Body odor
Body odor or body odour (BO) is present in all animals and its intensity can be influenced by many factors (behavioral patterns, survival strategies). Body odor has a strong genetic basis, but can also be strongly influenced by various factors, such as sex, diet, health, and medication.[1] The body odor of human males plays an important role in human sexual attraction, as a powerful indicator of MHC/HLA heterozygosity.[2][1] Significant evidence suggests that women are attracted to men whose body odor is different from theirs, indicating that they have immune genes that are different from their own, which may produce healthier offspring.[3]
Causes
In humans, the formation of body odors is caused by factors such as diet, sex, health, and medication, but the major contribution comes from
The main components of human axillary odor are unsaturated or hydroxylated branched fatty acids with E-3-methylhex-2-enoic acid (E-3M2H) and 3-hydroxy-3-methylhexanoic acid (HMHA), sulfanylalkanols and particularly 3-methyl-3-sulfanylhexan-1-ol (3M3SH), and the odoriferous steroids androstenone (5α-androst-16-en-3-one) and androstenol (5α-androst-16-en-3α-ol).[6] E-3M2H is bound and carried by two apocrine secretion odor-binding proteins, ASOB1 and ASOB2, to the skin surface.[7]
Body odor is influenced by the actions of the
Factors such as food, drink, gut microbiome,[13] and genetics can affect body odor.[5]
Function
Animals
In many animals, body odor plays an important survival function. Strong body odor can be a
Body odor is an important feature of animal physiology. It plays a different role in different animal species. For example, in some predator species that hunt by stalking (such as big and small
Humans
In humans, body odor serves as a means of chemosensory signal communication between members of the species. These signals are called pheromones and they can be transmitted through a variety of mediums. The most common way that human pheromones are transmitted is through bodily fluids. Human pheromones are contained in sweat, semen, vaginal secretions, breast milk, and urine.[1] The signals carried in these fluids serve a range of functions from reproductive signaling to infant socialization.[16] Each person produces a unique spread of pheromones that can be identified by others.[2] This differentiation allows the formation of sexual attraction and kinship ties to occur.[2][17]
Sexual selection
Pheromones are a factor seen in the mating selection and reproduction in humans. In women, the sense of olfaction is strongest around the time of ovulation, significantly stronger than during other phases of the menstrual cycle and also stronger than the sense in males.[20][21] Pheromones can be used to deliver information about the major histocompatibility complex (MHC).[2] The MCH in humans is referred to as the Human Leukocyte Antigen (HLA).[22] Each type has a unique scent profile that can be utilized during the mating selection process. When selecting mates, women tend to be attracted to those that have different HLA-types than their own.[2][22] This is thought to increase the strength of the family unit and increase the chances of survival for potential offspring.[2]
Studies have suggested that people might be using odor cues associated with the immune system to select mates. Using a brain-imaging technique, Swedish researchers have shown that homosexual and heterosexual males' brains respond in different ways to two odors that may be involved in sexual arousal, and that homosexual men respond in the same way as heterosexual women, though it could not be determined whether this was cause or effect. When the study was expanded to include lesbian women, the results were consistent with previous findings – meaning that lesbian women were not as responsive to male-identified odors, while responding to female odors in a similar way as heterosexual males.[23] According to the researchers, this research suggests a possible role for human pheromones in the biological basis of sexual orientation.[24]
Kinship communication
Humans can olfactorily detect blood-related kin.[17] Mothers can identify by body odor their biological children, but not their stepchildren. Preadolescent children can olfactorily detect their full siblings, but not half-siblings or step-siblings, and this might explain incest avoidance and the Westermarck effect.[25] Babies can recognize their mothers by smell while mothers, fathers, and other relatives can identify a baby by smell.[5] This connection between genetically similar family members is due to the habituation of familial pheromones. In the case of babies and mothers, this chemosensory information is primarily contained within breastmilk and the mother's sweat.[26] When compared to that of strangers, babies are observed to have stronger neural connections with their mothers.[27] This strengthened neurological connection allows for the biological development and socialization of the infant by their mother. Using these connections, the mother transmits olfactory signals to the infant which are then perceived and integrated.[27]
In terms of biological functioning, olfactory signaling allows for functional breastfeeding to occur. In cases of effective latching, breastfed infants are able to locate their mother's nipples for feeding using the sensory information enclosed in their mother's body odor.[28] While no specific human breast pheromones have been identified, studies compare the communication to that of the rabbit mammary pheromone 2MB2.[29][30] The perception and integration of these signals is an evolutionary response that allows newborns to locate their source of nutrition. Signaling contains a level of precision that allows babies to differentiate their mother's breasts from that of other women.[26] Once the baby recognizes the familiar olfactory signal, the behavioral response of latching follows.[26] Over time the infant becomes habituated to their mother's breast pheromones which increases latch efficiency.[28]
Beyond a biological function, a mother's body odor plays a role in developing a baby's social capabilities. The ability of an infant to evaluate the properties of human faces stems from the olfactory cues given from their mother.[16] Frequent exposure to the pheromones exuded by their mother allows the connection between vision and smell to form in infants.[27] This type of connection is only found between mothers and babies and over time it socializes the ability to recognize the features that distinguish human faces from inanimate objects.[16]
Environmental threats
The connection between olfactory and visual cues has also been observed outside of familial relationships. Evolutionarily, body odor has been used to communicate messages about potentially dangerous stimuli in the environment.[1] Body odor produced during particularly stressful situations can produce a cascade of reactions in the brain. Once the olfactory system is activated by a threatening stimuli, heightened activity in the amygdala and occipital cortex is triggered.[31][1] This chain reaction serves to help assess the nature of the threat and increase chance of survival.
Humans have few olfactory receptor cells compared to dogs and few functional olfactory receptor genes compared to rats. This is in part due to a reduction of the size of the snout in order to achieve depth perception as well as other changes related to bipedalism. However, it has been argued that humans may have larger brain areas associated with olfactory perception compared to other species.[18]
Genes affecting body odor
MHC
Body odor is influenced by
Experiments on animals and volunteers have shown that potential sexual partners tend to be perceived more attractive if their MHC composition is substantially different. Married couples are more different regarding MHC genes than would be expected by chance. This behavior pattern promotes variability of the immune system of individuals in the population, thus making the population more robust against new diseases. Another reason may be to prevent inbreeding.[5]
ABCC11
The ABCC11 gene determines axillary body odor and the type of earwax.[6][32][33][34] The loss of a functional ABCC11 gene is caused by a 538G>A single-nucleotide polymorphism, resulting in a loss of body odor in people who are specifically homozygous for it.[34][35] Firstly, it affects apocrine sweat glands by reducing secretion of odorous molecules and its precursors.[6] The lack of ABCC11 function results in a decrease of the odorant compounds 3M2H, HMHA, and 3M3SH via a strongly reduced secretion of the precursor amino-acid conjugates 3M2H–Gln, HMHA–Gln, and Cys–Gly–(S) 3M3SH; and a decrease of the odoriferous steroids androstenone and androstenol, possibly due to the reduced secretion of dehydroepiandrosterone sulfate (DHEAS) and dehydroepiandrosterone (DHEA), possibly bacterial substrates for odoriferous steroids; research has found no difference, however, in testosterone secretion in apocrine sweat between ABCC11 mutants and non-mutants.[6] Secondly, it is also associated with a strongly reduced/atrophic size of apocrine sweat glands and a decreased protein (such as ASOB2) concentration in axillary sweat.[6]
The non-functional ABCC11 allele is predominant among
However, research has observed that this allele is not solely responsible for ethnic differences in scent. A 2016 study analyzed differences across ethnicities in volatile organic compounds (VOCs), across racial groups and found that while they largely did not differ significantly qualitatively, they did differ quantitatively. Of the observed differences, they were found to vary with ethnic origin, but not entirely with ABCC11 genotype.[36]
One large study failed to find any significant differences across ethnicity in residual compounds on the skin, including those located in sweat.[37] If there were observed ethnic variants in skin odor, one would find sources to be much more likely in diet, hygiene, microbiome, and other environmental factors.[38][36][39]
Research has indicated a strong association between people with axillary osmidrosis and the ABCC11-genotypes GG or GA at the SNP site (rs17822931) in comparison to the genotype AA.[34]
Ethnic groups | Tribes or inhabitants | AA | GA | GG |
---|---|---|---|---|
Korean | Daegu city inhabitants | 100% | 0% | 0% |
Chinese | Northern and southern Han Chinese | 80.8% | 19.2% | 0% |
Mongolian | Khalkha tribe | 75.9% | 21.7% | 2.4% |
Japanese | Nagasaki people | 69% | 27.8% | 3.2% |
Thai | Central Thai in Bangkok | 63.3% | 20.4% | 16.3% |
Vietnamese | People from multiple regions | 53.6% | 39.2% | 7.2% |
Native American | 30% | 40% | 30% | |
Filipino |
Palawan | 22.9% | 47.9% | 29.2% |
Kazakh | 20% | 36.7 | 43.3% | |
Russian | 4.5% | 40.2% | 55.3% | |
White Americans | From CEPH families without the French and Venezuelans | 1.2% | 19.5% | 79.3% |
African |
From various sub-Saharan nations | 0% | 8.3% | 91.7% |
African Americans | 0% | 0% | 100% |
Genotype ABCC11 |
Sex | Ethnic population | Age | Net weight sweat (g)/2 pads |
HMHA–Gln (μmol/2 pads) |
3M2H–Gln (μmol/2 pads) |
Cys–Gly conjugate
of 3M3SH (μmol/2 pads) |
---|---|---|---|---|---|---|---|
AA | F | Chinese | 27 | 2.05 | ND' | ND | ND |
AA | F | Filipino | 33 | 2.02 | ND | ND | ND |
AA | F | Korean | 35 | 1.11 | ND | ND | ND |
GA | F | Filipino | 31 | 1.47 | 1.23 | 0.17 | Detectable, < 0.03 μmol |
GA | F | Thai | 25 | 0.90 | 0.89 | 0.14 | Detectable, < 0.03 μmol |
GA | F | German | 25 | 1.64 | 0.54 | 0.10 | Detectable, < 0.03 μmol |
GG | F | Filipino | 45 | 1.74 | 0.77 | 0.13 | Detectable, < 0.03 μmol |
GG | F | German | 28 | 0.71 | 1.30 | 0.19 | 0.041 |
GG | F | German | 33 | 1.23 | 1.12 | 0.16 | 0.038 |
* ND indicates that no detectable peak is found on the [M+H]+ ion trace of the selected analyte at the correct retention time.
* HMHA: 3-hydroxy-3-methyl-hexanoic acid; 3M2H:
Alterations
Body odor may be reduced or prevented or even aggravated by using
Industry
As many as 90% of Americans and 92% of teenagers use antiperspirants or deodorants.[44][45] In 2014, the global market for deodorants was estimated at US$13.00 billion with a compound annual growth rate of 5.62% between 2015 and 2020.[46]
Medical conditions
Osmidrosis or bromhidrosis is defined by a foul odor due to a water-rich environment that supports bacteria, which is caused by an abnormal increase in perspiration (hyperhidrosis).[33] This can be particularly strong when it happens in the axillary region (underarms). In this case, the condition may be referred to as axillary osmidrosis.[33] The condition can also be known medically as apocrine bromhidrosis, ozochrotia, fetid sweat, body smell, or malodorous sweating.[47][48]
Trimethylaminuria (TMAU), also known as fish odor syndrome or fish malodor syndrome, is a rare metabolic disorder where trimethylamine is released in the person's sweat, urine, and breath, giving off a strong fishy odor or strong body odor.[49]
See also
- Drug resistance
- Foot odor
- Halitosis (bad breath)
- Old person smell
- Olfactophilia
- Olfactory fatigue
- Pheromone
- Sweat gland
References
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In one study about smell and romance, straight women preferred the body odor of straight men whose immune systems were different enough that any offspring would have healthy immune systems. For most of human history, infectious disease has been our greatest threat. In modern times we may seek life-partners that satisfy a multitude of needs, but more fundamentally, if you could produce babies with immune systems able to fight a potpourri of pathogens, then your progeny—and your genes—stand a better chance at survival.
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External links
- Flores G (November 4, 2004). "Immunity, smell linked". The Scientist Magazine.
- Dunning B (October 25, 2022). "Skeptoid #855: Sniffing for Human Sex Pheromones". Skeptoid.