Why Can Underarm Odor Have a Metallic Note?

When underarm odor was smelled compound by compound, strong notes resembling mushrooms and metal appeared among sour and other odors. A 2003 study investigated the trace compounds thought to be responsible and confirmed in the laboratory that reactions between fatty acids and iron produced the same types of compounds.

The research went beyond searching for odor-causing compounds to examine their formation and inhibit the reaction using plant extracts. It approached underarm odor through lipid oxidation as well as microbial decomposition.

Classifying underarm odor from 66 people by character and intensity

Sixty-six men washed with unperfumed body soap, then wore T-shirts with deodorized gauze sewn into the underarm areas for 24 hours. The researchers assessed the odor collected on the gauze and divided it broadly into five types: cumin-like spicy, sour, musty, milky, and green.

The sour type was the most common, while the cumin-like type smelled strong. Some people had overlapping characteristics, however. Rather than neatly assigning everyone to five boxes, the classification organized the range of underarm odors.

Samples from four men with particularly distinctive, strong odors were analyzed in detail. The researchers collected compounds released from the gauze into the air and also used solvent extraction, separating compounds by gas chromatography and combining mass spectrometry with human olfactory assessment.

Metallic and mushroom-like compounds that stand out even in trace amounts

About 60 compounds were identified, but the researchers focused on two distinctive positions at which odors were perceived in samples from all four men. One compound was confirmed by mass spectrometry as 1-octen-3-one, abbreviated OEO in the paper, which has metallic and mushroom-like characteristics.

The other strong candidate was cis-1,5-octadien-3-one, or ODO, which has a metallic and musty odor. It could not be identified by mass spectrometry in the human samples. Instead, the evidence was that the same odor appeared at the same retention position as the reference standard even when separation columns with different properties were used. Although the paper’s conclusion discusses the two compounds together, their identification in human samples differed in certainty.

Both compounds can be perceived strongly in small amounts. A compound can make a noticeable contribution to odor even without producing a large analytical peak. In fact, the overall instrument patterns were very similar across the five odor types, leading the authors to suggest that differences in trace compounds accounted for differences in impression.

Testing the hypothesis that iron promotes fatty-acid oxidation

Drawing on earlier research into metallic off-odors in foods and other work, the researchers focused on reactions between unsaturated fatty acids and iron. They conducted model experiments in which iron was added to linoleic or linolenic acid and allowed to react at 40°C for one hour.

They confirmed that linoleic acid produced OEO and linolenic acid produced ODO. More OEO formed with ferrous iron than with ferric iron. Within the range tested, large changes in the amount of iron made little difference to production, whereas more linoleic acid produced more OEO. The authors interpreted iron as a catalyst promoting the reaction rather than a material consumed in large quantities.

The chemical explanation is that long fatty-acid molecules undergo oxidation and cleavage, producing smaller molecules that evaporate more readily. The paper presents a reaction pathway consistent with this idea.

Whole-body sweat from the three men used for iron measurements contained approximately 10 ppb of iron in every case. This connects the presence of iron in sweat with the production of candidate compounds in an iron–fatty-acid model. It was not, however, an experiment tracking the reaction on actual underarm skin, nor did it examine the relationship between individual iron levels and odor intensity.

Inhibiting oxidation also reduced odor-compound formation

The next aim was to inhibit this formation pathway. First, the researchers oxidized linoleic acid at 50°C and measured the time required for an oxidation indicator to reach a specified level, comparing the antioxidant effects of plant extracts. Extracts of mulberry, great burnet, and Houttuynia cordata substantially delayed that point.

They also compared these three extracts in a model producing OEO from iron and linoleic acid. With the no-extract control set at 100, the index of OEO production was approximately 30 with an extract from mulberry root bark. Houttuynia and great burnet also yielded values below the control, with mulberry the lowest among the values shown. Its value was also lower than that for the comparator, tocopheryl acetate.

The figure reports three replicates, but no statistical tests between groups are described. The authors considered mulberry extract promising and discussed possible effects of its flavonoids, while leaving the molecules involved and their specific points of action for future investigation. What was demonstrated was inhibition of production in a model reaction, not a comparative human trial of a mulberry-containing deodorant.

The authors’ proposed distinction between oxidation odor and decomposition odor

The authors proposed a time-dependent model distinguishing decomposition odor, which increases with microbial activity, from oxidation odor, including OEO formed through lipid oxidation. In this model, oxidation odor contributes to the sharp impression when sweating begins, with microbially derived odors joining later.

The original paper also estimates a contribution of about 20% to odor after 24 hours, but this report does not present a validation that directly separates and measures that proportion. The useful takeaway is less a fixed percentage or universal timetable than the perspective that underarm odor can arise through multiple pathways.

Summary

This study of underarm odor in 66 men confirmed metallic, mushroom-like OEO in samples from four men with distinctive odors, and identified ODO as another strong candidate. These compounds were also produced in model reactions involving iron and unsaturated fatty acids, and adding extracts such as mulberry root bark reduced OEO formation.

The authors outlined a research approach that acts at the stage when lipids oxidize into odor compounds, rather than merely covering an odor once it is perceived. The paper’s significance lies in connecting analysis of human samples, models of formation reactions, and the search for inhibitory materials.

At the time of the study, four authors were affiliated with Lion Corporation and four with Takasago International Corporation. No separate research-funding or conflict-of-interest statement was found in the paper.

Reference

Iida, S., Ichinose, N., Gomi, T., Someya, K., Hirano, K., Ogura, M., Yamazaki, S., & Sakurai, K. (2003). Mechanism and Regulation of Body Malodor Generation (1): Effect of Iron on Axillary Malodor and of Anti-Oxidant on Deodorant. Journal of Society of Cosmetic Chemists of Japan, 37(3), 195–201. Original paper