Gut bacteria and hair loss
Gut bacteria and hair growth


The human gut harbours a vast array of bacteria that comprises approximately 160 species in the small and large intestine.  There are very few species shared between unrelated individuals, but the functions carried out by these species appear to be similar in everybody’s gastrointestinal tract.

Your gut bacteria carry out several metabolic reactions not encoded by the human genome but necessary for human health.  Gut bacteria are responsible for the extraction of energy from food, the production of essential vitamins and the regulation of the immune system.

In addition to this important role in the intestine, evidence indicates gut bacteria contributes to physiological regulation in other areas of the body such as the liver, skin and the hair follicles of the scalp.

Everyone is provided with a unique gut bacteria profile that plays specific functions in host nutrient metabolism, maintenance of intestinal structural integrity, modulation of the immune system, and protection against pathogens.

The personal core native bacteria remain relatively stable in adulthood but differ between individuals.  There is no unique optimal gut bacteria profile as metabolic requirements differ from person to person.  However, a healthy host-bacteria balance must be struck in order to optimally perform metabolic and immune functions and prevent development of chronic disorders like insulin resistance, obesity, irritable bowel syndrome and hair loss.


Gut bacteria are part of the gut microbiota which are composed of several species of microorganisms, including bacteria, yeast and viruses.  Bacteria are classified according to phyla, classes orders, families, genera and species.  Only a few phyla account for more than 160 species.  The dominant phyla that make up gut bacteria are Firmicutes, Bacteroidetes, Actinobacteria, Proteobacteria, Fusobacteria and Verrucomicrobia, with the two phyla Firmicutes and Bacteroidetes representing 90% of gut bacteria.

Intestinal bacteria are made up of three basic types.   Most bacteria in the digestive tract are probiotic, meaning they are “good” or “friendly”.  Examples of good bacteria are Bifidobacteria and Lactobacillus.   “Good” bacteria help with the breakdown and absorption of nutrients in the gut.  They help detoxify substances and manufacture antibiotics such as acidophilin, which are effective at controlling Streptococcus and Staphylococccus.  These probiotic bacteria provide a front-line defence for the immune system.

The second type of bacteria found in the gut are “commensals”, they are neutral and neither harmful nor helpful.  Commensals are a critical and active inducer of regulatory immune responses.  Most notably, the establishment of tolerance and the active suppression of inflammatory responses to food and other ingested substances that could activate the immune system.

The third type of bacteria are “bad” bacteria, this harmful bacterium can lead to illness.  Examples of pathogenic bacteria are Clostridium, Salmonella, Staphlococcus, Proteus, Campylobacter and Listeria.  Harmful bacteria are often found in small quantities and are harmless in small numbers.  These bacteria are weak, but if they are given the opportunity to thrive, they can cause intestinal dysbiosis.


Dysbiosis in the small and large intestine results in metabolic problems for the host that can lead to hair loss conditions such as androgenic alopecia, alopecia areata and telogen effluvium.

Gut bacteria populations change constantly in response to the environment of the gut. There are several factors that can alter the population of the gut.

The most common causes of dysbiosis are:

  • A change of gastrointestinal pH
  • Antibiotic use
  • Oral contraceptives
  • Pesticides, herbicides, preservatives, solvents
  • Chemotherapy and radiation exposure
  • Stress
  • Refined sugars and grains
  • Excessive alcohol consumption (especially fermented alcohol such as wine, beer and cider)
  • Food additives
  • pylori infection
  • Heavy metal exposure
  • Non-steroidal anti-inflammatory drugs

Gut dysbiosis reduces the metabolic functions of the bacteria in the gut.  Dysbiosis is a counter state to symbiosis, where the human benefit from the bacterial presence.  In a state of dysbiosis, the bacteria in the gut can contribute to chronic conditions and commensal bacteria may become pathogenic under the right circumstances.

Pathogenicity relates to the potential ability to cause disease.  The pathogenic potential of “bad” bacteria depends on the capacity of a given microbe, including the ones comprising our own gut bacteria, to trigger or promote disease is highly dependent on the localisation of the microbe and the nutritional state and genetic predisposition of the host.

A large fraction of the immune systems function is aimed at controlling the relationship with the bacteria resident in the body.  This means the highest number of immune cells in the body are resident in the gut, scalp and the skin.

The gut bacteria keep a delicate balance in the immune system by eliminating invading pathogens, while still maintaining self-tolerance is critical for the body’s health.  The alteration of the gut bacteria to more pathogenic bacteria can cause immune dysregulation leading to autoimmune disorders such as alopecia areata.  Gut bacteria has a role beyond the local immune system and impacts many systemic immune components.  Changing even a single bacterial species can alter the outcome or virulence of an autoimmune disease by tipping the balance between a pathologic or protective immune response.

The lining of the gastrointestinal tract is a natural protective barrier against invading pathogens.  When the lining of the gut becomes irritated or inflamed due to an abundance of “bad” bacteria or an absence of “good” bacteria that provides fuel for intestinal cells, its natural function is significantly impaired leading to malabsorption of the proteins, carbohydrates, fats, vitamins and minerals required for healthy hair growth


When the gut bacteria community shifts, the repercussions can be far reaching.  The gut bacteria community can shift in response to a change in diet, alcohol consumption, stress, medication or illness.  These community shifts can occur in as little as a day and can also be recovered as quickly.  Metabolic and functional effects of long term dysbiosis can take much longer to correct and in some cases require interventions over and above a change of diet and lifestyle.

Three specific hair loss conditions linked to a change in the levels of “good” gut bacteria are alopecia areata, androgenic alopecia and telogen effluvium.


Alopecia areata is characterised as an autoimmune disease.  Patients with alopecia areata experience patchy hair loss, with spontaneous relapses and recovery.  Alopecia areata is a T-cell mediated autoimmune disease caused by T-cell infiltrating and destroying the connective tissue around the hair bulb.  Gut bacteria regulate innate and adaptive immune response which directly affect regulation, balance and behaviour of T-cells.

A study by Dr James Chen showed that wiping out the gut microbiota with antibiotics, prevented alopecia areata in a study of mice.  Other researchers have found increased permeability of the intestine due to dysbiosis and /or inflammation places increased stress and triggers the disease in those genetically predisposed to alopecia areata [1].  Gut dysbiosis also effects T-cell tolerance, this is a key factor in the development of alopecia areata as it is a T-cell mediated autoimmune disease [2].

Patients with alopecia areata have been found to have increased populations of Holdemania filiformis, bacteria in the Erysipelotrichacea and Lachnospiraceae families, Parabacteroides johnsonii, Clostridiales vadin BB60 group, Bacteroides eggerthii, and Parabacteroides distasonis (Moreno-Arrones).  The same study also found an association between these colonisations and the presence of alopecia areata.  Finally, the presence of Lactobacillus species in the gut has been shown to be essential for the progression of this type of hair loss.


Androgenic alopecia is an androgen mediated disease, characterised by an increased level of DHT in the hair follicle.  In the body, DHT upregulates hair growth, in the scalp, DHT reduces hair growth and leads to a recession at the temples and the frontal area of the scalp.  The severity of androgenic alopecia can range from mild to severe. This type of hair loss can start any time after the onset of puberty.

The sensitivity of the hair follicle to DHT is determined by several factors including metabolic and genetic factors.  Finasteride, the main medication prescribed for androgenic alopecia, prevents the formation of DHT by inhibiting 5-alpha reductase, an enzyme that metabolises DHT from testosterone.

Gut bacteria is a major regulator of androgen metabolism as it plays a role in the hormonal clearance pathway.  When testosterone and DHT are due to be excreted from the body, they are conjugated in the liver for easy removal.  This process, glucuronidation, increases water solubility and allows androgens to be excreted via bile to the small intestines.

Bifidobacterium, Lactobacillus, Escherichia coli, Clostridium, Bacteroides fragilis and other Bacteroides species, can produce an enzyme called beta-glucuronidase, this frees testosterone and DHT in the intestinal tract.  A study looking at the effect of deglucronidation found levels of testosterone did not increase in response to higher free levels of testosterone but did show elevated levels of circulating DHT [3].  High fat, high protein and low fibre diets have been associated with higher beta glucuronidase activity when compared to diets with a high fibre content.


Telogen effluvium is a common type of hair loss characterised by thin and slow growing hair.  This type of hair loss is often associated with metabolic disruption such as an imbalanced lipid profile, anemia, iron deficiency, stress, medication, food intolerance and IBS.  Because there are a vast number of triggers for this type of hair loss it is difficult to characterise a specific community that contribute to this type of hair loss.

The change in bacterial community in telogen effluvium is likely to be similar to those in patients with IBS. Studies confirmed and revealed some phyla and genera variations in IBS patients compared to healthy controls: an increase in the Firmicutes to Bacteroidetes ratio, a decrease in some Firmicutes families (LactobacilliFaecalibacterium) and the Actinobacteria population (Bifidobacteria, Collinsella), and an increase in some Firmicutes families (Veillonella, Streptococciand Ruminococcus) and in Proteobacteria (Enterobacteriaceae).

These findings demonstrate a loss of microbial richness that are involved in amino acid synthesis, the integrity of cellular junctions, and inflammatory response.  A reduction in amino acid synthesis means a reduction in protein availability for the hair structure.  Inflammation reduces nutrient uptake and overall metabolism contributing to disrupted iron metabolism.


The cells that make up the hair follicle have a high energy requirement due to the number of rapid divisions per cell.  Carbohydrate fermentation is a core activity of gut bacteria, driving the energy and carbon metabolism of the body.

The sophisticated relationship that has evolved between humans and the bacteria that reside in the gut allows for effective utilisation of dietary carbohydrates.  In the small intestines, simple sugars (such as glucose) are absorbed and disaccharides (e.g., lactose) are broken down into monosaccharide components so they too can be absorbed.  A significant portion of dietary carbohydrates including plant-derived complex carbohydrates and starch, normally pass undigested through to the large intestine.

It is here that dense bacteria populations are equipped to hydrolyse these complex carbohydrates.  Many of the enzymes required to digest and absorb complex carbohydrates are not encoded by the human genome whereas the “good” bacteria in the gut are abundant in such enzymes.  Utilisation of complex carbohydrates via fermentation by anaerobic bacteria in the large intestines results in the accumulation of short chain fatty acids.

Short chain fatty acids are absorbed in the large intestine, where butyrate provides energy for intestinal cells, and acetate and propionate provide an energy source for the liver, peripheral organs and the hair follicle.

Given the energy and nutritional requirements for healthy and robust hair growth, the effective breakdown of carbohydrates is essential and protects against hair loss.


Gut bacteria are key regulators of digestion along the gastrointestinal tract; commensal bacteria play an important role in the extraction, synthesis and absorption of many nutrients and metabolites, including bile acids, lipids, vitamins, amino acids, and short chain fatty acids.

Dietary habits are a major factor in influencing the diversity of gut bacteria.  By using germ free mice that were faecal transplanted with human gut bacteria, one study demonstrated that switching from a low-fat diet rich in complex carbohydrates to a high-fat, high sugar diet can shift the configuration of gut bacteria in one day.  This diet-altered gut bacteria were enough to induce obesity in mice within 2 weeks [4].

In another study, gut bacteria were compared in stool samples of children from Europe and rural Africa [5].  The diet of African children is rich in fibre, starch and plant polysaccharides (starch, cellulose, pectins) and low in animal fat and animal protein, whereas the diet of European children is high in sugar, starch and fat and low in fibre.  When compared with the European children in the study, the gut bacteria of African children had a significant depletion in Firmicutes and an increase in Bacteroidetes.  Interestingly, species of the bacteria Prevotella and Xylanibacter, which are known to encode genes for metabolising plant polysaccharides were observed in African children but completely absent in European children.  A significantly higher level of anti-inflammatory molecules, like short chain fatty acids were also found in African children.

High protein diets are often thought to contribute to healthy hair growth.  The fermentation of amino acids can provide much needed short chain fatty acids, but this process also produces a range of potentially harmful compounds.  Studies show that compounds such as ammonia, phenols, p-cresol, certain amines and hydrogen sulphide play important roles in the initiation or progression of increased intestinal permeability and inflammation.

High levels of fat in the diet are also problematic to the gut bacteria community.  A high-fat diet changes the bacterial communities and increases biomarkers of inflammation.  The mechanisms that make a high-fat diet problematic are likely to be more complex that the simple concept of energy balance.  Emerging evidence shows that high levels of fat in the diet lead to a decrease in Bacteroidetes and an increase in Firmicutes that are associated with several chronic disease and the onset of hair loss.


The most common form of dysbiosis is the overgrowth of a yeast called Candida albicans.  candida normally inhabits the mucus membranes of the body including the gastrointestinal lining, the urinary tract, the sinus cavity and the vagina.  Candida is harmless in small quantities, but when candida grows in population size this will become intolerable to the host.

A weak immune system allows candida to flourish and dominate the healthy bacteria in its human host. Candida feeds of decaying matter in the intestinal tract such as partially fermented carbohydrates, especially from grains.  Whilst candida is regarded as an opportunist yeast, it does serve to remove carbohydrates from the digestive tract.

Candida thrives on sugar, a diet high in refined sugar and grains will promote the growth of yeast.  The body requires several nutrients to maintain the balance between good bacteria and pathogens such as yeast.  An overgrowth of yeast further inhibits the ability to assimilate nutrients from the diet.  Nutrient deficiencies including zinc, iron and B12 are linked to high candida levels through its effect on the immune system.


In one study, supplementation with probiotic Lactobacillus reuteri increased anagen hair follicle counts by 106% when compared to a control group.  Researchers determined that the probiotic-fed male mice had 74% of their hairs in the active, anagen phase, while the control group only had 36%. In comparison, in the control group 64% of hairs were in the non-growth, telogen phase [1].

Probiotics and prebiotics are popular due to the associated nutritional claims and reported health benefits such as improving digestive transit time, supporting weight loss, relief of depression or anxiety and boosting immunity.   Rigorous laboratory studies looking at improvements in specific parameters can involve a small number of biomarkers and quality control during the manufacturing process can be somewhat lax.  Of the studies that can show differences in biomarkers (such as cholesterol or glucose levels), none show any clinical benefit; rather assumptions are made from extrapolated laboratory values.

A study looking at the effects of probiotics that generated short chain fatty acids, butyrate, showed probiotics did not reverse chronic alopecia areata in 16 weeks of treatment, but there was a 15% improvement in T-cell counts compared to an improvement of 12% in the control group [6].

From 2016 to 2017 the FDA inspected more than 650 probiotic facilities and found 50 percent had violations including low purity, low strength and incorrectly categorised bacterial strains in the final product [7].

In Italy, a national survey of probiotic suppliers claimed that bifidobacterial was present when none contained the bacteria at all [8].  Another study found of 16 probiotic products, only the contents of one matched its label claims, some contained little or no viable bacteria [9].  A third study investigating 26 probiotics products found misidentification at species (27 incorrectly identified) and genus levels (19 incorrectly identified) [10, 11].

Safety concerns have been raised concerning the use of probiotics and the resulting abnormal read counts (number of times a particular bacterium was identified) in otherwise healthy subjects.  A study looking at the impact of probiotics found a significantly lower abundance of Bifidobacteria which could potentially have a detrimental effect on health.

There are potential harms as well as some benefits with consuming probiotics and prebiotics.  Some prebiotic supplements have been found to contain harmful bacteria that produce toxins (such as Bacillus cereus).

Probiotics can adversely modify gut bacteria and clinical studies on their effects are questionable.  Whilst probiotics and prebiotics can offer some health perks, they may not be appropriate for long term change and stability in resident gut bacteria diversity and species richness.   Zinc supplementation, however, is a solid and proven method of increasing gut bacteria diversity within the species parameters that are correct for the individual.


Zinc is an essential micronutrient involved in many cellular functions in both humans and bacteria.  The availability of zinc in the body impacts the survival and pathogenicity of gut bacteria.  Recent studies show that levels of dietary zinc alter the composition if gut bacteria.

A study probing the effects of zinc in the gut bacteria population showed that supplementation with zinc sulphate (a combination of zinc and sulphur) resulted in significant changes in gut bacteria.

Zinc deficiency has a significant effect on the bacteria population and diversity in the intestines.  A study by Koren and Tako aimed to characterise specific gut bacteria populations in zinc deficiency [12].  They found that zinc deficiency decreases overall species richness and diversity establishing a bacterial profile like that of other diseased states.  Through gene analysis of the reduced gut bacteria available, they determined a significant depletion of macro- and micronutrient uptake [13].

Zinc deficiency leads to an overgrowth of bacteria able to survive in a low zinc concentration.  Recent studies have shown high doses of zinc increased the presence of gram negative facultative anaerobic bacterial groups, the concentration of short chain fatty acids, and overall richness and diversity [14, 15, 16].

Likewise, others have found enriched gut bacteria populations, specifically Lactobacillus, leading to favourable changes on metabolic activity.  The study found a decrease in the detoxification of bacterial toxins, ethylbenzene degradation, linoleic acid metabolism, tetracycline biosynthesis, lipid metabolism, carbohydrates digestion and absorption, and mineral absorption [17].


A team of Japanese scientists gave mice a diet with, and without biotin, and observed no hair loss.  The experiment was then repeated after administration of antibiotics and this time hair loss was observed in the (now) bacteria-free mice.  The scientists discovered that a particular type of bacteria, Lactobacillus murinus, had expanded after the antibiotic treatment.  This bacterium could not produce biotin and was the cause of the biotin deficiency that led to the hair loss.

Hair loss was reversed with biotin injections.  The study was able to demonstrate that gut bacteria can manufacture essential nutrients for hair growth, even when these nutrients are completely absent from the diet.  Bacteria also provide other essential nutrients such as vitamin K, vitamin B12, niacin and folic acid.


An analysis of the gut bacteria population of the participants of the American Gut Project [18] concluded that increasing the frequency of moderate exercise from never to daily causes greater diversity among the Firmicutes phylum (including Faecalibacterium prausniizii and species from the genus Oscillospira, Lachnospira and Coprococcus) which contribute to a healthy gut environment.

Although very little is known about the contribution of gut bacteria to an individual’s exercise performance.  Several studies show that exercise alone induces modifications in the gut bacteria composition.


  1. You find it difficult to lose weight; Studies have linked a lack of beneficial bacteria, such as Lactobacillus or Bifidobacterium, to issues with weight management.
  2. You’re experiencing gas and/or bloating; A lack of “good” bacteria can cleave gassy by-products in the gut; this can lead to gas or bloating.
  3. You have digestive issues; Some people experience digestive problems like constipation or diarrhoea, due to inflammation and nutrient malabsorption caused by dysbiosis.
  4. You have hair loss; Imbalances in the gut microbiome (as well as a genetic disposition) can lead to a loss of species richness that may be involved in amino acid synthesis and an increase in inflammatory response.


The gut microbiome has the capability to regenerate itself to host an array of beneficial bacteria.  Through diet, nutritional supplementation, and exercise, most incidences of gut bacteria dysbiosis can be resolved.

  1. Consume a wide range of foods; A ‘healthy hair diet’ consisting of many different high quality and unprocessed food types can lead to a diverse population of beneficial gut microbiota.  Reduce intake of grains, refined sugars, alcohol, caffeine and high fat food. Add as much fruit and vegetables to your diet as you can comfortably digest.
  2. Supplement with zinc; Boost your gut population by taking a high-quality zinc supplement
  3. Start sweating; Recent studies show that exercise is beneficial for the gut microbiota. It is linked to an increase in the number of beneficial microbial species, an increase in microbial diversity, as well as improved short-chain fatty acid synthesis, and carbohydrate metabolism.


Our gut bacteria is necessary for the uptake of many nutrients vital for healthy hair growth.

The simplest way to support bacterial diversity within the gut is to take a high-quality zinc supplement, eat high quality whole foods and exercise regularly.


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  14. Dose-dependent effects of dietary zinc oxide on bacterial communities and metabolic profiles in the ileum of weaned pigs. J. Anim. Physiol. Anim. Nutr. 2012,96, 825–833.
  15. Bar-Coded Pyrosequencing of 16S rRNA Gene Amplicons Reveals Changes in Ileal Porcine Bacterial Communities Due to High Dietary Zinc Intake. Appl. Environ. Microbiol. 2010, 76, 6689–6691.
  16. Chronic Dietary Zinc Deficiency Alters Gut Microbiota Composition and Function.
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