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Can Intermittent Fasting Cause Hair Loss?

Can intermittent fasting cause hair loss? What a 2025 Cell study found in mice and one small human trial, why diet-related shedding shows up months later, and what to check.

By Published 11 min read

It starts with the shower drain. Then the hairbrush, then the pillow. You’ve been fasting for a few months, the scale is finally moving, and now there seems to be more hair in places it shouldn’t be. A quick search turns up headlines saying fasting slows hair growth, and suddenly the eating window looks like the obvious suspect.

Fasting might be involved, but the timing alone cannot tell you why. Rapid weight loss, inadequate nutrition and unrelated medical conditions all deserve attention. The new study raises a separate question about growth, which is easy to confuse with shedding.

The short version

Rapid weight loss and inadequate nutrition can trigger delayed hair shedding, including during a fasting routine. A 2025 Cell study also found slower short-term hair growth with 18:6 fasting, but its human hair analysis included only 26 people across three groups. It does not establish the risk of noticeable hair loss during everyday fasting. If your hair is thinning or shedding worries you, review recent diet and health changes with a GP or dermatologist rather than assuming the eating window is the cause.

Normal shedding, and the kind that follows a diet

Losing hair every day is normal. The American Academy of Dermatology puts it at 50 to 100 hairs a day, and the NHS gives the same range. Most of it goes unnoticed because it’s spread across the scalp and across the day.

One cause of increased shedding is telogen effluvium. Counting hairs at home cannot establish that diagnosis. Telogen is the resting stage of the hair cycle, and the hairs that fall out in this condition are ones that had already stopped growing. The AAD lists the common triggers: losing 20 pounds or more, giving birth, a high fever, surgery, an illness or a stretch of heavy stress. The NHS includes weight loss and iron deficiency among the causes of temporary hair loss.

Diet is one of the classic triggers. A dermatology review by Guo and Katta calls acute telogen effluvium “a well-known effect of sudden weight loss or decreased protein intake.” A second review, by Almohanna and colleagues, describes nine crash dieters from a 1976 case report who developed it two to five months after starting an aggressive weight-loss program, having lost between 11.7 and 24 kg. That’s a small and old case series, but the pattern it describes is the one dermatologists still look for.

None of this requires fasting. A fast can contribute in a more ordinary way: if a short eating window leaves you eating far less food, or much less protein, than you need, you’ve built the conditions these reviews describe. That makes food intake worth reviewing, but it does not rule out an effect of fasting itself or another cause of hair loss.

Why the timing fools people

The awkward part is the delay. Shedding usually shows up months after whatever set it off, so the thing you changed most recently gets the blame.

The AAD gives childbirth as its example: shedding is often noticeable around two months after the birth and peaks around four months. As excessive shedding settles, the AAD says normal fullness tends to return within six to nine months. That is general recovery guidance, not a deadline measured from the first day of a fasting routine. If the trigger continues, the shedding can continue too. The crash-diet cases followed a similar delay, two to five months after the diet began.

Why the timing misleads

Shedding arrives
months after the trigger.

Two reported patterns on the same scale. Month 0 is the event that set the shedding off.

After a crash diet

9 patients, 1976 case series · month 0 = diet began

Shedding began 2 to 5 months in

After childbirth

AAD example · month 0 = birth

  • About 2 months: shedding noticeable
  • About 4 months: usually peaks

If the trigger continues, shedding can continue too.

Ranges as reported by Almohanna et al., 2019 (describing a 1976 crash-diet case series) and the American Academy of Dermatology. The rows start from different events and are not a forecast for any one person. Neither source establishes an onset or recovery timetable for intermittent fasting.

So if you started fasting in January and lost weight quickly through February and March, shedding in April or May could fit a delayed response. This is an illustrative example, not a way to diagnose the cause. It also means the shedding can start just as the diet gets easier, which makes it feel random.

What the 2025 hair-follicle study found

The paper came from Bing Zhang’s group at Westlake University (Chen et al., 2025). Its main experiments were in mice. Fasting activated a hormone-and-fat-cell pathway that damaged newly activated hair-follicle stem cells and delayed regrowth; blocking parts of that pathway reduced the damage. Donor human cells exposed to fatty acids in the laboratory also showed damage. These experiments support a possible mechanism, not a diagnosis for a person noticing hair in the drain.

What the human trial could and couldn’t show

In the same paper, 49 healthy adults were randomized to usual eating, 18:6 eating from 10 am to 4 pm without a calorie target, or 1,200–1,500 kcal daily without the same time restriction. Of 35 people who completed the 10-day intervention, 26 had hair measurements: 9 usual-diet participants, 8 fasting participants and 9 calorie-restriction participants.

Investigators measured three days of regrowth on a shaved 1 cm² scalp patch before and near the end of the intervention. The fasting group’s growth rate was reported as 18% slower than the usual-diet group’s, a difference of 0.075 mm/day (p = 0.0028). That percentage describes growth speed, not the fraction of hair lost. Hair-density changes were not statistically significant; that does not establish safety or equivalence.

Hair was a secondary outcome; the sample size had been planned around glucose. Missing measurements and the small remaining groups limit confidence in the result. The paper’s displayed uncertainty uses standard errors and box plots, not a confidence interval for that 18% estimate. We have not converted those displays into a new interval.

The trial registry confirms the dates, eating-window instructions and a hair outcome, but also describes breakfast comparisons and a crossover design. The paper reports the three-group hair experiment above. Those descriptions should not be treated as interchangeable, and no summary results are posted in the registry.

One study, three kinds of evidence

A mouse mechanism.
A short human trial.

  1. Mice

    Detailed experiments

    1. Fasting raises adrenal stress hormones
    2. Fat cells in the skin release fatty acids
    3. Stem cells burn them and build up reactive oxygen species
    4. Newly activated stem cells die; regrowth slows

    Fasted mice ate about as many calories as the controls.

  2. Human cells in a dish

    Lab support

    Donor hair-follicle stem cells exposed to fatty acids showed oxidative damage and cell death.

    Shows human cells can respond this way, not what happens on a scalp.

  3. People

    One small, short trial

    Who
    49 enrolled; 26 with hair measurements
    How long
    10 days
    Diets
    Eat 10 am–4 pm (18:6), or 1,200–1,500 kcal a day, or usual diet
    Measured
    Three-day hair growth in mm/day on a shaved patch; secondary outcome
    Reported
    18% slower growth versus usual diet (8 vs. 9 people)

    Important limit: 14 withdrawals; another 9 completers had no hair measurements

    Still unanswered: long-term visible hair loss, other eating windows, or what happens after stopping

Based on Chen et al., 2025, Figure 7, supplementary Figure S7 and STAR Methods. Arrows describe the mouse experiments. The growth-rate comparison is not a measure of hair loss or proof of the same mechanism in living people.

Several things this trial didn’t do matter for anyone reading the headlines:

  • It measured growth, not loss. Hair growing more slowly on a shaved patch over 10 days is not the same as thinning or shedding you can see.
  • It tested 18:6 for 10 days. It says nothing direct about 16:8, 12:12 or a routine kept for a year.
  • The participants were healthy adults. The registry excluded people with diabetes, high blood pressure and heart disease, and the paper reports a median age of 27. Whether people with obesity respond the same way is untested.

How much to trust this

The study gives a reason to investigate fasting and hair growth. It cannot tell you how often fasting causes noticeable thinning, which people might be susceptible, or whether changing a schedule would reverse a problem. A statistically significant short-term finding can still leave the practical question unanswered.

Do not treat the animal mechanism as a treatment recommendation. In particular, experiments that protected mouse follicles do not establish that taking antioxidants or applying vitamin E will prevent hair problems in people who fast. The human experiment did not test those treatments.

The nutrition evidence answers a different question. Dermatology reviews describe shedding after major dietary restriction, and clinical guidance recognizes weight loss and nutrient deficiencies among possible triggers. Neither that history nor this fasting trial tells us which explanation is most likely for an individual reader. Both can matter when a clinician takes your history.

You also cannot compare the trial’s three-day growth measurement directly with the months-long shedding timeline above. One asks how quickly existing hairs grow; the other concerns hairs entering a resting phase and later falling out. Keeping those outcomes separate prevents a small experimental result from becoming a much broader claim about baldness.

Who should get checked

The NHS recommends seeing a GP if you are worried about hair loss. You do not need to wait several months. In particular, arrange an assessment if:

  • The loss is patchy, you have bald spots, a receding hairline or a widening part. Those patterns suggest other causes, such as inherited hair loss or alopecia areata, that a change of eating schedule won’t fix. The AAD separates shedding from hair loss for this reason.
  • Shedding keeps going for more than a few months after your eating and weight have settled.
  • You have other new symptoms, such as unusual tiredness. The AAD lists thyroid disease and nutrient deficiencies among possible causes. Your clinician can decide whether blood tests are appropriate after reviewing your history and examining your scalp.
  • You’ve had an eating disorder, or your eating window has become a way to eat very little. Our guide to who should not do intermittent fasting covers this in more detail.

Ask a clinician what needs checking before buying a hair supplement. Guo and Katta recommend screening based on your history and an exam, and their review found insufficient evidence to recommend supplements without a documented deficiency. Almohanna and colleagues add that high-dose biotin can make thyroid and other lab tests read falsely high or low, so tell your doctor if you take it. Too much vitamin A or selenium can itself cause hair loss, according to the AAD.

What to do with this

  • Check your rate of loss. Tell your clinician how much weight changed and over what period. Rapid loss or restrictive eating is a reason to review the whole plan, including whether fasting is appropriate for you.
  • Eat enough protein in the window. A short window makes it easy to eat one rushed meal. Build each meal around a protein source, the same habit that helps protect muscle while fasting.
  • Don’t treat the window as a calorie cap. A 16:8 schedule isn’t meant to be a starvation diet. If you break the fast with something small and then stop, what to eat when breaking a fast has better options.
  • Be patient with timing. Shedding that began a few months after a big change often settles over the following months once the cause is gone. New research on fasting itself is too thin to say whether loosening your schedule changes the outcome.
  • Keep a simple record. Noting when you started, how your weight changed and when shedding began helps a clinician see the pattern. If you use Atomic Fast’s optional food log, it shows estimated protein for each item when you review a meal. Those estimates can help you discuss what you have been eating. They cannot diagnose a deficiency, establish that your intake is adequate or identify the cause of hair loss.

Frequently asked questions

Does 16:8 fasting make your hair fall out? The study discussed here does not answer that: its human experiment tested 18:6 for 10 days. It measured short-term growth, not long-term visible hair loss. Rapid weight loss or inadequate nutrition can contribute to shedding on many kinds of diet, so your own symptoms still deserve assessment.

How long after starting a diet does hair shedding begin? Usually months, not days. Dermatology sources describe shedding a few months after the trigger, and in one case series of crash dieters it began two to five months after the diet started. That delay is why people often blame whatever they changed most recently.

Will my hair grow back? Shedding from a temporary trigger usually settles. The AAD says hair tends to regain its normal fullness within six to nine months, although shedding can continue if the trigger does. Patchy or patterned loss is different and needs a medical assessment.

Should I take biotin or a hair supplement? Not without checking with a clinician first. The reviews discussed here do not support routine biotin for people without deficiency. High doses can distort blood tests, and too much vitamin A or selenium can itself cause hair loss.

Should I stop fasting if I’m losing hair? You do not need to keep a restrictive routine while seeking advice. Discuss your eating pattern, weight changes and symptoms with a clinician, especially if you are struggling to eat enough. This trial cannot tell us whether stopping or shortening fasting would resolve your hair problem.

Sources

Sources checked: October 19, 2026.

The hair-follicle study

Shedding, nutrition and when to get help

Product notes: the food log’s per-item protein estimate was checked against Atomic Fast’s current iOS and Android source code. This was not a runtime test, and the app has no hair or nutrient-deficiency feature.

Atomic Fast is a free fasting timer with an optional food log. Download it on the App Store or Google Play.

This article is for general education and isn’t medical advice. If you’re pregnant, managing a health condition, taking medication, have a history of disordered eating, or are considering fasts longer than 24 hours, talk to a healthcare professional first.

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