All Soaps Are Not Created Equal (And Which Are Best For Your Skin)

Brian Stanton

You park the car, walk towards the store. This should be a quick trip. All you need is a new bar of soap.

But when you reach the cosmetics aisle, you hesitate. There are literally dozens of soaps, and you’re unsure which are best for your skin.

The seconds tick by. An octogenarian arrives, makes her selection, and leaves - all while you vacillate over the great wall of soap.

So how do you choose a soap that doesn’t dry out, or irritate, your skin? Well, it all comes down to understanding the cleansing chemicals within each bar.

We’ll get there in a bit. But first, a little history on soap making.

A Brief History of Soap

Some 5000 years ago, the ancient Babylonians invented soap. This soap was formed by boiling fat with ashes, and was used for medical and cooking purposes. Not for personal hygiene.

In fact, it wasn’t until the the second century AD, according to records from the Greek physician Glen, that soap began to clean the human body. But even with Glen’s notes on record, soap wouldn’t be widely used for many years to come. That’s because the secret to making soap — a chemical recipe called saponification — wasn’t made public until 1775.

Saponification involves combining an acid like coconut oil or tallow with a base like lye (lye is derived from ashes) under hot conditions. This reaction forms a salt with soapy cleansing power.

When saponification became public knowledge in the late 18th century, an English company was the first to begin mass producing soap. By the early 19th century, several large American companies joined in and started pumping out soap too. The 19th century also saw the advent of modern plumbing. This plumbing, in the form of miles and miles of iron piping, contributed to the rise baths, showers and sinks across the civilized world. Which meant more soap was used.  

But it was only recently that we, as a species, leaped forward in soap technology.

That leap happened in 1955, when we created the first non-saponified soap bar. Like prior soaps, this new soap was derived from natural fats like coconut oil. But unlike prior soaps, this soap’s active ingredient — sodium cocoyl isethionate — wasn’t formed via the age-old soap-making process of saponification, but rather through a synthetic process called esterification.

These new soaps were called syndets and syndets were not only better at removing surface grime and dirt than saponified soaps, but were also less irritating to the skin. In other words, compared to saponified soaps, syndet soaps were a win-win.

Today almost all bar soaps can be categorized as either saponified soaps or syndet soaps. Now let’s review how these two categories of soap affect your skin.

Saponified vs. Syndet Soaps for Skin Health

Hold on a second. Aren’t “natural”, saponified soaps healthy for your skin? They were used by our ancestors, after all.

But our ancestors aren’t always the best models of behavior. In fact, research dating back to the early 1980s suggests that saponified soaps are more irritating than syndet soaps. While both kinds of soaps can potentially unbalance skin moisture and skin bacteria, syndet soaps appear to be milder and less drying to the skin itself.

Why? Because saponified soaps and syndet soaps have different active ingredients — different surfactants — powering their cleansing action.

Surfactants are compounds that penetrate and cleanse the skin. Saponified soaps like glycerin use natural surfactants —sodium tallowate, for instance — while syndet soaps use synthetic surfactants like sodium cocoyl isethionate. Research has shown that saponified surfactants tend to be more irritating than syndet surfactants. Let’s explore the science underlying this effect.

 First, compared to syndet soaps, saponified soaps bind to more proteins — stratum corneum proteins, to be precise — on the skin’s surface. This protein-binding action alters skin barrier function, causing it to dry out, lose elasticity and itch after the soap is washed off.

Surfactants not only impact proteins, but also skin lipids that help keep the skin moist and healthy. And saponified surfactants (surprise surprise) have been shown to deplete these lipids more than syndet surfactants.

Finally, the product’s PH, or alkalinity, matters. Alkaline cleansers, studies suggest, negatively impact both skin proteins and skin lipids. What cleansers tend to be alkaline? You guessed it: saponified soaps. Syndet soaps usually have a lower PH, ranging from acidic (5.5) to neutral (7).

Yet the skin benefits of syndet soaps aren’t merely theoretical. Researchers have shown, in fact, that regular skin cleansing with syndet soaps improves skin health - even in individuals with sensitive skin.

But how does soap affect skin bacteria? Let’s see.

Soap and Skin Bacteria

Saponified soap not only dries out skin, but may also impact the bacteria colonizing the skin’s surface.

Take, for instance, Ammonia-Oxidizing (AO) bacteria. AO bacteria consume sweat and urea on the skin, and help balance the skin microbiome. Unfortunately, since saponified soap appears to be toxic to these helpful critters, it means that humans have likely been abusing our skin biomes since the time of the pharaohs.

This isn’t to say that syndet soaps support healthy levels of bacteria on the skin. Even if syndet soaps are less irritating than saponified soaps, we still don’t fully understand their effects on skin microbes. For instance, researchers have found that skin-based microbial colonies stay fairly stable, even at skin sites - like the palm - exposed to frequent washing. Bottom line: there’s still much to learn about what affects the skin biome.

The Reality of Bar Soap Today

Today much of the world uses bar soap. (Most of the US population, in fact!). And what kind of soap does the world use? If you guessed saponified soap, you’re right.

And yet saponified soaps have been drying out skin for thousands of years, and perhaps even destroying skin biomes for thousands of years too. The Babylonians can be excused for not knowing this. But here in 2018, we can claim ignorance of this fact no longer.  

How to Choose a Soap

You’re back in the cosmetics aisle, facing the wall of soap. You probably, after reading this article, want a syndet soap. But how can you tell which soaps are which?

Try this. Look for sodium cocoyl isethionate (the most commonly used surfactant in syndets), and avoid sodium tallowate and sodium cocoate from saponified soaps. That should help narrow things.

And whatever soap you choose, no need to wash five wash times a day. Just cleanse when needed, and not only will your bar soap live longer, but your skin bacteria might live longer too.

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