Prolonged Water Fasting and Cancer: What the Science Says, What I Do, and What I Would Never Recommend

Medical disclaimer: As always… I’m not a doctor. Nothing in this post constitutes medical advice. I’m a Stage IV cancer patient describing what I do, why I do it, and what the published research I have found along my journey says.

Prolonged water fasting carries serious risks – particularly for cancer patients – and should not be attempted without medical supervision. If you are considering fasting during cancer treatment, please make sure to speak to your oncology team first.

In this guide:

• Why I Fast
• My Fasting History
• The Science: What Fasting Does at a Cellular Level
• Differential Stress Resistance: Protecting Normal Cells
• Autophagy: The Cellular Clean-Up
• Cancer Stem Cells and Circulating Tumour Cells
• The Glucose-Ketone Index: How I Monitor
• Immune System Regeneration
• The Risks: What Can Go Wrong
• Where the Experts Disagree
• What I Actually Do During a Fast
• The Honest Summary
• Frequently Asked Questions

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Why I Fast

I’ll be direct about this. I fast because the research suggests it may help target two things that conventional oncology largely ignores: circulating tumour cells (CTCs) and cancer stem cells (CSCs).

My scans (April 2026) show no measurable disease. That’s particularly extraordinary given where I started – a nearly 10cm tumour, secondaries, metastatic spread to multiple lymph nodes, an 11.5-month prognosis. But “no measurable disease” isn’t the same as “no disease.” CTCs are cancer cells that have broken away from the primary tumour and circulate in the bloodstream. Cancer stem cells are the subpopulation of cells that can self-renew, resist treatment, and seed new tumours. Standard imaging cannot detect either of them.

My oncologist is focused on what she can see on a scan. I’m focused on what she cannot.

That distinction matters, because it informs everything I do with fasting – and everything I’m about to share in this post.

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My Fasting History

I didn’t arrive at prolonged fasting overnight. I’ve mainly been eating a vegetarian ketogenic diet since shortly after my diagnosis in October 2024, which means my body has been running for long periods on ketones as its primary fuel source for well over a year. That metabolic flexibility matters – it means my system is already adapted to low glucose states before I begin a fast.

My fasting history with cancer treatment:

• 48-72 hour fasts before chemotherapy – I fasted before four of my six CAPOX chemotherapy cycles. I broke the fast on the evening of each infusion because the oral chemotherapy component (capecitabine) needed to be taken with food. The purpose was to leverage differential stress resistance – a concept I’ll explain in detail below.
• Multiple 10-day water fasts – completed before, during (in breaks in chemo) and after my treatment period.
• Current fast – as I write this, I’m actually on day five of what I’m planning to be a 15-20-day water fast, though in my experience I’ll likely reach c.10 days before my body tells me to stop.

The pre-chemo fasts had a noticeable effect on my tolerance of treatment. The oxaliplatin infusions – which are notoriously brutal – were markedly more tolerable when I had fasted beforehand. I can’t prove that was causal, but I can tell you it was consistent across multiple cycles.

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The Science: What Fasting Does at a Cellular Level

There are several distinct mechanisms through which prolonged fasting appears to affect cancer biology. I’m going to take each one separately, because they are often conflated, and the evidence base for each is different.

The three I consider most relevant are:

1. Differential stress resistance (DSR) – protecting healthy cells while sensitising cancer cells to treatment
2. Autophagy – the cellular recycling and clean-up process
3. Effects on cancer stem cells and circulating tumour cells

There are others – immune regeneration, inflammation reduction, IGF-1 suppression – which I’ll also cover. But those three are the reason I fast.

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Differential Stress Resistance: Protecting Normal Cells

This is the concept that first convinced me fasting had a role in my protocol, and it comes primarily from the work of Professor Valter Longo at the University of Southern California.

The principle is elegant. When you fast, your healthy cells respond to the drop in nutrients and growth factors by entering a protective, stress-resistant state. They essentially shut down growth and redirect energy toward maintenance and repair. Cancer cells, however, can’t do this. Their oncogene mutations – the very mutations that make them cancerous – lock them into a growth-and-proliferate mode. They cannot downregulate. They cannot enter that protective state.

The result is a differential: normal cells become more resistant to stress (including chemotherapy), whilst cancer cells become more vulnerable to it.

Longo’s team first demonstrated this in 2008, showing that short-term starvation provided up to 1,000-fold better protection to normal yeast cells against oxidative stress compared to cells expressing oncogene equivalents. In mice, short-term starvation provided complete protection against high-dose etoposide while the injected neuroblastoma cells remained vulnerable (Raffaghello et al., 2008).

A 2012 study in Science Translational Medicine extended this further, showing that fasting cycles were as effective as chemotherapy drugs in delaying tumour progression across multiple cancer types, and that combining fasting with chemotherapy resulted in long-term cancer-free survival in neuroblastoma mouse models – something neither treatment achieved alone (Lee et al., 2012).

The clinical evidence is building. In a 2009 case series, 10 cancer patients who voluntarily fasted 48-140 hours before chemotherapy reported reduced fatigue, weakness, and gastrointestinal side effects. Fasting didn’t prevent tumour reduction from the chemotherapy itself (Safdie et al., 2009).

The landmark clinical trial came in 2020 – the DIRECT trial. This phase 2 randomised controlled trial of 131 breast cancer patients found that those using a fasting-mimicking diet alongside neoadjuvant chemotherapy were significantly more likely to achieve a radiological response (OR 3.168) and, in per-protocol analysis, were more likely to achieve 90-100% tumour cell loss (OR 4.109). The FMD also significantly reduced chemotherapy-induced DNA damage to healthy T-lymphocytes (de Groot et al., 2020).

This is why I fasted before chemotherapy. The logic is straightforward: if fasting makes my healthy cells harder to kill and my cancer cells easier to kill, then timing a fast around treatment creates a window of maximum differential.

Longo’s comprehensive 2018 review in Nature Reviews Cancer summarises the state of the field: “the combination of FMDs with chemotherapy, immunotherapy or other treatments represents a potentially promising strategy to increase treatment efficacy, prevent resistance acquisition and reduce side effects” (Nencioni et al., 2018).

BUT… I also want to be honest about the limitations – The DIRECT trial used a fasting-mimicking diet (roughly 200-800 calories per day), not a full water fast. Most of the dramatic results are from animal models. The human evidence is promising but not yet definitive. That said, the mechanistic logic is sound, and the clinical direction of travel is consistent.

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Autophagy: The Cellular Clean-Up

If differential stress resistance is about protecting healthy cells during treatment, autophagy is about what happens during the fast itself.

Autophagy – from the Greek for “self-eating” – is a process by which cells break down and recycle damaged components: misfolded proteins, dysfunctional organelles, accumulated cellular debris. Yoshinori Ohsumi won the Nobel Prize in Physiology or Medicine in 2016 for elucidating the mechanisms of autophagy.

Fasting and calorie restriction are among the most potent non-genetic stimulators of autophagy. A comprehensive 2018 review concluded that “both fasting and CR have a role in the upregulation of autophagy, the evidence overwhelmingly suggesting that autophagy is induced in a wide variety of tissues and organs in response to food deprivation” (Bagherniya et al., 2018).

The relationship between autophagy and cancer is complex – and this is where honest uncertainty matters. Autophagy can be both tumour-suppressive and tumour-promoting depending on the context:

• In early-stage cancer, autophagy appears to be protective – clearing damaged cells and preventing the accumulation of mutations that lead to tumour formation.
• In established tumours, some cancer cells may use autophagy as a survival mechanism, recycling their own components to survive nutrient stress.

This dual role means the picture sadly isn’t straightforward. A 2016 review in Frontiers in Oncology proposed that fasting-induced autophagy may enhance the efficacy of chemotherapy through several mechanisms: mobilising the autophagic apparatus before treatment so it can be repurposed for clearing chemo-damaged components, improving immune surveillance through better epitope processing, and modulating inflammation (van Niekerk et al., 2016).

A 2023 study from Longo’s group added an interesting wrinkle – in a leukaemia model, the fasting-mimicking diet actually inhibited autophagy in cancer cells (not induced it), and this inhibition synergised with chemotherapy to promote T-cell-dependent cancer-free survival (Buono et al., 2023).

So the autophagy story is nuanced. What I take from it is this: prolonged fasting triggers a deep cellular housekeeping process in healthy tissue, and the interaction between fasting-induced metabolic stress and cancer cell survival mechanisms creates vulnerabilities that may be therapeutically useful – particularly in combination with conventional treatment.

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Cancer Stem Cells and Circulating Tumour Cells (CTCs)

This is the primary reason I’m fasting now – beyond the chemo phase, in maintenance immuno.

Cancer stem cells are a small subpopulation within a tumour that possess the ability to self-renew and differentiate. They are thought to be responsible for treatment resistance, relapse, and metastatic spread. Standard chemotherapy often kills the bulk of differentiated cancer cells but leaves cancer stem cells intact – which is why such a large proportion of cancers come back.

A 2021 study published in Cell Metabolism demonstrated that a fasting-mimicking diet lowered glucose-dependent protein kinase A (PKA) signalling and stemness markers in cancer stem cells, reducing their number and increasing mouse survival in a triple-negative breast cancer model. Importantly, metastatic patients with lower baseline glycaemia survived longer than those with higher glycaemia (Salvadori et al., 2021).

The mechanism appears to be metabolic: cancer stem cells are particularly dependent on glucose and glycolytic metabolism. When you fast, glucose drops, and these cells lose a critical fuel source. Normal stem cells, by contrast, can switch to fatty acid oxidation – a 2018 Cell Stem Cell study showed that fasting enhanced intestinal stem cell function through exactly this fatty acid oxidation pathway (Mihaylova et al., 2018).

This is the logic behind my current extended fast. My latest scan was clear. But I can’t see CTCs or cancer stem cells on any scan, and I know that cancer stem cells are the mechanism by which Stage IV oesophageal cancer returns. If prolonged fasting creates a metabolic environment that’s hostile to cancer stem cells while promoting normal stem cell function, then I want to create that environment – repeatedly, periodically, and measurably.

I must caveat that the cancer stem cell research is largely preclinical. There are no complete phase 3 trials confirming that fasting eliminates cancer stem cells in humans. But the mechanistic rationale is strong, the preclinical results are consistent, and I don’t have the time nor patience to wait for a trial that may take a decade to complete when the intervention is free and – with appropriate monitoring – manageable.

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The Glucose-Ketone Index: How I Monitor

I don’t fast blind. I measure glucose and ketone levels in my blood at least three (often many more) times a day throughout every extended fast, using a dual glucose-ketone meter (I use the keto mojo testing kit)

The metric I track is the Glucose-Ketone Index (GKI), developed by the brilliant Professor Thomas Seyfried at Boston College (He of DOAC fame – link in case you’ve not yet seen it). The GKI is calculated by dividing your blood glucose (in mmol/L) by your blood ketones (in mmol/L). It gives a single number that indicates your metabolic state:

• GKI above 9 – normal fed state, high glucose relative to ketones
• GKI 3-9 – moderate ketosis
• GKI 1-3 – high therapeutic ketosis
• GKI below 1 – deep therapeutic ketosis (the zone Seyfried’s research suggests is most hostile to cancer cell metabolism)

Seyfried’s team published the GKI calculator in 2015 as a tool for monitoring the efficacy of metabolic therapy, demonstrating a clear relationship between GKI values and therapeutic outcomes in brain cancer models using ketogenic diets and calorie restriction (Meidenbauer et al., 2015).

A major 2024 consensus paper in BMC Medicine – authored by Seyfried and 46 co-authors – proposed the GKI as a standardised biomarker for assessing biological compliance in ketogenic metabolic therapy, ideally via real-time monitoring. The paper describes how GKI-adjusted ketogenic diets, calorie restriction, and fasting aim to increase substrate competition and normalise the tumour microenvironment (Duraj, Seyfried et al., 2024).

I can tell you from experience that on a prolonged water fast, my GKI drops into the 1-2 range by day two and can reach below 1 by day three or four. I’m metabolically flexible from over a year of ketogenic eating and multiple previous fasts, which means I transition into deep ketosis faster than someone attempting this from a standard Western diet for the first time.

I’m happy to share my daily readings if it’s useful. The data tells a story that blood tests and scans can’t.

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Immune System Regeneration

One of the most compelling findings in this field is what happens to the immune system during prolonged fasting.

Longo’s team showed in 2014 that prolonged fasting reduced circulating IGF-1 levels and PKA activity, leading to signal changes in long-term hematopoietic stem cells that promoted stress resistance, self-renewal, and lineage-balanced regeneration. Multiple cycles of fasting reversed the immunosuppression caused by chemotherapy and reversed age-dependent immune decline in mice (Cheng et al., 2014).

A 2019 study published in Cell found that short-term fasting dramatically reduced circulating inflammatory monocytes by activating AMPK in liver cells and suppressing systemic inflammatory signalling – without compromising the body’s ability to mount an immune response to acute infection or injury (Jordan et al., 2019).

A 2015 Cell Metabolism study demonstrated that periodic fasting-mimicking diet cycles reduced cancer incidence, rejuvenated the immune system, and promoted multi-system regeneration in mice, with a pilot human trial showing decreased risk factors for cancer and other age-related diseases (Brandhorst et al., 2015).

For me, this matters because I’m on pembrolizumab – an immunotherapy drug that works by enabling my immune system to recognise and attack cancer cells. If fasting regenerates and rebalances the immune system, then periodic fasting may complement what the immunotherapy is doing. I don’t have proof of that. But the logic is consistent with the mechanisms.

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The Risks: What Can Go Wrong

It would be highly irresponsible of me to write about the potential benefits of prolonged fasting without being equally thorough about the risks. These are real, they are serious, and some of them can kill you.

Muscle Wasting and Cachexia

This is the risk that keeps oncologists awake at night – and rightly so. Cancer patients are already at elevated risk of cachexia (a wasting syndrome involving loss of muscle mass and body weight) and sarcopenia (age-related muscle loss). Fasting accelerates protein catabolism once glycogen stores are depleted.

A 2023 review in the Journal of the National Cancer Institute Monographs stated plainly: “An outstanding concern regarding the use of intermittent fasting among cancer patients is that fasting often results in caloric restriction, which can put patients already prone to malnutrition, cachexia, or sarcopenia at risk” (Kalam et al., 2023).

This isn’t theoretical for me. I monitor my body composition, my strength, and my weight throughout every fast. When I start shaking during routine movements, when fatigue becomes debilitating rather than manageable, when electrolyte imbalances manifest as muscle tremors – that is when I stop. Not on a schedule. When my body tells me.

Refeeding Syndrome

Refeeding syndrome is a potentially fatal condition that can occur when nutrition is reintroduced after prolonged fasting. The abrupt shift from catabolism to anabolism causes rapid movement of electrolytes – particularly phosphorus, potassium, and magnesium – from the bloodstream into cells, leading to dangerous drops in plasma levels. This can cause cardiac arrhythmias, respiratory failure, seizures, and death.

Refeeding syndrome occurs within five days of refeeding after prolonged fasting and is caused by the abrupt metabolic transition, with hypophosphataemia being the hallmark biochemical finding (Borriello et al., 2025).

I break every extended fast slowly and carefully – starting with clean and simple vegetable broth and small amounts of easily digestible food (often tofu and steamed broccoli or wilted spinach), reintroducing calories gradually over 24-48 hours. This isn’t optional. It’ a non-negotiable safety protocol.

Electrolyte Imbalances

Sodium, potassium, magnesium, and phosphorus all shift during prolonged fasting. Symptoms range from muscle cramps and weakness to cardiac irregularities. During my fasts I supplement heavily with Baja Gold mineral salt in my water and take magnesium. Even so, by the second week I typically experience periods of shaking and instability that tell me I’m approaching my limit.

Hypoglycaemia

Blood glucose drops significantly during a prolonged fast. For someone already on a keto diet, the body is adapted to running on ketones, which mitigates the risk. For someone fasting from a standard diet or for the first time, the glucose drop can be steep and dangerous – particularly for diabetics or anyone on glucose-lowering medication.

Drug Interactions

I continue some medications during my fast – metformin, propranolol, and berberine on occasion, alongside selenium in my water. Each of these has different pharmacokinetics in a fasted state. Metformin in particular can exacerbate the glucose-lowering effect of fasting. I’ve done this enough times to know how my body responds, but this is an area where individual variation is significant, and I’d not recommend others follow my lead here. At the very least speak to a health professional to confirm what may or may not work for you beforehand.

Who Should Not Do This

I want to be unequivocal:

• If you have never fasted before, do not start with a prolonged water fast. Build up gradually – 24 hours, then 48, then 72 – and learn how your body responds.
• If you’re underweight or losing weight involuntarily, prolonged fasting could be dangerous.
• If you’re on active chemotherapy without prior fasting experience, talk to your oncology team. The DIRECT trial used a controlled fasting-mimicking diet, not a full water fast.
• If you have diabetes, kidney disease, or are on medications that affect electrolytes or glucose, medical supervision is essential.
• If you don’t have the means or discipline to monitor glucose, ketones, and symptoms throughout, do not attempt an extended fast.

I do my fasts unsupervised – not because I think that’s ideal, but because I’ve done enough of them to know my body’s signals intimately. I wouldn’t recommend that approach to anyone who has not built up significant experience and feels confident in doing so.

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Where the Experts Disagree

I want to address something honestly – Not everyone in the integrative cancer community agrees on prolonged fasting.

Jane McLelland – whose work on the metabolic approach to cancer I’ve enormous time and respect for (as anyone hwo’s followed my journey knows), and with whom I’m a co-trustee of the Beyond the Standard charity – has expressed reservations about extended water fasting. Her concern, as I understand it, relates to the metabolic stress it places on the body and the potential for muscle wasting in cancer patients.

I respect Jane’s position. Her Metro Map framework – which targets the metabolic pathways cancer cells depend on using repurposed pharmaceuticals – is one of the most rigorous and evidence-informed pieces of work in this space to date. But, on this particular point, we simply see it differently – as those of you whom attended our Astron Health hosted podcast last year will have witessed.

My view is that the research on differential stress resistance, cancer stem cell vulnerability, and immune regeneration supports periodic prolonged fasting as a component of a broader metabolic protocol – provided the person doing it has the experience, the monitoring capability, and the metabolic flexibility to do it safely. Jane’s view, as I understand it, is that the risks outweigh the benefits for most cancer patients.

I think both positions are defensible and genuinely support Jane in her right to a difference of opinion.

And, the honest answer is that the science is not yet settled, and reasonable people looking at the same evidence can reach different conclusions. That is how it should work.

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What I Actually Do During a Fast

For transparency, here is exactly what a prolonged water fast looks like for me:

Intake:

• Water – as much as I want, typically 3-4 litres per day
• Baja Gold mineral salt – added to water throughout the day for sodium and trace minerals
• Magnesium supplement
• Selenium – added to water
• Occasional: metformin, propranolol, berberine (not every fast, and not every day within a fast)

Monitoring:

• Blood glucose and blood ketones measured three times daily (morning fasted, midday, evening)
• GKI calculated from each reading
• Body weight daily
• Subjective symptom tracking: energy levels, clarity, muscle tremors, sleep quality

Stopping signals:

• Persistent muscle shaking or tremors (electrolyte depletion)
• Debilitating weakness or fatigue (beyond the manageable tiredness of days 2-4)
• Sustained dizziness or cognitive impairment
• Any chest pain or cardiac symptoms

Breaking the fast:

• Veg broth first
• Small portions of easily digestible food
• Gradual reintroduction over 24-48 hours
• No large meals or high-carbohydrate foods for at least 48 hours after breaking

What I do not do:

• I don’t ever set a rigid end date and push through warning signs to reach it
• I don’t fast during active chemotherapy infusion days (I broke the pre-chemo fasts on the evening of infusion)
• I don’t take all of my usual pharmaceutical protocol during fasting – some medications require food or have altered effects in a fasted state and could be genuinely dangerous

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The Honest Summary

I want to be clear about what I know and what I don’t know.

What the science supports:

• Fasting and fasting-mimicking diets create a metabolic environment that protects healthy cells while potentially sensitising cancer cells to treatment (differential stress resistance)
• Prolonged fasting induces autophagy – cellular recycling – across a wide range of tissues
• Fasting reduces cancer stem cell markers and stemness signalling in preclinical models
• Prolonged fasting promotes immune system regeneration, including reversal of chemotherapy-induced immunosuppression
• The GKI is an emerging standardised biomarker for monitoring metabolic therapy in cancer

What remains uncertain:

• Whether full water fasting is significantly more effective than fasting-mimicking diets (most clinical trials use FMDs, not complete fasts)
• Whether the cancer stem cell effects observed in mouse models translate to meaningful clinical outcomes in humans
• The optimal duration, frequency, and timing of fasting for different cancer types
• Whether fasting during immunotherapy (as opposed to chemotherapy) provides additional benefit

What is genuinely risky:

• Muscle wasting and cachexia in cancer patients who are already nutritionally compromised
• Refeeding syndrome if the fast is broken incorrectly
• Electrolyte imbalances during extended fasts
• Dangerous hypoglycaemia in non-keto-adapted individuals or those on glucose-lowering medications

I fast because, when I weigh the evidence and the risks against my specific situation – metabolically adapted, experienced, monitored, and with no measurable disease to lose ground on – the potential benefit of targeting cancer stem cells and maintaining metabolic pressure outweighs the manageable risks.

That is my calculation. It should NOT automatically be yours.

If you’re a cancer patient considering fasting, read the research. Talk to your medical team. Start small. Monitor everything. And understand that what works in a preclinical model isn’t the same as what works in a human body – but also understand that waiting for a completed phase 3 trial means waiting while cancer doesn’t.

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References

All references retrieved from PubMed.

1. Raffaghello L, Lee C, Safdie FM, et al. Starvation-dependent differential stress resistance protects normal but not cancer cells against high-dose chemotherapy. Proc Natl Acad Sci USA. 2008;105(24):8215-20. DOI: 10.1073/pnas.0708100105
2. Lee C, Raffaghello L, Brandhorst S, et al. Fasting cycles retard growth of tumors and sensitize a range of cancer cell types to chemotherapy. Sci Transl Med. 2012;4(124):124ra27. DOI: 10.1126/scitranslmed.3003293
3. Safdie FM, Dorff T, Quinn D, et al. Fasting and cancer treatment in humans: A case series report. Aging. 2009;1(12):988-1007. DOI: 10.18632/aging.100114
4. de Groot S, Lugtenberg RT, Cohen D, et al. Fasting mimicking diet as an adjunct to neoadjuvant chemotherapy for breast cancer in the multicentre randomized phase 2 DIRECT trial. Nat Commun. 2020;11(1):3083. DOI: 10.1038/s41467-020-16138-3
5. Nencioni A, Caffa I, Cortellino S, Longo VD. Fasting and cancer: molecular mechanisms and clinical application. Nat Rev Cancer. 2018;18(11):707-719. DOI: 10.1038/s41568-018-0061-0
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19. Di Tano M, Longo VD. Fasting and cancer: from yeast to mammals. Int Rev Cell Mol Biol. 2022;373:81-106. DOI: 10.1016/bs.ircmb.2022.02.005
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This post is part of The Life Organic – where I write about the integrative protocol I’ve built alongside conventional cancer treatment. Read my full protocol overview for context on the thermal therapy, pharmaceutical, and supplement components.

I’m not selling fasting. I’m simply telling you what I do, why I do it, and what the research says. Make your own decisions with your own medical team.

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Frequently Asked Questions