Does fasting cause hair loss? It can — and understanding why is key to protecting your hair health. Whether you follow intermittent fasting, a very low-calorie diet, or a more prolonged fast, significant caloric restriction places physiological stress on the body. As a result, non-essential processes such as hair growth may be deprioritised, potentially triggering a form of diffuse shedding known as telogen effluvium. This article explains the mechanisms behind fasting-related hair loss, the nutritional deficiencies involved, and when to seek medical advice from your GP.

How Fasting Affects the Body and Hair Growth Cycle

Fasting can trigger telogen effluvium by causing hair follicles to prematurely enter the resting phase, with shedding typically appearing six to twelve weeks after caloric restriction begins. Severity correlates with the degree and duration of restriction.

Fasting — whether intermittent, prolonged, or as part of a very low-calorie diet (VLCD) — places the body under a degree of physiological stress. When caloric intake is significantly reduced, the body prioritises essential functions such as cardiac activity and brain function. Non-essential processes, including hair growth, may be deprioritised as a result. This biological triage can directly influence the hair follicle cycle.

Hair growth follows a structured cycle with three main phases:

• Anagen – the active growth phase, lasting two to seven years

• Catagen – a short transitional phase lasting around two weeks

• Telogen – the resting phase, after which hairs are shed

During periods of significant caloric restriction or nutritional stress, a larger proportion of hair follicles may prematurely enter the telogen (resting) phase. This can result in increased shedding several weeks to months after the fasting period begins — a phenomenon known as telogen effluvium (TE). Shedding is typically delayed, meaning hair loss may not become apparent until six to twelve weeks after the triggering event, which can make the connection to fasting less obvious. This pattern is described in guidance from the British Association of Dermatologists (BAD) and the Primary Care Dermatology Society (PCDS).

Not sure if this is normal? Chat with one of our pharmacists →

The severity of hair loss related to fasting tends to correlate with the degree and duration of caloric restriction. Based on current evidence, short-term or moderate intermittent fasting — such as a 16:8 eating pattern — appears less likely to cause significant hair loss than prolonged fasting or crash dieting, though direct evidence is limited and the risk rises with severe or prolonged restriction. Individual responses vary, and genetic predisposition, baseline nutritional status, and overall health all play a role.

Important: who should seek medical advice before fasting

Certain groups should consult a GP or other healthcare professional before undertaking any fasting or very low-calorie regimen, as the risks — including nutritional deficiency and hair loss — may be greater:

• People who are pregnant or breastfeeding

• Children and young people under 18

• People who are underweight

• People with diabetes, particularly those taking insulin or other hypoglycaemic medicines

• People with an active or recent history of an eating disorder

• People with chronic health conditions or taking regular prescribed medicines

The NHS provides guidance on the safe use of very low-calorie diets, and a healthcare professional can help you assess whether a fasting approach is appropriate for your circumstances.

Nutritional Deficiencies Linked to Hair Loss During Fasting

Fasting can deplete iron, protein, zinc, biotin, vitamin D, and ferritin — all nutrients essential for healthy hair follicle function. Supplementation should only follow confirmed deficiency, as excess intake of some nutrients can itself cause harm.

One of the primary mechanisms by which fasting may contribute to hair loss is through the depletion of key nutrients that are essential for healthy hair follicle function. Even short periods of restrictive eating can reduce the availability of micronutrients that hair follicles depend upon for normal growth and structural integrity.

Several specific nutritional deficiencies are recognised in the context of hair loss:

• Iron – Iron deficiency, particularly in premenopausal women, is one of the most common nutritional causes of hair thinning. Fasting diets that restrict red meat or overall caloric intake can reduce iron stores, impairing the delivery of oxygen to hair follicles. The NHS provides guidance on recommended iron intake and dietary sources.

• Protein – Hair is composed almost entirely of keratin, a structural protein. Inadequate dietary protein during fasting reduces the availability of amino acids needed for hair shaft production.

• Zinc – Zinc plays a role in hair tissue growth and repair, and deficiency has been associated with hair loss. However, zinc testing and supplementation should only be considered where there is clinical suspicion of deficiency, as excess zinc can cause copper deficiency and other adverse effects.

• Biotin (Vitamin B7) – True biotin deficiency is uncommon in people eating a varied diet. Very restrictive diets may reduce intake, and biotin supports keratin infrastructure. Important: the MHRA has issued a Drug Safety Update warning that high-dose biotin supplements can interfere with a range of laboratory tests, including thyroid function tests and troponin assays, potentially causing false results. If you are taking biotin supplements, inform your GP or any clinician arranging blood tests before the sample is taken.

• Vitamin D – Low vitamin D levels have been observed in some studies of people with hair loss conditions, including alopecia areata; however, this association is largely observational and causality has not been established. NICE public health guidance (PH56) recommends that adults in the UK consider a daily supplement of 10 micrograms of vitamin D during autumn and winter, regardless of diet. Testing is recommended where deficiency is clinically suspected.

• Ferritin – Low ferritin (stored iron) has been linked to diffuse hair shedding in some studies, even when haemoglobin remains within normal limits. The evidence is variable and a universally accepted threshold has not been established, but serum ferritin is a useful investigation in the context of hair loss.

A note on supplementation

Supplementation should not be started without appropriate assessment. Testing first and using targeted replacement where a deficiency is confirmed is the recommended approach. Unnecessary supplementation may be harmful: for example, excess vitamin A is a recognised cause of hair loss, and high-dose biotin can interfere with laboratory tests as noted above. The NHS vitamins and minerals pages provide evidence-based information on safe intake levels. A GP or registered dietitian can arrange relevant blood tests and advise on whether supplementation is appropriate for your individual circumstances.

Types of Hair Loss Associated With Caloric Restriction

Telogen effluvium is the most common type of hair loss linked to fasting — a diffuse, non-scarring, generally reversible shedding that resolves within three to six months once nutrition is restored. Prolonged restriction may lead to chronic telogen effluvium lasting more than six months.

Not all hair loss is the same, and understanding the type most commonly associated with fasting can help distinguish it from other causes. The form of hair loss most frequently linked to caloric restriction and nutritional stress is telogen effluvium (TE) — a diffuse, non-scarring form of hair shedding that is generally reversible. This is described in patient information from the BAD and in PCDS guidance.

Telogen effluvium occurs when a physiological or psychological stressor causes a large number of hair follicles to simultaneously shift from the anagen (growth) phase into the telogen (resting) phase. Approximately two to four months later, these hairs are shed, often in noticeable quantities. Individuals may observe:

• Increased hair on pillowcases, in the shower drain, or on hairbrushes

• Diffuse thinning across the scalp rather than patchy or localised loss

• Reduced hair volume or density overall

Telogen effluvium associated with fasting is typically temporary. Once adequate nutrition is restored and any underlying deficiencies are corrected, hair growth usually resumes within three to six months, though full recovery of hair density may take longer.

In some cases, prolonged or repeated cycles of restrictive dieting may contribute to chronic telogen effluvium, where shedding persists for more than six months. This is more likely in individuals who engage in repeated crash dieting or who maintain very low-calorie intakes over extended periods.

It is also important to consider that fasting may unmask or exacerbate other underlying conditions associated with hair loss, such as androgenetic alopecia (pattern hair loss) or alopecia areata (an autoimmune condition). Regarding the latter, evidence linking nutritional stress to triggering autoimmune hair loss is limited and causality has not been established. NICE CKS guidance on alopecia areata and PCDS resources can help clinicians differentiate between these conditions. If the pattern of hair loss is unclear, a GP can refer for specialist assessment.

It is also worth noting that postpartum telogen effluvium — hair shedding following childbirth — is a common and distinct trigger that should be considered in the differential diagnosis where relevant.

When to Seek Medical Advice About Hair Loss

See your GP if hair loss is patchy, persists beyond three to four months, affects quality of life, or is accompanied by symptoms such as fatigue or cold intolerance. Suspected scarring alopecia or tinea capitis requires urgent dermatology referral to prevent permanent loss.

Whilst some degree of hair shedding during or after a period of fasting may be expected and self-limiting, there are circumstances in which it is important to seek professional medical advice. Early assessment can help identify reversible causes, rule out underlying conditions, and prevent unnecessary distress or prolonged hair loss. The NHS hair loss (alopecia) page provides a useful patient-facing overview of when to seek help.

Contact your GP if you notice any of the following:

• Hair loss that is patchy, asymmetrical, or associated with scalp redness, scaling, pustules, or broken hairs — these features may suggest tinea capitis (scalp ringworm) or a scarring alopecia, both of which require prompt assessment to prevent permanent hair loss

• Shedding that continues for more than three to four months without improvement

• Significant thinning that affects your confidence or quality of life

• Hair loss accompanied by other symptoms such as fatigue, weight changes, cold intolerance, or irregular periods — which may suggest an underlying thyroid disorder or hormonal imbalance

• Hair loss in children or adolescents following restrictive eating

• Suspected eating disorder, as hair loss can be an early physical sign of conditions such as anorexia nervosa (see NICE NG69: Eating disorders — recognition and treatment for guidance and support pathways)

Suspected scarring alopecia or tinea capitis/kerion warrants urgent or expedited referral to dermatology, as delayed treatment can result in permanent hair loss.

Investigations in UK primary care

Your GP will tailor investigations to your clinical history and examination findings. For diffuse hair loss, standard baseline tests typically include a full blood count (FBC), serum ferritin, and thyroid function tests (TSH). Additional tests — such as vitamin D, B12, zinc, or a coeliac screen — are usually arranged only where the history or examination suggests deficiency or malabsorption, rather than as a routine panel for all presentations.

Medicines as a potential cause

Your GP may also review your current medicines, as a number of prescribed drugs can trigger telogen effluvium. These include retinoids, anticoagulants (such as warfarin and heparin), beta-blockers, and sodium valproate, among others. Do not stop any prescribed medicine without first speaking to your GP.

Referral and specialist support

Depending on findings, your GP may refer you to a NHS dermatologist for specialist assessment of hair and scalp conditions, or to a registered dietitian if nutritional optimisation is required. In some cases, referral to an endocrinologist may be appropriate if a hormonal cause is suspected. Please note that whilst some private trichologists offer hair and scalp assessments, trichologists are not medical doctors and are not regulated by the NHS or a statutory healthcare regulator; any concerns about hair loss are best assessed initially by your GP, who can arrange appropriate NHS referral.

Reporting suspected side effects

Experiencing these side effects? Our pharmacists can help you navigate them →

If you suspect that a medicine, vaccine, herbal remedy, or supplement is contributing to hair loss or another adverse effect, you can report this via the MHRA Yellow Card Scheme (yellowcard.mhra.gov.uk). This applies to both prescribed and over-the-counter products.

Sustainable, balanced dietary approaches are generally preferable to extreme caloric restriction for both overall health and hair health. A healthcare professional can help you fast safely whilst minimising the risk of adverse effects.

Frequently Asked Questions