests that people with sensitive skin who normally find AHAs too harsh may respond well to products containing a low dose — around five to eight percent — of lactic acid. Most over-the-counter milk-based products contain anywhere from five to 20 percent lactic acid, whereas professional treatments can contain as much as 50 percent. Sapra recommends starting with a low-dose product to see how it feels on your skin. "Microscopic swelling is important," he says. "It will plump up the skin, reducing the appearance of fine lines. But if there is noticeable swelling, redness or pain, you’ve used too strong a product." Allow your skin enough time to heal before trying a lower-dose product.

A good daily regimen, according to Sapra, consists of a facial scrub and a mild cleanser containing a low dose of lactic acid, then an AHA-free day cream with an SPF of 15 or more, followed by a night cream with a low dose of lactic acid to slough the skin while you sleep.

As it exfoliates, lactic acid diminishes blemishes, like sunspots and acne scars, but it also induces collagen and elastin production. "Studies have shown, histologically, that AHAs make the epidermis thicker and healthier by stimulating chemical reactions in the skin and improving the way the epidermis actually grows," says Sapra. "You’ll get smooth, glowing skin in very little time."

Products with a higher percentage can be used "once every couple of months or for a few days before a big event," says Kendall-Reed. But don’t let good results spur you on to more frequent treatments or higher-dose products. "Your skin can become irritated," says Sapra.

But you’ll know you’re using lactic acid in just the right concentration when your "compliment ratio" goes up. "I use this term in reference to AHAs, peels, Botox and collagen," explains Sapra. "If people are telling you that you look better, you are doing something right. What they are really noticing is your skin!"

Lactic-acid infused products top to bottom:
• Bliss Minty Moisture Milk
• Yves Rocher Bio Specific Active Sensitive Derma-Care Skin Defense
• Lothantique Milk Shea Butter Soap