Fasting and Mental Health: What 4-21 Day Fasting Studies Do and Don’t Show

If you’ve been struggling with persistent mood issues, chronic fatigue, or that sense that conventional treatments aren’t quite reaching the root of your problems, you’re not alone. You may have heard that fasting can lift mood. Here is what the research actually shows, what it does not show, and why fasting beyond a day or two is not a psychiatric treatment.

An observational study published in PLOS ONE followed 1,422 people through medically supervised fasting periods lasting 4 to 21 days at a German fasting clinic. It had no control group, and the participants were clinic guests, not people being treated for depression or another psychiatric condition, so it cannot show that fasting treats mental illness.

What This Research Really Shows

What participants reported (self-reported, uncontrolled):

• 93% of participants reported no hunger feelings during their fast
• Both emotional and physical well-being improved significantly across all fasting durations
• 84% of people with pre-existing health complaints reported improvement
• Adverse effects occurred in less than 1% of participants
• Benefits appeared regardless of whether someone fasted for 5 days or 20 days

Participants rated their emotional well-being higher at the end of their fast. Without a comparison group, we can’t tell how much of that came from fasting itself versus rest, time away from daily stress, the clinic setting, or expectation.

What Researchers Think Might Be Happening

Your brain on extended fasting undergoes several important changes:

Metabolic Reset After about 12-24 hours without food, your body shifts from burning glucose to burning fat and producing ketones. Your brain, which normally runs on glucose, begins using these ketones as fuel. Some people describe this as “mental fog lifting,” though research on ketones and mood is still early.

Inflammation Reduction Chronic low-grade inflammation contributes to depression, anxiety, and cognitive problems. Some studies suggest fasting can lower certain inflammatory markers, but whether that improves mood in people with depression has not been shown.

Stress Response Improvement Fasting is itself a physical stressor. Some people report feeling calmer during a supervised fast, while others feel irritable, anxious or unable to sleep, and the effect on stress resilience in psychiatric patients is unknown.

Cellular Renewal During extended fasting, your body activates autophagy—essentially a cellular cleanup process that removes damaged components and regenerates healthy ones. Most autophagy research comes from animal and cell studies, and its role in human mood is not established.

What to Expect: A Realistic Timeline

Days 1-2: The Adjustment Phase

• Some people experience mild headaches or fatigue as blood sugar stabilizes
• Hunger typically peaks and then begins to diminish
• Sleep may be temporarily disrupted

Days 3-5: The Transition

• Hunger often disappears entirely (this surprises most people)
• Energy levels frequently increase
• Mental clarity often begins to improve
• Some people notice mood lifting

Days 6 and Beyond: The Adaptation

• Physical and emotional well-being continue to improve
• Many report feeling more emotionally stable
• Some people report changes in chronic symptoms, which may not last after eating resumes
• Sleep quality typically improves

Who Might Benefit Most

Researchers are interested in fasting for the following, but evidence in psychiatric patients is limited and none of these is a reason to fast without medical supervision:

• Treatment-resistant depression or anxiety
• Chronic fatigue and brain fog
• Inflammatory conditions affecting mood
• Metabolic issues contributing to mental health problems
• Multiple health complaints that haven’t responded to conventional treatment

Important Safety Considerations

Fasting beyond a day or two is not a psychiatric treatment. The study participants were monitored by physicians throughout their fasting periods, and extended fasting should never be attempted without medical supervision. Risks include dehydration, low blood pressure, electrolyte problems and refeeding syndrome, a dangerous shift in fluids and minerals when eating resumes.

Do not fast without medical supervision if any of these apply to you:

• A current or past eating disorder, or being underweight (BMI under 20)
• Pregnancy or breastfeeding
• Severe kidney, liver, or heart disease
• Diabetes treated with insulin or other glucose-lowering medicines
• Taking lithium, antipsychotics or other psychiatric medication, whose levels and side effects can change with fasting and dehydration

Getting Started: Your First Steps

1. Find Qualified Medical Support Start with your psychiatrist and your primary care physician. They can tell you whether any fast is safe for you, given your diagnoses and medications. Never change or skip psychiatric medication in order to fast.
2. Complete Comprehensive Health Screening Before any extended fast, you’ll need blood work, physical examination, and review of your medical history and current medications.
3. Start with Preparation Most protocols include a preparation phase where you gradually reduce food intake and eliminate processed foods, caffeine, and alcohol.
4. Plan Your Support System Extended fasting works best when you can rest, reduce stress, and focus on the process. Plan accordingly with work and family obligations.

What Makes This Different from Crash Diets

Supervised fasting in research settings differs from restrictive dieting in several ways, though it carries its own risks:

• It’s time-limited with clear endpoints
• Medical monitoring reduces, but does not remove, the risks
• It is studied as a health intervention, not a weight-loss plan
• Careful refeeding is needed to prevent refeeding syndrome
• It is not a substitute for evidence-based psychiatric treatment and does not address root causes on its own

Real-World Application

Stories about fasting and mood can be striking, but in a mental health practice we weigh them carefully: they are not evidence that fasting treats depression, and people who felt worse or relapsed are rarely the ones who tell their stories.

For depression, anxiety and PTSD, the treatments with the strongest evidence remain psychotherapy, medication when appropriate, sleep, physical activity and a balanced diet. Controlled trials of prolonged fasting in these conditions are small and few.

The Refeeding Process: Equally Important

How a fast ends matters for safety. The study participants followed a supervised 4-day refeeding period, gradually reintroducing food. After a prolonged fast, eating too much too fast can cause refeeding syndrome, which needs medical monitoring.

Looking Ahead

Extended fasting is not a treatment for depression, anxiety or PTSD. Research on fasting and mood is early, mostly uncontrolled, and rarely includes people with psychiatric diagnoses.

If conventional approaches haven’t provided the relief you’re seeking, talk with your psychiatrist about next steps, including nutrition and metabolic health. Any fast longer than a day or two needs medical supervision and realistic expectations.

Next Steps

Ready to talk about nutrition, metabolic health and your mood?

• Schedule a consultation to discuss your health history and goals with our integrated team
• Get comprehensive lab work to assess your current health status
• Review your medications and health history before any change to how you eat
• Build a plan that fits your diagnosis and your treatment

Remember: healing takes time, and every person’s journey is different. Fasting research is still early, and any change should be made with your clinician.

We combine evidence-based psychiatric care with attention to nutrition and metabolic health. Your clinician will explain which options are appropriate for you and whether any form of fasting is safe given your health and medications.

Ready to learn more? Contact us to schedule a consultation where we can discuss your specific situation and determine if this approach might be right for you.

This information is for educational purposes and should not replace professional medical advice. Extended fasting should only be undertaken with qualified medical supervision.

References

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de Cabo, R., & Mattson, M. P. (2019). Effects of intermittent fasting on health, aging, and disease. New England Journal of Medicine, 381(26), 2541-2551. https://doi.org/10.1056/NEJMra1905136

Longo, V. D., & Mattson, M. P. (2014). Fasting: Molecular mechanisms and clinical applications. Cell Metabolism, 19(2), 181-192. https://doi.org/10.1016/j.cmet.2013.12.008

Mattson, M. P., Moehl, K., Ghena, N., Schmaedick, M., & Cheng, A. (2018). Intermittent metabolic switching, neuroplasticity and brain health. Nature Reviews Neuroscience, 19(2), 63-80. https://doi.org/10.1038/nrn.2017.156

Michalsen, A., & Li, C. (2013). Fasting therapy for treating and preventing disease – current state of evidence. Forschende Komplementärmedizin, 20(6), 444-453. https://doi.org/10.1159/000357765

Murta, L., Seixas, D., Harada, L., Damiano, R. F., & Zanetti, M. (2023). Intermittent fasting as a potential therapeutic instrument for major depression disorder: A systematic review of clinical and preclinical studies. International Journal of Molecular Sciences, 24(21), 15551. https://doi.org/10.3390/ijms242115551

Nugraha, B., Riat, A., Ghashang, S. K., Eljurnazi, L., & Gutenbrunner, C. (2020). A prospective clinical trial of prolonged fasting in healthy young males and females—Effect on fatigue, sleepiness, mood and body composition. Nutrients, 12(8), E2281. https://doi.org/10.3390/nu12082281

Skurvydas, A., Istomina, N., Dadeliene, R., Valanciene, D., Majauskiene, D., Mickeviciene, D., … & Brazaitis, M. (2025). Physiological and psychological responses to five-day fasting. PLoS One, 20(6), e0324929. https://doi.org/10.1371/journal.pone.0324929

Stapel, B., Fraccarollo, D., Westhoff-Bleck, M., Widder, J., Bereiter-Hahn, J., Günther, S., … & Bauersachs, J. (2022). Impact of fasting on stress systems and depressive symptoms in patients with major depressive disorder: A cross-sectional study. Scientific Reports, 12(1), 7642. https://doi.org/10.1038/s41598-022-11639-1

Wilhelmi de Toledo, F., Grundler, F., Bergouignan, A., Drinda, S., & Michalsen, A. (2019). Safety, health improvement and well-being during a 4 to 21-day fasting period in an observational study including 1422 subjects. PLoS One, 14(1), e0209353. https://doi.org/10.1371/journal.pone.0209353