Smellmaxxing: The Science of Old People Smell (2-Nonenal Explained)

Scientific concepts, discoveries, and nature/biology phenomena

“Old people smell” linked to a specific skin-emitted odorant

• The video argues that a large portion of age-associated body odor is driven by 2-nonenal (spelled in the subtitles as “non-enol/nonenol/non-enal”; the intended compound is 2-nonenal, an aldehyde).
• Onset claim: detectable on human skin starting around age ~40.
• Human prevalence claim (from one cited fabric/odor study):
  • Under 40: 0/9 subjects had detectable 2-nonenal odor
  • Over 40: 9/13 subjects had detectable odor
  • Interpreted as a likely increase with age, with a caveat that detectable levels vary and not everyone in older groups shows the smell in that specific study.

How the odor is described and measured

• Descriptors used across sources include “greasy” and “grassy”; others add “musty” and “cardboard.”
• The video claims the “wet cardboard” note is associated with 2-nonenal’s taste/smell chemistry:
  • Same compound associated with stale beer/cardboard flavor.
  • It asserts a sensitivity difference: far less is needed to detect in a glass than on human skin, framed as “30,000 times less on a glass than on skin.”

Cultural/language framing of age odor

• Japan is said to have a term “keshu” (aging odor), and the video claims it exists because related research is in Japanese.

Biological mechanism (proposed/triangulated across claims)

The video describes a mechanism involving:

• Skin oil and unsaturated fats undergoing oxidation to yield aldehydes like 2-nonenal.
• Distinctions in the subtitles between omega-7 vs omega-10 and between omega-7 olefin-derived vs omega-10-derived oxidation products.
• Supply claim: overall skin-oil output may not increase with age (and may decline in women after menopause), while the specific odorant may rise.

Claims about “where it shows up” on the body

• The video emphasizes that 2-nonenal emission is strongest in high-oil regions, especially the nape of the neck.
• It also mentions behind ears and the forehead (citing a “Satato” study in the subtitles).
• The argument is that deodorant industry targeting armpits may miss the main emission source for this specific odorant.

Environmental/behavioral modulation (oxidation-related)

• Odor formation is linked to oxidation (sunlight increasing peroxide on the skin surface).
• Reported influences include:
  • Sunlight increasing relevant oxidative precursors (peroxide increase claimed as ~60-fold in subtitles).
  • Dietary fat vs vegetables affecting forehead odor emission later (from a cited study design, but presented with caveats).
  • Acute stress altering odor ratings/emissions (small effect mentioned in subtitles).
• Smoking is reported as not moving the odor in at least one test.

Evidence scale/credibility ladder used by the narrator

The narrator ranks evidence for “fixes” using a ladder:

• A: measured in people (human trial on 2-nonenal)
• B: small/single human study
• C: test-tube/in vitro, not on skin/humans
• D: mechanism suggests it should work
• E: sold by someone
• ?: unmeasured (not validated for the endpoint)

A recurring critique: many proposed interventions have not been tested against 2-nonenal on human skin specifically.

Fabric/clothing retention

• The video claims cotton removes more 2-nonenal during washing than polyester.
• It suggests oily aldehydes tend to remain more in hydrophobic polyester.
• Cited points include:
  • Higher release from cotton into air after washing (as described in subtitles).
  • Accumulation in polyester across multiple wear/wash cycles (noted as a lab rig plus mixed data sources).

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Methodologies / study procedures mentioned (as described)

2-nonenal detection via shirt-and-air sampling (key study protocol)

• Use clean cotton shirts for baseline (air above shirts “empty/odorless”).
• Wear on bare skin for 3 nights.
• Cut a rectangle from the shirt back.
• Pump/collect air above the region for ~18 hours (tube/chamber setup).
• Assess whether 2-nonenal odor is present (judges/rating in the study described).

Odor participant preparation (alternative cited study protocol)

• Collect emission by inserting pads sewn into shirts.
• Wash with odorless detergent.
• Wear while sleeping alone for 5 nights.
• Freeze samples at -80°C.
• Blind raters score odor intensity/unpleasantness.

Intervention study example: black currant powder feeding

• Dose: black currant powder ~6 g/day for 7 days.
• Measure neck emission before/after.
• Design limitations noted in subtitles:
  • No placebo
  • Control week always ran first
  • Uncontrolled diet/composition
  • Small number of subjects and many screened compounds

Sun/oxidation manipulation example

• Increase peroxide on skin surface via sunlight exposure (90 minutes claimed).
• Evaluate whether odor-related endpoints change (with the video arguing the odor molecule itself was not directly measured in that test).

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Researchers/sources featured (as named in subtitles)

Named authors / research contributors

• Hajime(s) / Hayas(u) Gou Nakamura (as transcribed) — lead author
• Unknown co-authors — subtitles indicate “four co-authors” / “seven authors total,” but names aren’t individually listed
• Mettr / Metro Gordon Olsen (as transcribed; likely Gordon Olsen) — author in a 2012 study
• Lundström (as transcribed) — author in a 2012 study
• POS1 (2012) Manell Chemical Senses Center — source center named in subtitles (specific researcher names beyond Olsen/Lundström not given)
• Finnegan 2024 — author in a skin-measurement study (subtitles note 2-nonenal detection/lack of 2-nonenal in analysis)
• Kuiper study (2011) — referenced as measuring a different molecule in urine after vitamin C
• Keiper study / “Kuiper” (spelled as Kuiper in subtitles)
• Satato study (2021) — referenced for high concentration behind ears/forehead

Funders (named)

• National Institutes of Health (NIH)
• Swedish Research Council
• Subtitles also refer to “Ni(h)/Swedish funders.”

Company/patent entities mentioned (cited sources, not necessarily academic researchers)

• Shiseido (company; cited for patent/fabric study affiliations)
• Fragrance House / Chaseedo / Shidto (as transcribed; likely Shiseido/fragrance-industry partners)
• Deodorant/cosmetics/soap companies (multiple unnamed brands in subtitles)
• US cotton industry body (donor of fabric for a lab study)
• Black currant trade association (source of study funding/powder donation)