What’s sweet to your gastrointestinal tract may not be so kind to your skin. Papain, an enzyme derived from the papaya fruit, has long been relied upon, in supplement form, to aid the digestive process, especially the breakdown and absorption of proteins. It’s generally considered to be a safe digestive aid, although there can be some allergic reactions, especially in people allergic to mangoes or kiwis. If most people can put the stuff inside their bodies, you might figure there should be no problem putting it on skin, right?

Well, here’s a surprise. A new study from Vienna, Austria, found that when this natural enzyme is applied topically, it can cause allergic skin reactions. What’s worse, it’s in so many products that you may not be aware of exactly what’s causing your skin to itch.

REMOVING DEAD SKIN…AND CAUSING ALLERGIES

The cosmetic industry uses papain in many different skin-care products, shampoos and conditioners and even in enzymatic contact lens cleaners to remove protein deposits. When it’s applied to the skin, it helps remove dead cells from the surface, revealing fresh, healthy cells that lie beneath and improving skin texture and appearance. It may also open clogged pores. No wonder it’s used in exfoliating products such as facial scrubs, body cleansers, facial masks and peels. In hair-care products, papain conditions and softens dry or damaged hair, But there’s a dark side.

In a recent study, Austrian researchers put papain directly on the skin of mice as well as on human skin cells in the petri dish. Within a short time (30 to 120 minutes), the papain…

• Compromised the integrity of the skin barrier by degrading the “tight junctions” that join skin cells together. That makes it easier for other compounds…including nasty ones…to penetrate your skin’s natural protective barrier.

• Increased water loss, contributing to drier skin.

• Induced vasodilation—a widening of blood vessels—which can cause skin to become warm, red and itchy.

• Stimulated the release of inflammatory cells in the skin (which can lead to irritation)…including mast cells, which release histamine as part of allergic reactions.

After repeated exposure, the mice developed antibodies to papain—a sure sign of an allergic response. (Papain, it turns out, is structurally very similar to a very common dust mite allergen.) All of these effects are problematic on their own but, adding insult to injury, compromising the skin barrier function could allow other chemicals to penetrate the skin more deeply, setting the stage for further irritation or allergic responses. This is a particular concern for people who have eczema, whose skin is already susceptible to bacteria, fungi and viruses. The researchers’ conclusion: Papain has all the characteristics of a strong allergen.

The FDA, it turns out, already strongly warned manufacturers against selling unapproved ointments that contain papain as a treatment for serious skin conditions such as diabetic ulcers and traumatic wounds, based on allergic reactions. That was back in 2008. The new research makes us wonder if we should rethink its place in everyday cosmetics, too.

The take-home message: If you have sensitive skin or you’re prone to skin redness or irritation, especially after using new products, read product ingredient lists and steer clear of skin- and hair-care products that contain papain. The allergy is related to latex allergies, the researchers find, so that if you’re allergic to latex, you should avoid papain-containing products as well. (Look for other names as well—papainase, papaine, summertrin, tromasin, vegetable pepsin, velardon.)

If your skin is at all sensitive, you are prone to allergic skin reactions or you have a skin condition such as eczema, there’s no need to take a chance on papain.