A recent Scientific Reports study investigated whether body odor is protective against diseases.
Study: Human scent as a first-line defense against disease. Image Credit: Cast Of Thousands/Shutterstock.com
Background
Behavior is the first line of defense against infection across phyla. Behavior is the most cost-effective approach to preventing and reducing disease by avoiding contaminated people. This behavioral or reactive avoidance of diseased individuals is called the behavioral immune system.
Several studies have indicated that unhealthy animals release an odor that causes avoidance by conspecifics.
In an animal-based laboratory experiment, test rats were introduced to lipopolysaccharide (LPS), an endotoxin, via injection. The LPS exposure induced the innate immune system in the rat and resulted in inflammation.
Interestingly, the LPS exposed sick rats, and other healthy rats avoided their urine. In another study, human participants exposed to LPS could be easily identified through their body odor. This olfactory cue was due to a systemic immune activation after LPS exposure.
LPS-exposed humans could be used to study sickness response, as these participants experience headaches, tympanic temperature, and increased pro-inflammatory cytokine levels, which indicates sickness.
The body odor samples obtained from these patients were significantly more intense, unhealthy, and unpleasant compared to the healthy or non-LPS exposed control group. This unpleasant odor could be because LPS-exposed individuals release more volatile components or modify the profile of volatile compounds.
Body odor samples were also analyzed using gas chromatography-mass spectrometry (GC–MS), which revealed that the number of volatile substances was almost the same as that of control samples.
About the study
The current study investigated whether a lower-grade systemic inflammation induced by a 25% weaker LPS dosage than in previous experiments could also trigger unpleasant body odor.
A total of fifty participants (23 men and 27 women) were recruited in this study for donation of body odor. Some of the inclusion criteria of the participants were that all individuals must be between 18 and 50 years old, medication-free, right-handed, non-smokers, and without a history of chronic pain or psychiatric disorders. The average age of the participants was 28 years.
Participants were injected with either LPS or saline solution. Twenty-nine participants received 0.6 ng/kg LPS, and twenty-one received saline injection.
The body odor sample was collected using a tight t-shirt, which was made of 50% cotton and 50% polyester. Participants wore the t-shirt immediately after being injected with LPS or saline and returned it at the end of the session.
All participants followed strict dietary restrictions that included avoidance of spicy food and alcohol or the use of any odorized products a day before sample collection/session. Urine samples were also collected.
After five hours of LPS injection, the participants’ body temperature increased by 1°C. Plasma samples were collected at different time intervals starting at baseline. GC-MS was conducted to analyze the odor samples.
This study also performed a body odor perception test using sixty-nine healthy participants.
Study findings
The current study assessed whether a lower systemic inflammation leads to decreased body odor pleasantness. Individuals exposed to LPS revealed varied levels of inflammation-related markers compared to those who received saline injections.
This finding reveals a significant activation of the innate immune system post-LPS injection. Consistent with previous studies, participants exposed to LPS had more unpleasant body odor than the control or placebo group.
Unpleasant body odor after LPS injection has been linked to the alteration of the chemical composition of body odor instead of any changes in chemical abundance.
Consistent with previous studies, this study revealed a decrease in body odor pleasantness within hours of the onset of systemic inflammation.
GC-MS analysis revealed that the levels of 6-methyl-5-hepten-2-one significantly differed in response to endotoxin and subsequent inflammation. This study detected the presence of this compound both in human sweat and fabric substrates.
It must be noted that 6-methyl-5-hepten-2-one is not correlated with body temperature. This compound plays a significant role in inducing human reaction to illness.
Conclusions
One of the key strengths of this study is the utilization of a lower dose of LPS, with respect to previous studies, to induce unpleasant body odor and systematic inflammation. In contrast to previous studies, the LPS dose used in this study enabled the activation of donors’ immune systems without making them physically too ill.
The current study emphasized the existence of a behavioral defense against diseases by detecting sickness via olfactory cues and subsequent avoidance. A behavioral defense is also associated with triggering behavior that reduces the risk of contamination.
Even minor changes in inter-individual contact patterns could lead to a significant reduction in disease transmission or infection burden.
Taken together, a systemic inflammation response leads to a generation of unpleasant body odor that induces behavioral defense against disease.
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