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The Science Behind Smelling Good


Summer is coming and with-it images of people sweating too. Whether you are working out in the gym or having a good run in the park, it is likely that you have sometimes wondered why do we sweat? Sweating is a physiological thermoregulatory phenomenon that governs the core temperature of the body. It can be caused by various factors such as high temperature, stress, anxiety, food, etc. It occurs throughout the body; however, certain places are more prone to sweating than others [1].


Sweat itself is odourless but it can cause unpleasant body odour. How? The body odour is produced by the skin microbiome - which thrives in the moist environment of the skin. These bacteria break down certain proteins and lipids, resulting in the generation of compounds that create odour [1].


The key players involved in the production of body odour are as follows:


1) Sweat gland: Body odour depends on the type of sweat gland. There are two types of sweat glands such as called eccrine glands and apocrine glands. Eccrine glands are found throughout the skin. They squeeze sweat directly to the surface of the skin through a duct. The sweat produced by eccrine glands is mainly salt and other electrolytes [1]. As it lacks the proteins and lipids compounds that can be broken down by bacteria and produce malodorous metabolites, it is less likely to produce a smell. Contrary to it, apocrine glands empty into a hair follicle instead of a duct. These glands are found in selected areas of the body such as the armpits, groin, and pubic area. The sweat produced by the apocrine gland comprises water, salts, electrolytes, and a considerable amount of lipids, glycosaminoglycans, proteins, and steroids that produce malodorous substances when broken down by the bacteria. Therefore, the sweat produced by apocrine glands is mainly responsible for body odour [2].


In addition, it is important to note that apocrine glands start functioning at puberty, which is the main reason why babies do not smell even if they sweat [3].

Unpleasant body odour [6]

2) Different types of skin bacteria: Human skin is home to millions of bacteria. It exhibits a diversity of ecological niches, varying in moisture and nutrient availability [4]. The microbial population of the human armpit plays a critical role in the development of axillary odour by biotransforming odourless natural secretions of the apocrine gland into volatile compounds that produce unpleasant odours. According to the research, the axillary microbiota is mainly dominated by Staphylococcus, Corynebacterium, and Propionibacterium. Among these, Corynebacterium spp. and Staphylococcus spp. predominate in the most humid parts of the axilla [2].


Researchers have established a link between the microbial composition/load on the body and the strength of body odour, with Corynebacterium spp. found to be the primary source of unpleasant axillary odour [2].


3) Various types of metabolites produced: The components that provide the greatest potency of unpleasant odour are volatile sulphur compounds, which resemble the characteristic odour of onion. One of the most potent compounds found is thioalcohol, 3-methyl-3-sulfanylhexan-1-ol (3M3SH), which is secreted as a precursor molecule by the apocrine gland. Once the bacteria cleave the carbon and sulphur bonds, the precursor is subsequently broken down, producing the odour. In addition, any formation of short and medium-chain fatty acids with 3-hydroxy-3-methylhexanoic acid (HMHA), the most abundant fatty acid found in axillary secretions, possesses pungent characteristics. The human olfactory sensitivity to these molecules is high, which contributes to the perception of a bad odour in the axilla [2].


Deodorants and antiperspirants are used to reduce or prevent body odour. However, do you know what is the science underlying these products, and how they work?


Deodorant and antiperspirant to combat unpleasant body odour [7]

Deodorants are products that contain antimicrobial compounds, which work to prevent the growth of odour-causing bacteria. The deodorant formulation contains three essential ingredients such as (1) antimicrobial agents, (2) fragrance, and (3) odour absorbers. The compounds, such as triclosan or chlorhexidine, attack bacteria and prevent them from multiplying. This reduces the amount of odour-causing bacteria on the skin, thus reducing body odour. Additionally, perfume, fragrance, and essential oils are often found in a deodorant formulation to mask body odour. Deodorants work by changing the pH of the skin to make it less hospitable for bacteria [3].

Antiperspirants, on the other hand, work by blocking sweat glands and reducing the amount of sweat produced. Aluminium-based compounds, such as aluminium chloride, are often used in antiperspirants. The mechanism of action of antiperspirant is the formation of gel plugs in sweat pores. A gel plug is formed as a result of the interaction between aluminium salts and the biomolecules in the sweat solution. This reduces the amount of sweat produced resulting in the elimination of the food source for bacteria, thus reducing body odour [3].


The products on the market are still based on a group of well-known and established active ingredients for both deodorant and antiperspirant introduced over 50 years ago. Therefore, attempts are in progress to find out natural and synthetic alternative active ingredients for body odour reduction [3].


Naturally derived ingredients:


1) Bacterial extracts (acetic acid bacteria (AAB) extracts): Two membrane-bound enzymes-alcohol dehydrogenase and aldehyde dehydrogenase (ALDH)-of acetic acid bacteria convert the malodorous aldehydes to carboxylic acid, thus highlighting the potential use of AAB extracts as deodorizing agents[3].


2) Plant extract: The extracts of plants such as Terminalia spp. show antimicrobial activity in addition to antiperspirant activity due to the presence of tannins. However, there is a certain limitation to this approach as tannins could lead to the staining of clothes [3].


3) Essential oils: Essential oils extracted from plants such as Satureja species, Salvia lanigera Poir., Carum copticum, Origanum vulgare Linnaeus, and Asiasarum heterotropoides consist of antimicrobial and odour-absorbing agents [3].


Synthetic ingredients:


1) Anticholinergic agents: They block the action of the neurotransmitter acetylcholine (Ach) by inhibiting its binding to the muscarinic acetylcholine receptor (mAchRs). This receptor is responsible for smooth muscle contraction and increased secretion of sweat and sebum, making anticholinergics a potential alternative treatment for hyperhidrosis (excessive sweating) [3].


2) Block copolymers: Block copolymers, such as a polyquaternium-16 (PQ-16) formulation, decrease bacterial count and axilla odour intensity more effectively than the commercially available roll-on, and are as effective as the aluminium-based formulation [3].


3) Silver nanoparticles (AgNPs): AgNPs show antimicrobial activity. A hybrid material comprising AgNPs has been found to have higher odour-absorbing activity than currently used compounds such as zinc ricinoleate [3].


4) Deodorizing fabrics: The photocatalysts and nanofibers have the potential to reduce the concentration of malodorous gas through physical absorption. Exploiting this inherent characteristic of photocatalysts and nanofibers, fabrics containing photocatalysts (nanoscale-TiO2) or phthalocyanine nanofibers that possess the ability to reduce body odour have been developed. However, this technology is still under development and awaits FDA approval [3].


5) Hygroscopic materials: Hygroscopic materials, such as propylene glycol, have also been studied and found to be effective in forming a gel plug to obstruct sweat flow [3].


There are several different types of products available in the market to reduce or prevent body odour [5]. As Deodorants and antiperspirants include distinct active chemicals, each of which works differently to decrease body odour. Knowing the science of deodorant and antiperspirant is important for consumers to ensure that they are using a product that is most effective for their individual needs.


Additionally, understanding the science behind deodorant and antiperspirant can help consumers to avoid potential skin irritation caused by certain ingredients and to make more informed decisions when it comes to choosing a product.


References:

1) Baker, L. B. (2019). Physiology of sweat gland function: The roles of sweating and sweat composition in human health. Temperature, 6(3), 211–259. https://doi.org/10.1080/23328940.2019.1632145

2) De Oliveira, E. M. N., Salvador, D. S., De Souza Santos Holsback, V., Shultz, J. D., Michniak-Kohn, B., & Leonardi, G. R. (2021). Deodorants and antiperspirants: identification of new strategies and perspectives to prevent and control malodor and sweat of the body. International Journal of Dermatology, 60(5), 613–619. https://doi.org/10.1111/ijd.15418

3) Teerasumran, P., Velliou, E., Bai, S., & Cai, Q. (2023). Deodorants and antiperspirants: New trends in their active agents and testing methods. International Journal of Cosmetic Science. https://doi.org/10.1111/ics.12852

4) Grice, E. A., & Segre, J. A. (2011e). The skin microbiome. Nature Reviews Microbiology, 9(4), 244–253. https://doi.org/10.1038/nrmicro2537

5) Reserved, C. ©. K. C. a. R. (2020c, February 5). Which Type of Deodorant Is Best for You? | Ban. Ban Deodorants. https://www.bandeodorant.com/en-us/blog/deodorant-101/types-of-deodorant/#:~:text=Types%20of%20Deodorant%201%20Spray%20Deodorant%20This%20convenient,Solid%20Deodorant%20...%206%20Invisible%20Solid%20Deodorant%20. Accessed on 2nd April 2023


6) Image sourced from Freepik: Image by <a href="https://www.freepik.com/free-vector/teen-girl-with-smelly-armpits_39207511.htm#query=stinking%20underarms&position=35&from_view=search&track=ais">Image by brgfx</a> on Freepik

7) Image sourced from Freepik: Image by <a href="https://www.freepik.com/free-vector/hand-drawn-hair-keratin-illustration_32283324.htm#page=2&query=perfume%20applying%20girl&position=17&from_view=search&track=ais">Freepik</a>


Keywords: Sweat, sweating, sweat gland, bacteria, microbial population, deodorant, antiperspirant, body odor.


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