Why do feet smell?

“If your feet and your nose runs, you must be upside down!”

A common question from patients to podiatrists is “How do you put up with all those smelly feet?” reflecting the belief that foot odour is a common issue. In truth, as most podiatrists know, most feet don’t really smell – but a few do! This article examines the science behind foot odour and its management.

Foot Odour can have a significant effect on quality of life for the sufferer and those around them.

Sweating and foot odour – best friends?

Foot odour (FO) generally follows excessive sweating of the feet. It can be in isolation but can be part of a wider, generalised sweating problem (hyperhidrosis) affecting the armpits, palms, soles and sometimes the face. Estimates from the United States suggest between 2- 4% of the population are affected [1, 2] - most commonly younger adults.  In most cases generalised hyperhidrosis has no identifiable cause (primary hyperhidrosis) but secondary causes include a range of conditions including [2]:

·         Febrile illness

·         Endocrine and metabolic disorders – diabetes, acromegaly, hyperthyroidism etc.,

·         Drug side effects

·         Menopause

·         Neoplastic disease

·         Spinal cord injuries

·         Respiratory disease

·         Infection

Stress can often exacerbate the condition. Generalized sweating which is a problem for the patient requires a medical assessment to elucidate any underlying cause.

Sweaty feet do not always lead to foot odour

Sweating localised to the foot, most commonly occurs in adolescents, those in physically demanding occupations or sports, during pregnancy and the menopause.  Sweat is composed of water, salt with a few amino-acids (including serine, leucine, glycine, alanine) [3], polypeptides, lactic acid and in itself, has no real odour.

For those who have FO it can have a significant effect on a person’s quality of life as studies have shown [4]. In a study of 729 military recruits, 42% (n=309) self-reported foot odour as a problem. The worse the foot odour, the more effect on the person it had using the Dermatology Life Index as a measure (see my earlier blog).

So why do feet smell?

Foot odour has been investigated in a few studies over the years, and it arises as primarily the interaction between bacteria and sweat on the outer most layer of the skin on the foot. Sweat on the skin is oxidised and broken down by bacteria producing a host of chemicals including those that have the recognisable malodour.

What is causing the odour?

Scientific studies investigating this issue have been a bit sparse of the years, but collating what is available gives a consistent picture. In 1880, Thin first medically described foot odour and demonstrated the presence of bacteria amongst the sweat on damp hosiery [5, 6]. Latterly, a study investigating the bacteria living on the feet of 19 males showed there was no particular organism associated with foot odour, although higher densities of gram-positive coryneform and staphylococcus were extracted from patients with foot odour [7]. Moreover, those bacteria that had the ability to secrete enzymes from the bacterial cell into the surrounding area (“exo-enzymes”) created the greatest odour.

In 1990, a study undertook mass spectrometry of the socks of 10 subjects, five with and five without foot odour [8]. The results demonstrated that a short chain fatty acid – isovaleric acid was discovered to be present exclusively in all the socks from malodourous subjects but not in the odour free socks. In addition, reproduction of isovaleric acid in the lab  was confirmed by 14 out of 20 assessors who smelt it to be the classic odour of smelly feet!

Are other chemicals involved?

 

In 2006, a study extracted sweat and bacteria from the feet of females aged 20-30 to identify the cause of the odour [9]. They discovered a range of short chain fatty acids were present on the plantar surface:

1.      Acetic acid (85%)

2.      Propionic acid (7.7%)

3.      Isobutyric acid (1.9%)

4.      Butyric acid (0.8%)

5.      Isovaleric acid (2.3%)

6.      Valeric acid (0.55%)

*those in bold denoting foot odour producing chemicals.

Interestingly, isovaleric acid & isobutyric acid, although in lower concentrations, were shown to be the most detectable to the human nose, even at very low concentrations, as the distinctive smell of foot odour. Acetic acid (vinegar) despite being in higher concentration is less detectable to the human nose.

 

Where does the smell come from?

Several studies have highlighted that the isovaleric and isobutyric acids are a product of sweat degradation – particularly breakdown of the amino-acid L-LEUCINE, naturally present in sweat. Not all bacteria possess the ability to degrade leucine, but bacteria of the genera Bacillus, Corynebacterium and Staphylococcus have this ability and consequently can produce the chemicals of foot odour [9]. Possessing the enzyme leucine dehydrogenase, they are able to liberate isovaleric acid to create the unpleasant odour. In addition, some can liberate isobutyric acid from degradation of the amino acid valine to create a chemical resembling foot odour. Propionic acid has also been found as another cause of malodour from Propionibacterium.

·         Staphylococcus epidermidis

·         Staphylococcus aureus

·         Corynebacterium minutissimum

·         Propionibacterium granulosum,

·         Bacillus subtilis

Bacteria isolated from the foot suspected of causing odour [9]

Pitted Keratolysis a clinical example of foot infection causing malodour

Figure 2 : Pitted Keratolysis is a common foot infection which has a distinctive odour of rancid butter

A relatively common superficial infection of the skin of the foot is pitted keratolysis (PK). It is caused when Corynebacterium sp. infect the outer most layer of the epidermis. It inhabits the skin, enzymatically dissolving the epidermis as a food source. Visually this is seen as little pits in the epidermis (figure 2) which become prominent when the foot is wet producing white patches on the soles of the feet [10]. A by-product of its feeding activities on the skin is the release of butyric ("butrym" derived from the Latin for "butter") acid, which has a pungent acidic, sour smell like rancid butter or..... sweaty feet.

Implications for treatment

Understanding the mechanism by which foot odour occurs can help inform the optimum management.

Eliminating the bacteria: Antisepsis would be the primary approach. Eliminating the odour causing bacteria reduces the foot odour as demonstrated in studies. A podiatrist has a few products at their disposal. Hypochlorous solution, in a review of its applications, has been reported to have significant effect in reducing bacterial load and particularly odour [11]. As an easily accessible, water-based solution with few side effects or allergic issues,  it makes for an excellent treatment. The use of chlorhexidine soap or scrubs may be considered as second line. It has a narrower spectrum of activity as it is less effective on gram negative organisms whilst allergy/sensitivity to it has been reported to be a growing issue [12].  

Prevent the sweating: at a basic level preventing sweat production would decrease the supply to the odour forming bacteria. On this basis, antiperspirants can and have a significant effect but are not often used on the feet. Aluminium based products can reduce sweating by as much as 50% [13].  For more severe sweating, treatments such as iontophoresis [14] may be more effective whilst newer therapies such as botulinum toxins are showing promise [15].

Keeping the feet and shoes dry: Regular washing and drying of feet will remove excess sweat and dry the skin. Shoes can be sprayed with hypochlorous solution to reduce the antimicrobial load and should be thoroughly dried before wearing – many shoe driers, including USB powered devices, are available online. Changing of socks regularly also prevents the accumulation of sweat next to the foot.

Masking the smell: Probably the most palliative measure is using another chemical to mask the smell of foot odour could be used. In a study by Ara and et al. [9], a number of botanical products were tested for just this ability. Citral, citronellal and geraniol were all found to be effective in laboratory testing.

References

1.            Doolittle J, Walker P, Mills T, Thurston J: Hyperhidrosis: an update on prevalence and severity in the United States. Arch Dermatol Res 2016, 308:743–749.

2.            Strutton DR, Kowalski JW, PharmD, Glaser DA, Stang PE: US prevalence of hyperhidrosis and impact on individuals with axillary hyperhidrosis: Results from a national survey. J Am Acad Dermatol 2004, 51:241–248.

3.            Hadorn B, Hanimann F, Anders P, Curtius HC, Halverson R: Free Amino-acids in Human Sweat from Different Parts of the Body. Nature 1967, 215:416–417.

4.            Rujitharanawong C, Bunyaratavej S, Leeyaphan C, Pattanaprichakul P: Impacts of Foot and Body Odor on Quality of Life. J Med Assoc Thai 2018, 101:1371–1374.

5.            Thin G: On the Cause of the Bad Odour Sometimes Associated with Excessive Sweating of the Feet, with Directions for Treatment. Br Med J 1880, 2:463–464.

6.            Thin G: On Bacterium foetidum: an organism associated with profuse sweating from the soles of the feet. Proceedings of the Royal Society 1880, 205:473–478.

7.            Marshall J, Holland KT, Gribbon EM: A comparative study of the cutaneous microflora of normal feet with low and high levels of odour. J Appl Bacteriol 1988, 65:61–68.

8.            Kanda F, Yagi E, Fukuda M, Nakajima K, Ohta T, Nakata O: Elucidation of chemical compounds responsible for foot malodour. Br J Dermatol 1990, 122:771–776.

9.            Ara K, Hama M, Akiba S, Koike K, Okisaka K, Hagura T, Kamiya T, Tomita F: Foot odor due to microbial metabolism and its control. Can J Microbiol 2006, 52:357–364.

10.         Bristow IR, Lee YLH: Pitted keratolysis: a clinical review. J Am Podiatr Med Assoc 2014, 104:177–182.

11.         Armstrong DG, Bohn G, Glat P, Kavros SJ, Kirsner R, Snyder R, Tettelbach W: Expert Recommendations for the Use of Hypochlorous Solution: Science and Clinical Application. Ostomy Wound Manage 2015, 61:S2–s19.

12.         Chiewchalermsri C, Sompornrattanaphan M, Wongsa C, Thongngarm T: Chlorhexidine Allergy: Current Challenges and Future Prospects. J Asthma Allergy 2020, 13:127–133.

13.         Darrigrand A, Reynolds K, Jackson R, Hamlet M, Roberts D: Efficacy of antiperspirants on feet. Mil Med 1992, 157:256–259.

14.         Kreyden OP: Iontophoresis for palmoplantar hyperhidrosis. J Cosmet Dermatol 2004, 3:211–214.

15.         Eilertsen T, Kvammen B, Grimstad Ø: Botulinum Toxin A and B for Palmoplantar Hyperhidrosis. Dermatol Ther (Heidelb) 2024, 14:805–811.