Question: Why does sweating smell?

Body odor


Body odor (American English) or body odour (British English) is present in humans and other animals, and its intensity can be influenced by many factors (behavioral patterns, survival strategies). Body odor has a strong genetic basis, but it can be also strongly influenced by various diseases and physiological conditions. Body odor is generally considered to be an unpleasant odor among many human cultures.
Causes

In humans, the formation of body odors is caused by factors such as diet, gender, health, and medication, but the major contribution comes from bacterial activity on skin gland secretions.[1] Humans have three types of sweat glands; eccrine sweat glands, apocrine sweat glands and sebaceous glands. Eccrine sweat glands are present from birth, while the two latter become activated during puberty.[2] Among the different types of human skin glands, the body odor is primarily the result of the apocrine sweat glands, which secrete the majority of chemical compounds needed for the skin flora to metabolize it into odorant substances.[1] This happens mostly in the axillary (armpit) region, although the gland can also be found in the areola, anogenital region, and around the navel.[3] In humans, the armpit regions seem more important than the genital region for body odor which may be related to human bipedalism. The genital and armpit regions also contain springy hairs which help diffuse body odors.[4]
The main components of human axillary odor are unsaturated or hydroxylated branched fatty acids with E-3M2H (E-3-methylhex-2-enoic acid) and HMHA (3-hydroxy-3-methylhexanoic acid), sulfanylalkanols and particularly 3M3SH (3-methyl-3-sulfanylhexan-1-ol), and the odoriferous steroids androstenone (5α-androst-16-en-3-one) and androstenol (5α-androst-16-en-3α-ol).[5] E-3M2H is bound and carried by two apocrine secretion odor-binding proteins, ASOB1 and ASOB2, to the skin surface.[6]
Body odor is influenced by the actions of the skin flora, including members of Corynebacterium, which manufacture enzymes called lipases that break down the lipids in sweat to create smaller molecules like butyric acid. Greater bacteria populations of Corynebacterium jeikeium are for example found more in the armpits of men whereas greater population numbers of Staphylococcus haemolyticus are found in the armpits of women. This causes male armpits to give off of a rancid/cheese-like smell whereas female armpits give off a more fruity/onion-like smell.[7] Staphylococcus hominis is also known for producing thioalcohol compounds that contribute to odors.[8] These smaller molecules smell, and give body odor its characteristic aroma.[9] Propionic acid (propanoic acid) is present in many sweat samples. This acid is a breakdown product of some amino acids by propionibacteria, which thrive in the ducts of adolescent and adult sebaceous glands. Because propionic acid is chemically similar to acetic acid with similar characteristics including odor, body odors may be identified as having a vinegar-like smell by certain people. Isovaleric acid (3-methyl butanoic acid) is the other source of body odor as a result of actions of the bacteria Staphylococcus epidermidis,[10] which is also present in several strong cheese types.
Factors such as food, drink, and diseases can affect body odor.[4] An individual's body odor is also influenced by lifestyle, sex, genetics, and medication. 
Function

Animals
In many animals, body odor plays an important survival function. Strong body odor can be a warning signal for predators to stay away (such as porcupine stink), or it can also be a signal that the prey animal is unpalatable.[11] For example, some animals species, who feign death to survive (like opossums), in this state produce a strong body odor to deceive a predator that the prey animal has been dead for a long time and is already in the advanced stage of decomposing. Some animals with strong body odor are rarely attacked by most predators, although they can still be killed and eaten by birds of prey, which are tolerant of carrion odors.
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 cats), the absence of body odor is important, and they spend plenty of time and energy to keep their body free of odor. For other predators, which use endurance running after the visually located prey as a hunting strategy (dogs, wolves), the absence of body odor is not critical. In most animals, body odor intensifies in moments of stress and danger.
Humans
Sebaceous and apocrine glands become active at puberty. This, as well as many apocrine glands being close to the sex organs, points to a role related to mating.[4] Compared to other primates, humans have extensive axillary hair and have many odor producing sources, in particular many apocrine glands.[12] 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.[13]
Humans can olfactorily detect blood-related kin.[14] 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.[15] Babies can recognize their mothers by smell while mothers, fathers, and other relatives can identify a baby by smell.[4]
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.[12]
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 their response to female cues was similar to heterosexual males'.[16]  According to the researchers, this research suggests a possible role for human pheromones in the biological basis of sexual orientation.[17]
Genetics

Body odor is largely influenced by major histocompatibility complex (MHC) molecules. These are genetically determined and play an important role in immunity of the organism. The vomeronasal organ contains cells sensitive to MHC molecules in a genotype-specific way.
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.[4]
The ABCC11 gene determines axillary body odor and the type of earwax.[5][18][19][20] 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.[20][21] Firstly, it affects apocrine sweat glands by reducing secretion of odorous molecules and its precursors.[5] 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 levels and secretion of DHEAS and DHEA (possibly bacterial substrates for odoriferous steroids).[5] 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.[5]
The non-functional ABCC11 allele is predominant among East Asians (80–95%), but very low in other ancestral groups (0–3%).[5]  Most of the world's population have the gene that codes for the wet-type earwax and normal body odor; however, East Asians are genetically predisposed for the allele associated with the dry-type earwax and a reduction in body odor.[5][18][20] East Asians (Chinese, Koreans, and Japanese) have fewer apocrine sweat glands compared to people of other descent, making East Asians less prone to body odor.[22][23] The reduction in body odor and sweating may be due to adaptation to colder climates by their ancient Northeast Asian ancestors.[18]
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.[20]


* 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: (E)-3-methyl-2-hexenoic acid; 3M3SH: 3-methyl-3-sulfanylhexan-1-ol.
Alterations

Body odor may be reduced or prevented or even aggravated by using deodorants, antiperspirants, disinfectants, underarm liners, triclosan, special soaps or foams with antiseptic plant extracts such as ribwort and liquorice, chlorophyllin ointments and sprays topically, and chlorophyllin supplements internally. Although body odor is commonly associated with hygiene practices, its presentation can be affected by changes in diet as well as the other factors.[26]
Industry
As many as 90% of Americans and 92% of teenagers use antiperspirants or deodorants.[27][28] In 2014, the global market for deodorants was estimated at 13.00 billion USD with a compound annual growth rate of 5.62% between 2015 and 2020.[29]
Medical conditions
The condition can be known medically as bromhidrosis, apocrine bromhidrosis, osmidrosis, ozochrotia, fetid sweat, body smell, or malodorous sweating.[30][31]
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).[19] This can be particularly strong when it happens in the axillary region (underarms). In this case, the condition may be referred to an axillary osmidrosis.[19]
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.[32]
See also
Butyric acid
Drug resistance
Foot odor
Halitosis (bad breath) 
Old person smell
Olfactophilia
Olfactory fatigue
Pheromone
Sweat gland
Trimethylaminuria




—from UC Berkeley, February 2007
 from Miami Children's Brain Institute

 - Medical article, December 2009.