Body odor is present in all animals, including humans, 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 diseases and physiological conditions. Though body odor has played an important role (and continues to do so in many life forms) in early humankind, it is generally considered to be an unpleasant odor amongst many human cultures.
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. 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. 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. This happens mostly in the axillary (armpit) region, although the gland can also be found in the areola, anogenital region, and around the navel. 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.
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). E-3M2H is bound and carried by two apocrine secretion odor-binding proteins, ASOB1 and ASOB2, to the skin surface.
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. Staphylococcus hominis is also known for producing thioalcohol compounds that contribute to odors. These smaller molecules smell, and give body odor its characteristic aroma. 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, which is also present in several strong cheese types.
Factors such as food, drink, and diseases can affect body odor. An individual's body odor is also influenced by lifestyle, sex, genetics, and medication.
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