Kareishu: Why Older Adults Smell Different

The Chemistry: What's Actually Happening

It's one of those things nobody wants to talk about. You walk into your mother's home and notice it immediately — a faint, musty, slightly waxy scent that wasn't there five years ago. It clings to the couch cushions, the pillowcases, the collar of her favorite cardigan. You've tried opening windows. She showers daily. It doesn't go away.

Most people assume this is a hygiene issue. It isn't. And here's what makes it harder: the person producing the odor may already be aware of it — and quietly mortified.

The distinctive scent associated with aging has a name: 2-nonenal. It's an unsaturated aldehyde — a volatile organic compound with a characteristic greasy, grassy, slightly musty odor. It was first identified in 2001 by researchers at the Japanese cosmetics company Shiseido, published in the Journal of Investigative Dermatology.

Here's the biochemistry in plain language:

The skin constantly produces an oily substance called sebum through sebaceous glands. Sebum contains omega-7 unsaturated fatty acids. In younger skin, the body's natural antioxidant defenses keep these fatty acids stable. But starting around age 40, two things change simultaneously: the skin produces more omega-7 fatty acids, and the antioxidant defenses that prevent their breakdown weaken.

When these unprotected fatty acids hit oxygen in the air, they undergo a process called lipid peroxidation — essentially, they oxidize and break apart. One of the key byproducts of that breakdown is 2-nonenal.

The Shiseido team found that 2-nonenal was detectable in subjects aged 40 and older, and that levels correlated directly with the concentration of omega-7 fatty acids and lipid peroxides on the skin surface. Subsequent research has shown that 2-nonenal levels can increase up to six-fold compared to adults under 40.

This isn't about how often someone showers. It's about age-related changes in skin biochemistry that are as inevitable as graying hair. Older adults often know something has changed. Some notice the smell on their own clothing or bedding. Others become aware through social cues — a grandchild's offhand comment, a sense that people keep a certain distance. This is not a sign of neglect or a hygiene failure.

Why Regular Soap Doesn't Solve It

Here's where most well-meaning advice falls apart — and where the chemistry matters.

Soap is an amphiphilic surfactant. It's specifically designed to solubilize fats and pull them into an aqueous solution so they rinse away. And regular soap does remove some nonenal from the skin's surface. So why doesn't frequent showering solve the problem?

Two reasons. First, 2-nonenal binds to skin proteins and embeds itself in the outer layers of the skin in ways that simple surface cleansing doesn't fully address. A shower reduces the odor, but doesn't eliminate it — some nonenal remains even after thorough washing. Second, nonenal is being continuously produced. Lipid peroxidation is an ongoing process driven by the skin's age-related loss of antioxidant capacity. Whatever nonenal is removed during a shower is being replenished within hours.

This is why extra scrubbing, baking soda baths, antibacterial soaps, vinegar rinses, stronger fragrances, and essential oils all fail: they either target the wrong odor pathway (bacterial breakdown of sweat), mask the smell temporarily, or incompletely remove a compound that is immediately replenished.

What the Research Actually Supports: Persimmon Tannins

The most promising approach comes from an unlikely source: the Japanese persimmon fruit (Diospyros kaki).

The Japanese word for age-related body odor is kareishu. Researchers have focused on persimmon tannins — polyphenolic compounds with strong antioxidant and astringent properties. The mechanism is fundamentally different from soap. Rather than simply washing nonenal off the skin's surface, tannins undergo a direct chemical condensation reaction with the aldehyde, binding to it and neutralizing it at the molecular level. Critically, tannin compounds can remain on the skin after rinsing, providing ongoing neutralization of newly produced nonenal between washes — something conventional soap cannot do.

This is the key distinction. Soap incompletely removes the compound temporarily. Persimmon tannins chemically deactivate it and continue working after the shower is over.

A study published in the Journal of the Japan Association on Odor Environment (Tatsuguchi et al., 2012) evaluated a soap formulated with polyphenols including persimmon tannin extract. Middle-aged and older adults used the soap daily for two to four weeks. The study found significant reductions in measured 2-nonenal concentrations on the skin and noticeable improvement in users' self-reported odor levels.

Additional research from Japanese cosmetic and pharmaceutical labs — including Shiseido and Lion Corporation — has demonstrated that persimmon tannins can bind and neutralize 2-nonenal through direct chemical interaction. More recent work (2025) on eggplant-derived phenolamides (Kim et al., published in Molecules) has shown promising nonenal-scavenging activity as well, though this is still in earlier stages of investigation.

Important caveats: Most of the persimmon-tannin research was conducted or funded by Japanese cosmetic companies, published in Japanese-language trade journals, and based on relatively small study populations or in-vitro testing. There are no large-scale, independent, randomized controlled trials specifically on persimmon tannin soap and nonenal reduction. The science is plausible and consistent, but the evidence base is not at the level of a pharmaceutical intervention.

One critical distinction: Not all persimmon-based products are equivalent. Concentrated persimmon tannin extract — which contains high levels of the active polyphenolic compounds — is fundamentally different from products that use diluted persimmon juice, which may contain as little as 0.1% tannins. The research specifically supports concentrated tannin formulations.

Mirai Clinical

The most established persimmon-based product available in the U.S. market is Mirai Clinical, a Japanese-founded company that positions itself as the sole U.S. subsidiary of a Japanese persimmon pharmaceutical lab. Their product line includes bar soaps, body washes, and deodorizing wipes formulated with patented persimmon tannin extract and green tea catechins. A bar runs approximately $20.

Mirai claims their concentrated tannin formulation eliminates up to 97% of nonenal compared to approximately 35% by conventional soaps. The company appeared on Shark Tank in 2019 and is available on Amazon and through their direct website. They recommend focusing application on five key areas where nonenal concentrates: the back of the neck, behind the ears, the chest, the back, and the scalp.

Multiple persimmon-based products are also available in the U.S. The underlying science of persimmon tannin interaction with nonenal is legitimate, and consumer reviews are broadly positive. But this should be understood as a well-supported skincare intervention, not a clinically proven medical treatment.

What Actually Helps

Switch to a persimmon tannin-based soap (Mirai Clinical or equivalent). This is the single most impactful intervention.

Prioritize natural fiber clothing. This is backed by textile science. Research published in Polymers and the Textile Research Journal has specifically demonstrated that polyester absorbs more nonenal than cotton and that nonenal is more effectively removed from cotton during laundering. The broader literature consistently shows that synthetic fibers retain body odor compounds more intensely than natural fibers, with the ranking from least to most odorous being: wool, cotton, then polyester. Cotton, linen, and wool are better choices.

Wash bedding and clothing more frequently in hot water with enzyme-based or grease-cutting detergents. Standard detergents may not fully remove nonenal from fabrics — especially synthetics.

Improve ventilation. Open windows daily. In enclosed spaces — especially in assisted living or memory care settings — nonenal accumulates in the ambient air.

Don't frame this as a hygiene issue. It isn't one. Approaching it that way causes shame and often prompts defensive withdrawal — the opposite of what you want.

Normalize the biology. "I learned that skin chemistry actually changes as we get older — there's a specific compound that starts forming after 40 that regular soap can't address. There's a Japanese soap designed for it. Want to try it?"

Consider introducing the product as something you also use. This removes the stigma. Nonenal production begins at 40 — plenty of middle-aged adults are producing it themselves.

Be aware this may already be a source of private distress. If an older loved one has been declining social invitations or seems newly self-conscious, a change in body odor they can't control may be contributing.

The Bottom Line

Aging odor is not a sign of neglect or a hygiene failure. It's a predictable biochemical process — the oxidation of skin lipids producing a volatile aldehyde that regular soap was never designed to neutralize. The Japanese have understood this for decades and developed targeted solutions grounded in persimmon tannin chemistry. Those solutions are now available in the U.S.

Addressing nonenal is a small intervention with outsized impact on dignity, confidence, and social engagement. And for older adults themselves — who may be more aware of this change than anyone realizes — having a solution that actually works can make a meaningful difference in how they feel about moving through the world.