Your feet are home to billions of bacteria. How often should you wash them?

Some people scrub them every day while others say letting water run over them in the shower is enough. So are you cleaning these important appendages enough?

When you jump into the shower and reach for the soap and loofah, it's fair to say that certain body parts probably receive more attention than others. The underarm region no doubt gets the full lather, rinse, repeat treatment. Your feet, by virtue of being located at the end of your body, can be easy to overlook. However, according to some experts, your feet are just as, if not more, deserving of attention.

Both the UK's National Health Service (NHS) and US Centre for Disease Control (CDC), for example, advise washing feet daily with soap and water. One reason for this meticulous care is to prevent odour. The soles of the foot contain 600 sweat glands per square centimetre of skin, more than any other region of the body.  Although sweat itself doesn't smell, it contains a nutritious broth of salts, glucose, vitamins and amino acids, which serves as an all-you-can-eat buffet for bacteria that live there. And there a lot of bacteria.

"The foot – especially between the toes – is quite a moist, humid, and warm environment, so it can be a breeding ground for microbes," says Holly Wilkinson, a lecturer in wound healing at the University of Hull in the UK. This is exacerbated by the fact that most people encase their feet in socks and shoes, trapping the moisture inside.

If you zoom in on any square centimetre of human skin you will find between 10,000 to one million bacteria living there. Warm and moist areas of the skin, such as the feet, are considered prime real estate and are home to the greatest numbers of species. Feet are idyllic havens for Corynebacterium and Staphylococcus bacteria, for example. When it comes to fungi, your sweaty feet are considered a utopia to genera including Aspergillus (a pathogen often found in soil), Cryptococcus, Epicoccum, Rhodotorula, Candida (a kind of yeast which naturally lives on the body but can become an opportunistic pathogen), Trichosporon and others. In fact, the human foot contains a greater biodiversity of fungal species than any other body region.

This is probably a good reason to clean your feet. In one study, researchers swabbed the soles of 40 volunteers. They found that foot washing had a significant impact on bacteria numbers. People who washed their feet twice a day had around 8,800 bacteria living in each square centimetre of skin. Those who reported washing every other day had over one million bacteria per square centimetre.

However just because the soles of your feet are brimming with microbial life, that doesn't mean that they are necessarily smelly or that there is anything to worry about. As always, it's not just the number, but the type of bacteria that's important.

Staphylococcus are the key players when it comes to producing the volatile fatty acids (VFAs) responsible for foot odour. Sweat glands on the skin of the feet release a heady mix of electrolytes, amino acids, urea and lactic acid. The Staphylococcus bacteria consider this a veritable feast and, in the process of feeding, convert amino acids into VFAs. The main chemical culprit is isovaleric acid, which has an unpleasant odour which has been described as having a "distinct cheesy/acidic note". The comparison is apt, as many cheeses contain a similar mix of volatile chemicals.

In one 2014 study, researchers swabbed the feet of 16 subjects and found that 98.6% of the bacteria present on the soles of the feet were Staphylococci. The levels of VFAs, including the key foot odour compound isovaleric acid, were also significantly increased on the sole of the foot compared to the bridge (top) of the foot. Overall, the study concluded that the intensity of foot malodour was correlated to the total number of Staphylococcus present – another reason to reach for the soap. 

However, washing your feet isn't just about preventing cheesy foot odour. Many diseases and foot problems can be prevented through good foot hygiene.

"Because of the small space between the toes, these areas are particularly at risk for microbial infections," says Joshua Zeichner, associate professor of dermatology at the Mount Sinai Hospital in New York. "This can lead to itching, swelling, and a foul smell. As the skin barrier becomes disrupted, this can also increase the risk of microorganisms invading the skin and causing more significant soft tissue infections known as cellulitis," he says. 

According to Zeichner, the most common problem is the development of athlete's foot, which is a superficial fungal infection of the skin on the feet. The fungi that cause athlete's foot thrive in warm, dark, and moist environments – hence why this condition most commonly affects the spaces in-between the toes. Keep this area clean and dry and you deprive the fungi of their perfect home. This is a good thing, as athlete's foot can cause a series of unpleasant symptoms such as itchiness, a scaly rash, flaky skin and cracking on the soles of your feet and between your toes.

Keeping your feet clean could also prevent skin infections, such as those caused by Staphylococcus or Pseudomonas bacteria. While these bacteria exist naturally on your skin, if they get into your bloodstream via a cut then it can lead to a serious infection. Even a minor staph infection can lead to boils – bumps of pus that form under the skin around hair follicles or oil glands.

"The feet are more prone to infections because there's quite a lot of biomass of bacteria there, and also if you do have cracks or injuries to your feet, it tends to heal much more slowly than other areas of the body," says Wilkinson. "In a situation like that, there's a greater chance that if you have an injury, pathogens could get into that wound, populate and overgrow."

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While skin infections can still occur if you have good foot hygiene, regularly washing your feet reduces the number of bacteria present. So, if you happen to get a cut, there will be less microbes around to get into the bloodstream.

Frequent foot washing is especially important if you suffer from diabetes, a condition that makes people prone to ulcers and skin infections. Research has shown the feet of diabetic patients contains a higher proportion of pathogenic bacteria residing on the skin. 

"They are there waiting for an opportunity to cause an infection. So, it's really important that people with diabetes are keeping on top of their foot hygiene, because they're at more risk of developing infection because of that," says Wilkinson.

To make matters worse, people with diabetes also have an impaired immune response, so if they do get an infection, their body can't fight it off. Diabetes patients are also prone to cuts, wounds, and sores in the feet that don't heal. If these aren't caught early, then toes, feet, or even limbs may need to be amputated.

"If you have uncontrolled diabetes, you may have damage to the nerves in your feet, so you can't feel your feet properly," says Wilkinson. "Just the act of washing allows you to properly check your feet for any minor abrasions or dryness that might contribute to having an infection."

For that reason, Wilkinson – and charities such as Diabetes UK – recommend that diabetes patients wash their feet every day.

But what about everyone else? Some experts argue that for most people, washing the feet every day has little health benefit, and can even raise the risk of skin problems.

After all, the skin relies on its community of helpful microbes to perform essential functions. They repel harmful bacteria, produce lipids that keep the skin hydrated and supple, and even help repair wounds. Intensive washing and scrubbing can remove these beneficial species, especially if the water is hot. As a result, skin can become dry, irritated, or itchy. Cracked skin may allow bacteria to breach the usually impenetrable skin barrier, increasing the likelihood of infections.

"Overwashing the skin can disrupt the skin barrier, stripping the skin of natural oils, contributing to dryness and inflammation," says Zeichner. This leads to itchy, dry skin and can exacerbate conditions like eczema.

"It is also important not to overly scrub or exfoliate the skin on the feet," says Zeichner. "Calluses develop because of daily trauma. But they actually protect the feet from the environment. Removing calluses takes away that protection."

There is also a concern that antibacterial soaps could upset the delicate balance of microorganisms on the skin, killing the beneficial species and allowing the emergence of hardier, pathogenic strains that are resistant to antibiotics. Finally, our immune system needs to be challenged to a certain extent by microbes in order to do its job. If we don't come into contact with a steady stream of bacteria and viruses in childhood, then our bodies don't learn how to properly respond to attack. Some experts believe that bathing or showering too frequently could actually be counterproductive for you for this very reason.

So that leaves us with the perennial question, how often should we wash our feet? The answer depends to some extent on the individual.

"For people with diabetes, it is 100% advised that you wash your feet every day," says Wilkinson. "But if you don't have any underlying conditions, then dermatologists tend to advise that every couple of days is more than enough to maintain good hygiene, without stripping too much of the natural oils on your skin."

However, Wilkinson points out that if you're somebody likes to run or work out at the gym, then you will obviously need to wash your feet more regularly than somebody who is less active. It's also not just the frequency of washing that's important either. How you wash and dry your feet also has health implications.

"A lot of people think that if you have a shower and you kind of just let the water trickle, that that's washing feet, but it isn't – you need to actually physically wash your feet with soapy water," says Wilkinson.

However, according to Dan Baumgardt, a GP and lecturer in neuroscience and physiology at the University of Bristol in the UK, the most important thing he stresses to patients is to make sure you dry your feet properly. "When you've got wetness or dampness in between the toes that's allowed to just remain there in a warm environment, that's when you're prone to developing things like athlete's foot and other fungal infections," says Baumgardt.

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