Chapter 4：Body Odor

Sense of smell and well-being

Of the 5 human senses, smell remains the most enigmatic, yet most under studied sense. This became apparent during the first year of the COVID-19 pandemic when a considerable portion of the patients infected by SARS-CoV-2 suffered from a loss of sense of smell (anosmia), or a distorted sense of smell (dysosmia), sometimes in combination with sense of taste impairments. The pandemic caused widespread disruption, isolation, fear of infection, financial challenges, and the loss of loved ones that all contributed to stress, anxiety, and depression in many individuals. The role of COVID related smell and taste impairments cannot be ruled out as contributing factor to a decline in mental well-being.

Individuals with an impaired sense of smell are more accident prone and can easily miss vital signs in dangerous situations. For example, the sense of smell helps us detect the presence of toxic volatiles such as smoke, gas leaks, but also spoiled food, or other potentially harmful substances in our environment. In addition, the sense of smell and taste are closely interlinked, and can influence our ability to distinguish flavors in food and beverages. Smell and taste impairments can impact the quality of life by diminishing the pleasure derived from eating and drinking. Moreover, these impairments can also have psychological and emotional effects. Smell is closely tied to our memories, and emotions. As a result, smell and taste impairments may lead to a decreased sense of connectedness and impact the quality of life, affecting the overall well-being and mental health. Smell impairment can also influence social interactions and intimacy. Our sense of smell plays a role in the way we perceive others, and it contributes to our attraction and emotional connections. The loss of sense of smell may impact one's ability to fully engage in social and romantic relationships, potentially leading to feelings of self-consciousness, decreased self-esteem, or isolation.

Body odor and the microbiome

Body odor is closely linked to the composition and activity of the microbiome; the collection of bacteria, fungi, and other microorganisms that reside in and on our skin. The microbiome interacts with our body and plays a role in various physiological processes, including the production of body odor. Especially skin residing bacteria produce volatile organic compounds (VOCs) as a byproduct of their metabolic activities, like the break down of lipids and amino acids present in sweat and talcum. These VOCs contribute to the distinct scent of body odor. The specific odor produced can vary depending on factors such as the types and abundance of microorganisms present, as well as other factors like diet, genetics, personal hygiene, and overall health. For example, certain foods like garlic, onions, and spices can alter body odor temporarily. Moreover, hormonal changes, stress, and certain medical conditions can also affect body odor directly or via their influence on the microbiome.

The bacteria responsible for body odor are primarily from the genus Corynebacterium and Staphylococcus. Different individuals may have different microbial communities on their skin, resulting in unique body odor profiles.

Corynebacteria are in particular responsible for foot odor, and an overgrowth of specific species of Corynebacteria can contribute to smelly feet, also known as bromodosis. The corynebacteria break down sweat components (especially amino acids) into VOCs, such as isovaleric acid and butyric acid, that have a strong and unpleasant odor.

Bromodosis (Foot Odor)

Bacteria
Staphylococcus epidermidis
Corynebacterium spp.
Brevibacterium linens
Molecules
Methanethiol, CH₃SH Propanoic Acid, CH₃CH₂COOH Isovaleric Acid, (CH₃)₂CHCH₂COOH

Halitosis commonly known as bad breath, refers to an unpleasant odor that originates from the mouth. It can be a temporary condition or a chronic problem, and it can have various causes like poor oral hygiene leading to overgrowth of bacteria producing VOCs, or infections in the mouth, such as gum disease (periodontitis) or tooth decay. Again, the metabolites produced by bacteria consist of foul-smelling gases that is perceived as bad breath. Other upper airway infections such as sinus infections, tonsillitis, or throat infections and esophageal pathology can also contribute to bad breath.

You often do not perceive your own body odor or your partners body odor, a phenomenon known as olfactory adaptation or olfactory fatigue. Olfactory adaptation occurs when the receptors in our noses become desensitized to a specific smell, leading to a reduced perception or complete unawareness of the odor, even if it is still present. This can be explained by 3 mechanisms: 1) Sensory receptor fatigue: Prolonged exposure to a particular odor can lead to a decrease in sensitivity of the olfactory receptors. 2) Neural adaptation: The brain’s response to continuous or repetitive sensory input decreases over time, and 3) Cognitive factors: Our attention and focus play a role in our perception of smells. If we are engaged in other activities or thoughts, we may become less aware of the smells around us. Olfactory adaptation helps prevent us from being overwhelmed by continuous or background smells and allows us to focus on detecting novel or potentially more relevant odors.

Halitosis

Bacteria
Treponema denticola
Porphyromonas gingivalis
Prevotella intermedia
Fusobacterium nucleatum
Molecules
Methanethiol, CH₃SH Hydrogen Sulfide, H₂S Dimethyl Sulfide, (CH₃)₂S

Genital odors

The genital area contains a specific microbiome, that varies among individuals and is responsible for a highly individual genital odor. When confronted with someone’s genital odor, the response is rarely neutral. Depending on the setting, it either induces a strong off-putting and averse response, or a highly attractive and arousing reaction. In this process body odor in general but genital odor in particular plays an important role in sexual selection.

Some of the common bacteria found in genital skin include Corynebacterium , Streptococcus and Staphylococcus species, and anaerobic bacteria. Anaerobic bacteria, which thrive in oxygen-limited environments, can produce a distinct fish like smell known as amine smell. The vaginal microbiome moreover contains lactobacilli and fungi. Lactobacilli help maintain the acidic pH of the vagina, which is important for preventing the overgrowth of harmful bacteria and maintaining a healthy vaginal environment. They also produce lactic acid and other substances that further inhibit the growth of potential pathogens. The balance and diversity of the vaginal microbiome play a crucial role in maintaining vaginal health. When the balance is disrupted, it can lead to an overgrowth of particular bacteria or fungi, resulting in conditions such as bacterial vaginosis (BV) or yeast infections. Bacterial vaginosis (BV) occurs when there is a shift in the vaginal microbiome, leading to an overgrowth of anaerobic bacteria that can cause a fishy odor, and vaginal irritation. In individuals with a penis, the area under the foreskin, is an important site for bacteria producing VOC’s that contribute to the genital odor. Circumcision thus has a profound effect on genital odor.

Bacteria

Vaginal Odor Bacteria
Gardnerella vaginalis
Mobiluncus spp.
Bacteroides spp.
Genitals Odor Bacteria
Prevotella spp.
Penis Odor Bacteria
Corynebacterium spp.
Staphylococcus spp.
Anaerobic bacteria

Molecule

Trimethylamine, TMA, (CH₃)₃N Propionic Acid, CH₃CH₂COOH
Isovaleric Acid, (CH₃)₂CHCH₂COOH Acetic Acid, CH₃COOH Butyric Acid, CH₃CH₂CH₂COOH

Body odor and sexual selection

Pheromones are chemical signals (like VOCs) that are released by organisms, including humans, to communicate with others of the same species. These chemical signals can have various effects, including influencing behavior and physiological responses in sexual partners. In the animal realm, pheromones play a role in sexual selection by conveying information about an individual's reproductive fitness, genetic compatibility, and overall attractiveness. Pheromones can be detected through specialized sensory organs or receptors, which are often highly sensitive to these chemical signals. Which sensory organs are involved in human pheromones is not known, and the role of pheromones in human sexual selection is still a topic of ongoing research and debate.

In a landmark study dubbed the “T-shirt experiment” in 1995 Wedekind et al investigated the role of body odor and human mate selection. The experiment involved participants wearing a clean T-shirt for a certain period of time and then having other individuals smell and rate the pleasantness and attractiveness of the T-shirt based on its odor. The researchers related the level of attractiveness of participants donating their worn T-shirt to their human leukocyte antigen (HLA) type. HLA types are a group of genes that play a crucial role in the immune system. These genes help distinguish between self and non-self cells, enabling the immune system to recognize and eliminate harmful bacteria, viruses, or transplanted tissues. The T-shirt experiment aimed to understand whether HLA types influence body odor and whether individuals are subconsciously attracted to the body odor of individuals with different HLA types, as this could potentially enhance offspring’s immune system diversity. The experiment showed that the body odor of people with different HLA types was perceived as more attractive or pleasant compared to those with similar HLA types. This preference for dissimilar HLA types in body odor may be an adaptive mechanism that promotes mate selection for genetic diversity and a stronger immune system in offspring. It’s important to note that while the T-shirt experiment provides insights into the potential role of HLA types and body odor in mate selection, it is just one aspect of a complex process. While pheromones may play a role in initial attraction, human mate choice is also influenced by a wide range of other factors, such as physical appearance, personality traits, social status, and shared interests. Cultural and societal norms, as well as individual preferences, strongly influence the process of human sexual selection.

Further reading: Olfactory memory networks: from emotional learning to social behaviors. Sullivan RM, et al. Front Behav Neurosci. 2015.

Bromidrosis (Armpit Odor)

Bacteria
Staphylococcus hominis
Micrococcus luteus
Corynebacterium spp.
Molecules
3-Hydroxy-3-Methylhexanoic Acid (S)-3-Methyl-3-Sulfanylhexan-1-Ol (E)-3-Methyl-2-Hexenoic Acid