Introduction – children´s body odor as a chemo-communication tool.

Did you ever hold a baby in your arms and noticed a pleasant odor followed by the urge to hold it closer and cuddle even more? Or on the contrary, entered a teenager’s room and smelled a rather unpleasant scent deciding to give them the privacy that teenagers usually request? Body odors change during the development of children and influence interpersonal communication with their parents. Parents can identify their infant by its body odor and prefer this smell over the smell of other children, however, identification ability and pleasantness ratings change when children reach puberty. From a functional point of view, it can be assumed that the ‘wonderful’ smell of babies facilitates parental affection and caregiving when children rely the most on it. The presence of parental preference towards the odor of the own infant as well as the absence of this preference for pubertal children has been demonstrated in several studies. However, it is still unknown which chemical cues are conveyed through the children´s body odor. Hence, we decided to conduct a chemical-analytical study to comparatively analyze the body odor composition of infants and post-pubertal children.

Sampling of body odor – not as easy as it sounds.

In the beginning of our study, the main question was: how can we sample the body odor of children, especially of infants? In literature different methods to sample body odors are reported, for example, using textiles, or different sorbents. Still, there is no standardized method since every method has its advantages but at the same time limitations, e.g., regarding the range of detected volatiles. Another aspect that we had to consider was that due to the geographical distance of the institute where the participants were recruited (Dresden, Germany) and the lab where I analyzed the samples (Erlangen, Germany), samples had to be collected, frozen and finally transported in a proper way to preserve the body odor compounds. Also, recruitment of participants takes time, but samples should not be stored for too long to ensure the preservation of odorants. We conducted several pre-tests to find the most suitable way for body odor sampling and at the same time compared the results of the chemical-analytical analysis. In the end, we decided to go for direct contact sampling with cotton textiles. This method was also often used for sensory evaluations and hence, we could link up to the previous work of my project partners in Dresden.

The essential combination of two methods in odor analysis: GC-MS and GC-O.

Body odor samples were analyzed using gas chromatography-mass spectrometry (GC-MS) and gas chromatography-olfactometry (GC-O), a combination of instrumental detection and the human nose as a detector. In my time working with GC-O (often referred to as ‘sniffing’), I realized how important this method is but also very specific to the field of odor analysis and therefore often needs to be explained for more clarification. Hence, I would like to quickly introduce this method here. While GC-MS confirms the identity of volatile compounds, GC-O provides information about the odor of such volatiles. The main advantage of GC-O is that even trace compounds with low odor thresholds can be detected with the human nose, while these can still remain under limit of detection of the instrumental detector. Moreover, by diluting the sample extract and sniffing it step by step (odor extract dilution analysis; OEDA), the potential contribution of an odor to the overall odor impression can be estimated. This means that not always the highest peaks in a chromatogram have the highest impact, but it is rather important to ‘follow your nose’ during analysis.

Results and discussion – what is the difference between body odor of infants and post-pubertal children?

In total, we analyzed axillary body odor samples of 18 infants (0-3 years) and 18 post-pubertals (14-18 years). In our first approach, we focused on odorous compounds identified with GC-O and estimated their impact on body odor. The main qualitative difference between both age groups was the detection of two odorous steroids in the samples of post-pubertals, namely 5α-androst-16-en-3-one (odor: sweaty, urinal, musk-like) and 5α-androst-16-en-3α-ol (sandalwood-like, musk-like). Moreover, according to OEDA analysis, it can be assumed that several carboxylic acids have a higher impact on the body odor of post-pubertals than of infants. Among these acids, dodecanoic acid (wax-like, soapy) and 4-ethyl octanoic acid (goat-like) were detected with the highest odor dilution factors in the post-pubertal sample extracts. Please see Figure 1 for an overview of compounds and respective odor attributes. These findings can be explained by developmental changes of axillary glands during puberty. Apocrine sweat glands become activated and further, sebaceous glands secret more sebum. Apocrine sweat glands are known to secrete odorless precursors of the mentioned steroids which are released through bacterial action. Precursors of carboxylic acids are also secreted by apocrine sweat glands. Additionally, higher amounts of sebum on post-pubertal skin get in contact with ambient air leading to multi-step oxidative processes of sebum constituents, resulting in higher amounts of short- and medium-chain carboxylic acids. In sum, these findings indicate that the odor-active steroids 5α-androst-16-en-3-one and 5α-androst-16-en-3α-ol and carboxylic acids could contribute to the altered body odor of post-pubertals. To prove this, reconstitution experiments will be needed to evaluate the odor quality of synthetic mixtures of the body odor compounds.

In our second approach we focused on one of the sebum constituents, namely squalene (SQ; odorless), and its oxidative degradation products 6-methylhept-5-en-2-one (6MHO; fruity) and geranyl acetone (GA; soapy), see also Figure 1, since one of our pilot studies indicated quantitative differences for these substances. Results revealed higher concentrations of SQ in the post-pubertal samples and similar concentrations of 6MHO and GA for both age groups. The higher concentrations of SQ confirmed that there is more sebum on post-pubertal skin, however, it was surprising that 6MHO and GA were not quantified in significantly higher concentrations in the post-pubertal body odor samples, as well. In a next step, we calculated the ratios 6MHO/SQ and GA/SQ, since SQ is considered as a precursor of 6MHO and GA. Results showed that these ratios were higher in the case of the infants’ samples. According to GC-O results, 6MHO and GA seemed to have a minor impact on children´s body odor. However, mixture effects of different odorants are not considered for GC-O analysis, and further, our sampling technique may discriminate these odorants. Further studies are necessary to clarify the contribution of 6MHO and GA to social chemical communication.

Conclusion

Our study set out to investigate differences in body odor composition of infants and post-pubertal children. We identified odorants that differ qualitatively as well as in a quantitative manner between both age groups. These odorants likely contribute to the changing body odor of children and thus confirm that sexual maturation of children goes along with changes in chemical body odor composition. This study was an important contribution to unraveling potential chemical cues that are conveyed by children´s body odor. For future studies model experiments with these odorants may be conducted to investigate the response of parents towards these odors and hence shed light on body odors influencing social interactions and relationships within families.