Frequently Asked Questions About Fasting

OMAD (one meal a day) means fasting for about 23 hours, then eating all your calories in one big meal. People love it because it’s simple and often leads to weight loss. Staying in fat-burning mode for most of the day lowers insulin levels, improves fat metabolism, and can boost focus.

But it’s not easy to sustain. Trying to eat 2,000–2,500 clean calories at once can overload digestion and cause a food coma. It’s also hard to get enough protein, fiber, and micronutrients in a single meal. Blood sugar swings after a giant meal can leave you sluggish, and socially, it’s tough to skip every lunch or family dinner.

If you try OMAD, make the meal nutrient-rich (protein, vegetables, healthy fats) and don’t be afraid to rotate with 16:8 or 20:4 fasting for balance. It can be a powerful short-term tool for weight loss, but rarely a sustainable long-term plan.

Some people follow a “rolling fast”: for example 3 days with no food, then 4 days of eating, and repeat. Three days is usually enough to reach ketosis, trigger autophagy, and burn fat, while the eating days let your body recover and refuel.

I see two concerns. First, digestion: it takes about 3 days for the system to fully shut down, and then you immediately restart it — week after week. That stop–start pattern can lead to bloating, constipation, or “refeed chaos.” Second, sustainability: socially, skipping meals three days every week is tough to maintain. And long-term, there’s the risk of muscle loss or nutrient gaps if the eating days aren’t managed well.

If you try it, break the fast gently (broth, veggies, light protein), eat nutrient-dense foods on feeding days, and add strength training. It can be a powerful short-term tool, but rarely works as a forever plan.

Dirty fasting means you fast but allow small things that technically break the fast — like coffee with cream, bulletproof coffee, zero-calorie sodas, artificial sweeteners, or broth. It makes fasting easier to stick with, especially for beginners, and can still support weight loss.

The trade-off is that insulin and digestion get triggered, so you won’t reach the same depth of ketosis, autophagy, or gut rest as with a clean water fast. Some people also notice sweeteners increase cravings or cause blood sugar swings.

Bottom line: dirty fasting works for adherence and mild benefits, but it’s a compromise. If your goal is the full reset — deep autophagy, strong fat adaptation, or true digestive rest — clean fasting is the way to go.

Dry fasting means no food and no water. It’s part of some religious traditions and sometimes promoted as “accelerated fasting,” since fat metabolism releases a little water. The problem is safety - without water, risks build up fast.

The main concerns are hypovolemia (low blood volume, which can cause dizziness or fainting), hypertonicity (blood becoming too concentrated, stressing the brain and kidneys), and hypoglycemia (low blood sugar, leaving you shaky, weak, or drained). Most people feel unwell within 24–48 hours, and unlike water fasting, dry fasting has a very narrow safety margin. I’ve tried it myself, but stopped quickly for exactly these reasons — I didn’t feel well, and I’m not convinced it’s good for the body.

It’s also much less researched than water fasting. If you want the metabolic benefits, stick with water fasting — it’s far safer.

The simple rule - anything with calories breaks a fast, anything without them does not. Water, sparkling and mineral water, black coffee, and plain tea (black, green, herbal) are all fine, and so are electrolytes - salt in water and zero-calorie mixes. Milk, cream, sugar, honey, juice, broth, and "bulletproof" coffee with butter or MCT oil all break it. The calories and the insulin response are the point of fasting, and these undo both. Zero-calorie sweeteners sit in a gray zone. They add no calories, but the sweet taste keeps cravings alive, so I skip them on extended fasts. Gum, mints, and flavored sparkling water usually sneak in a few calories and sweeteners, so check the label. Medication is a different question - never stop a prescribed medication to keep a fast clean. Ask your doctor how to fit it in.

No, it’s not! Fasting and starvation are two completely different physiological states. During fasting, your body transitions in a controlled way: it first burns glycogen (stored glucose) and then switches to fat for fuel. Insulin levels drop by up to 70%, while growth hormone can rise 5–10 times to preserve muscle mass.

Starvation, on the other hand, starts when fat reserves are nearly depleted — below essential levels (around 4–5% body fat in men and 10–13% in women). At that point, the body can no longer rely on fat and begins breaking down muscle and even organs for energy.

Most people have enough fat to fast safely for 7–10 days. In my case, it would take more than three weeks to reach true starvation mode. In short, fasting is a coordinated, protective process, while starvation is uncontrolled and damaging.

No! Fasting is not a panacea. Think of it as one tool in the health toolbox — a super-powerful tool, but still just one.

Some people use fasting to lose extra weight. And yes, it works. But if you don’t change your diet and lifestyle habits, that weight will come back quickly. Extended fasts can act like a cleanse or reset, making your body more efficient at many things, including digestion. The catch is that if you return to the same habits afterward, you’ll regain the weight really fast.

So while fasting offers a ton of benefits, it works best as part of a healthy lifestyle that also includes good nutrition, regular activity, quality sleep, and stress management.

Start at least three days before. Cut sugar and refined carbs, reduce coffee, and eat more fat and protein, so the switch to ketosis is gentler and day 2 to 3 hits softer. Buy your electrolytes ahead of time - salt for sodium, and a mix or supplements for potassium and magnesium. Pick a start day with a light schedule and no big social meals in the first days. Decide the end date and the first meal in advance, so neither becomes a decision made while hungry. Weigh yourself on the morning of day 1 for a baseline, and if you have a ketone meter, take a reading too. Last, pick the project you will finish during the fast. That is all preparation needs; what to do during the fast is a separate question.
Step-by-step guide to starting, breaking, and managing a fast: https://fasting.center/how-to-fast

Counterintuitive, but no - the difference is smaller than it looks. The first three days are the hard part - the body switches from glucose to burning fat, hunger peaks, sleep gets rough, and energy dips. It is like biking uphill. From day 4 on you are on flat ground - hunger fades, ketones are high, energy steadies, and that is when most of the benefits of extended fasting begin. Days 4 to 7 are usually easier than day 2. So a 3-day fast means doing all the climbing and stopping right at the top, which is why I am not a big fan of them. My take - either stay short with intermittent or 24 to 48 hour fasts, or go all in with 7 to 10 days. The trade-off is risk, not difficulty - after about 7 to 10 days, electrolyte imbalance, hormonal disruption, nutrient deficiencies, and lean mass loss start rising much faster, so around 10 days is the practical ceiling for most people, and anything beyond that belongs in a conversation with your doctor.
What each fast length gives you, by the research: https://fasting.center/fasting-benefits

Possible, but it is at the far end of what I consider sensible. The main risks are electrolyte depletion (the kidneys dump 3 to 5 g of sodium a day early in a fast, and low sodium drives headaches, dizziness, and fainting), muscle loss that grows the longer you go, refeeding problems, disrupted sleep, and a real risk of turning fasting into a performance goal instead of a health tool. If you have never gone past 5 to 7 days, do that first and see how your body responds. A 21-day fast only makes sense with daily electrolytes, daily weight checks, and a plan to stop early if you feel off. Anything beyond 10 days is worth discussing with a doctor first.
The full list of tradeoffs, with my own lab data: https://fasting.center/fasting-blog/fasting-risks-and-downsides

My rule of thumb - the longer the fast, the rarer it should be. A 7 to 10 day fast once or twice a year is plenty for most people. 3 to 5 day fasts can run monthly or quarterly, and 24 to 48 hour fasts weekly if they fit your life. Two reasons not to stack long fasts back to back. First, recovery - lean mass, testosterone, and cortisol all take weeks to return to baseline after a 10-day fast, and the body needs eating time to rebuild. Second, results - the fat loss from a long fast only sticks if the eating weeks are good, so the eating weeks are where the work happens. Between extended fasts, intermittent fasting keeps the benefits ticking. If you find yourself wanting the next long fast as soon as the last one ends, that is a signal to slow down, not speed up.
What one extended fast a year looks like in practice: https://fasting.center/fasting-blog/10-day-fast-results

Two different answers, and the difference matters. On the scale, about 5 to 7 days of water fasting for most people. My 10-day fast took me from 164 to 148.7 lb. But roughly half of early scale loss is water and glycogen and comes back after refeeding. Real fat loss runs around 0.5 lb per day of fasting, so 10 lb of fat is closer to 3 weeks of total fasting time, best split into several shorter fasts with normal eating in between. The Extended Fasting Calculator (on this site and in the app) gives your own numbers from your weight, height, age, and sex, separately for scale weight and fat. Intermittent fasting gets you there too, more slowly and with far less water swing, and the Intermittent Fasting Calculator shows that timeline.
Calculators: https://fasting.center/fasting-calculators
What 10 days looked like on the scale and on Dexa: https://fasting.center/fasting-blog/10-day-fast-results

Because the first thing your body burns is not fat. It is glycogen - the stored carbohydrate in your liver and muscles - and every gram of glycogen holds about 3 grams of water. Liver glycogen is mostly gone by 18 to 30 hours, muscle glycogen follows, and the water goes with it. Add the contents of your gut, which clear out over the first 72 hours, and you get the familiar 5 to 7 lb drop by day 2 or 3. It feels like a great start, and it is, but almost none of it is fat. Real fat loss runs around 0.5 lb per day and only becomes the main source of loss from day 3 on, which is why the scale slows down sharply after the first days. And because glycogen, water, and gut content all come back when you eat, so does most of that early weight. The numbers to watch are body fat and waist, not the day-2 scale reading. The Extended Fasting Calculator on this site models all four tissues separately so you can see what your own curve should look like.
Where the early weight loss actually comes from: https://fasting.center/fasting-blog/fasting-weight-loss

Most people reach light ketosis (0.5 to 1 mmol/L) 12 to 24 hours into a fast. From there it keeps climbing: 1 to 3 mmol/L by day 2 to 3, 3 to 5 mmol/L by day 4 to 5, and 5+ mmol/L on longer fasts. If you already eat low carb or fast regularly, expect the faster end of each step, because your glycogen stores start lower. Exercise early in the fast speeds it up; my 10-day fast with cardio and strength training in the first days reached deep ketosis much faster than a sedentary 9-day one. A blood ketone meter is the only reliable way to know where you are; urine strips stop working once your body adapts. If you log ketones in the app, you will see your own curve after a few fasts.
Real data from my 7-, 9-, and 10-day fasts: https://fasting.center/fasting-blog/fasting-ketones-timeline

Autophagy is the cell's cleanup and recycling process, and it never fully stops - there is always a basal level, even when you are eating. Fasting turns it up. The best-supported human and animal data put the meaningful rise after about 48 hours without food, with activity roughly 5 to 8 times the basal level, peaking around day 3, then staying elevated for as long as the fast continues. It also varies by tissue - highest in the liver, lower in muscle, and much lower in the brain. So the rule of thumb is simple - the longer the fast, the more autophagy, but the biggest jump happens in the first three days. Intermittent fasting and 24-hour fasts nudge it, they do not get you the full effect. One honest caveat - nobody can measure autophagy in a living person outside a lab, so any app or article claiming "autophagy started at hour 16" is estimating, including the Fasting Center app.
The mechanism, the timeline, and the research behind it: https://fasting.center/fasting-blog/fasting-and-autophagy

Hunger during a fast is not a straight line up. It comes in waves tied to your usual meal times, peaks in the first 24 to 48 hours, and then drops for most people, often sharply, as ghrelin adapts and ketones start feeding the brain. The hardest part is the beginning, not the end. Three things make the waves easier - salt and electrolytes (much of what feels like hunger is a sodium dip), black coffee or tea, and a good night's sleep the night before, since poor sleep raises ghrelin more than fasting does. When a wave hits, drink a glass of salted water and give it 20 minutes - it passes. And every fast gets easier - day 1 of your tenth fast feels nothing like day 1 of your first.
Timeline, mechanism, and the research behind it: https://fasting.center/fasting-blog/hunger-during-fasting

Work back from the daily targets, not from the label's serving suggestion. During an extended fast I aim for sodium 3,000 to 5,000 mg (more with sauna or heavy sweat), potassium 1,000 to 2,000 mg, and magnesium 300 to 400 mg per day. Take the potassium and magnesium per scoop from your mix's label and divide the targets by those numbers. For most mixes, including Re-Lyte and LMNT, that lands at 2 to 3 scoops a day, spread across the day in 16 to 32 oz of water each. Cover sodium separately with salt, since very few mixes carry enough. Go toward 3 scoops if you get headaches, cramps, or dizziness. Do not push potassium past 2,000 mg - it has the narrowest safety range. And skip any mix that carries calories or sugar - a 20-calorie serving is a sports drink, not a fasting electrolyte. My own setup is Himalayan salt plus 3 packets of Ultima a day.
Targets, mix comparison, and my bloodwork: https://fasting.center/fasting-blog/electrolytes-for-fasting

Nine times out of ten it is sodium, not hunger and not "detox". When insulin drops, the kidneys dump 3 to 5 g of sodium a day, and low sodium shows up as a headache, lightheadedness when you stand up, weakness, and brain fog. The fix is fast - half a teaspoon of salt in a glass of water, then keep sodium at 3,000 to 5,000 mg for the rest of the day. If the headache stays, check potassium and magnesium too, and cut coffee if you have been drinking more than usual. Dizziness on standing means stand up slowly and add salt and water. Mild symptoms should ease within an hour of electrolytes and rest. Fainting, a racing or irregular heartbeat, chest pain, repeated vomiting, confusion, or severe weakness are a different story - break the fast and contact a doctor.
Why it happens and the exact targets: https://fasting.center/fasting-blog/electrolytes-for-fasting

Poor sleep is one of the most common downsides of extended fasting, and it is hormonal, not something you did wrong. An extended fast is a stressor, and the body raises cortisol to break down stored energy. In my last 10-day fast cortisol went from 8.5 μg/dL before to 18.5 at the end of the fast, and back to 10.8 six weeks later - more than double. Elevated cortisol keeps you alert, and on top of that blood sugar swings in the first 3 days and poor electrolyte management make it worse. So protect your sleep during a fast - keep the room dark, minimize screens, keep a consistent schedule, get enough electrolytes (magnesium in the evening helps), don't overtrain, take a hot shower or bath, and stay mentally calm. It usually settles after day 3 to 4 as ketones come up. If sleep stays bad past a few days, shorten the fast and try again another week.
More on sleep and the other tradeoffs, with my lab data: https://fasting.center/fasting-blog/fasting-risks-and-downsides

Probably not. 60 to 80 mg/dL is normal during an extended fast, and readings in the 50s are common after day 3. Once ketones take over as the brain's main fuel, the body no longer needs to keep glucose in the fed-state range, so the number drifts down and stays there. My glucose dropped into the 60s and 70s on every extended fast, bottomed at 42 mg/dL during my 10-day fast, and I felt fine. Symptoms matter, not the number alone - shakiness, sweating, confusion, or a racing heart together with a low reading mean take salt and water, and if that does not settle it within 20 minutes, break the fast. A low number with no symptoms is just fasting physiology. The exception - if you have diabetes or take any glucose-lowering medication, this does not apply to you - talk to your doctor before fasting at all.
Why it drops to the 60s and even the 40s, with my daily readings: https://fasting.center/fasting-blog/blood-sugar-during-fasting

It is not placebo - there is biology behind it. By day 3 the switch to ketones is complete, and ketones are a steadier fuel for the brain than glucose, so the blood sugar swings that drive irritability and brain fog are gone. Several other things happen at the same time - neuroinflammation drops, GABA activity (the brain's calming signal) rises, the stress-response pathways recalibrate, and the digestive system is switched off, which frees a surprising amount of energy and attention. Many people describe it as less mental noise, a calmer mood, and long stretches of focus they rarely get while eating. That is also why the fast feels easier from day 4 - the same shift that steadies the brain settles hunger. It is one of the reasons people who have done one long fast tend to do another. The flip side - the same calm can tip into low drive or apathy in some people, and sleep may still be short, so use the focus for things you want done, not for decisions that need a full night's sleep.
The mechanisms, in more depth: https://fasting.center/fasting-blog/fasting-and-brain

Stay active. The instinct is to lie low and "save energy", but the goal of a fast is to spend energy, not conserve it - every calorie you burn comes from body fat, and movement speeds the switch into ketosis. My 10-day fast with light cardio and strength training in the first days reached deep ketosis much faster than a sedentary 9-day one, and I felt better throughout. The rule is active, not heroic - walks, easy cycling, light weights at reduced load, chores, a sauna if you like it. When you feel tired, rest without guilt, then get moving again. What to avoid is overtraining - long runs, heavy lifting, anything that leaves you wiped out - because it raises cortisol, wrecks sleep, and pulls more from muscle than from fat. Days 1 to 3 are the low-energy stretch, so plan the easy stuff then. From day 4 on, most people have surprising energy, and that is a good time to get things done. If activity makes you dizzy, it is a sodium signal, not a reason to stop moving - take salt and water first.
How the fast with training compared to the one without: https://fasting.center/fasting-blog/10-day-fast-results

For most people, light to moderate exercise during fasting is safe and often feels great. Walks, yoga, stretching, or even an easy run can boost energy and deepen fat burning. Many notice better focus and alertness.

Resistance training also helps, especially during extended fasts. It signals the body to hold onto muscle even without food. From my experience, I train every other day instead of 6–7 times a week, and keep the load around 50–70% of normal.

Intense workouts — heavy lifting, sprints, HIIT — are harder to sustain when glycogen is low. Recovery takes longer, and electrolytes matter more. If you’re new to fasting, start light and see how your body responds.

Electrolytes (sodium, potassium, magnesium) - yes! They’re not just safe, they’re recommended during longer fasts. They don’t break a fast and make it much easier by preventing headaches, fatigue, and muscle cramps.

When it comes to vitamins, it gets trickier. Water-soluble vitamins like vitamin C or B-complex are okay, but they can cause nausea on an empty stomach. Fat-soluble vitamins (A, D, E, K, omega-3 oils) usually come with fat and technically break the fast. And anything with calories — protein powders, collagen, or gummies — definitely breaks it.

Personally, during extended fasts I keep it simple: hot water with pink salt and electrolytes. Everything else waits until refeeding.

Most zero-sugar sodas don’t add calories, so they technically don’t break a fast. That said, many contain artificial sweeteners, which can spike insulin, trigger cravings, or negatively impact the gut microbiome.

From experience, the response is very individual — some people drink them without issue, while others notice blood sugar swings or increased hunger.

If your main goal is fat loss and making fasting easier to stick with, they’re probably fine in moderation. But if you’re fasting for gut rest, autophagy, or the full range of fasting benefits, water, black coffee, and plain tea are the cleaner choices.

Come up with an exciting project you want finished by the end of the fast, and make that the point of the fast. Something you are really excited about: learning a new language, building a website or an app, cleaning out and organizing the garage, writing the first chapter of a book, sorting twenty years of photos, planning your next trip in detail, or fixing everything in the house that has been broken for months (or years). Fasting frees the hours you normally spend on shopping, cooking, eating, and cleaning up, which is a surprising amount of time; give those hours a job. By the way, your exciting project will push fasting into the background and make it much, much easier.

Fasting isn’t a contest — knowing when to stop is just as important as knowing when to start. A few clear red flags mean it’s time to break the fast:

• Persistent dizziness or fainting that doesn’t improve with rest.

• Rapid or irregular heartbeat.

• Severe weakness, confusion, or very low blood sugar (shakiness, sweating, blurred vision).

• Ongoing nausea, vomiting, or diarrhea.

• Pain — especially chest or abdominal pain.

Milder symptoms like hunger, occasional fatigue, or feeling a little colder than usual are common and not necessarily reasons to stop. But if your body is clearly signaling distress, that’s your cue to refeed.

It depends on the length. After 16 to 24 hours, just eat a normal, protein-forward meal and skip the sugar rush. After 2 to 3 days, start with something light - broth and steamed vegetables - then a normal meal a few hours later. After 4 days or more, the goal of the first day is not calories or protein, it is gently restarting digestion. Start with warm broth (bone or veggie) plus a little boiled or steamed broccoli or cauliflower, chew slowly, small portions, no eggs or heavy protein yet. Over the next meals add steamed veggies, a little lean fish or chicken, and some fermented food like sauerkraut. Refeed for at least half the number of days you fasted - fast 6 days, refeed 3 - and keep sugar, processed food, and heavy dairy out the whole time. Rush this step and the bathroom gets involved.
Step by step, with what to avoid: https://fasting.center/fasting-blog/how-to-break-an-extended-fast

Overeating after a fast is very common, and mostly psychological. It fades after several fasts, once your brain learns that food is coming back. Until then, three things help. Break the fast with a small planned meal, something like bone broth, eggs, or boiled vegetables; that is how I start every refeed. Wait 2 to 3 hours before the second meal, so you can tell real hunger from the urge to make up for lost time. And decide the first two meals before the fast ends, so you are not choosing while hungry. Skip sugar and refined carbs on refeed day; they drive the strongest cravings and the biggest glucose swing.
How I refed after 10 days, with the data: https://fasting.center/fasting-blog/10-day-fast-results

Because most of what you lost was not fat. In the first days of a fast the body burns through glycogen, and every gram of glycogen holds about 3 grams of water, so the scale drops fast. On refeeding, the glycogen and water come straight back, and so does the food sitting in your gut. My 10-day fast took me from 164 to 148.7 lb, and within two weeks of eating I was back near baseline on the scale while the fat loss stayed on the Dexa scan. That rebound is normal and expected. What is not normal is regaining the fat too, and that only happens for one reason - eating the way you did before the fast. A fast is a reset, not a diet. If nothing changes afterwards, the weight comes back within weeks. Track body fat, not just weight, and you will see the real result.
My scale, Dexa, and refeed numbers: https://fasting.center/fasting-blog/10-day-fast-results

Yes, but less than people think. First, you cannot measure muscle directly outside a lab, so the most common way is measuring lean mass, which is muscle protein, plus water, glycogen, and gut content. During my 10-day fast lean mass dropped 7.8 lb on Dexa and came back fully, plus 1.5 lb, within 12 days of refeeding while the fat loss stayed. Most of that swing was water and glycogen, not muscle. Based on my calculation, I lost about 30 g of muscle protein a day after day 3, totalling about 210 g of protein, or roughly 2 lb of muscle tissue, over the whole fast. Protein use is highest early in the fast and drops once ketones take over, and studies show strength stays unchanged through a 7 to 10 day fast. Three things keep it that way - electrolytes, light strength training during the fast (reduced load, not zero), and plenty of protein in the eating weeks. Where muscle really goes is stacking long fasts back to back with no rebuilding time in between.
Three Dexa scans - before, end of fast, after refeed: https://fasting.center/fasting-blog/10-day-fast-results

Yes, temporarily, and it is not what it looks like. When you fast, the body breaks down stored fat and uses it for energy instead of dietary glucose, so more fatty acids and cholesterol are released into the bloodstream as fuel. As a result, total cholesterol, LDL, ApoB, and triglycerides usually go up during a fast. That reflects fat being moved around, not more cholesterol being produced - LDL particles are the delivery trucks carrying fat and cholesterol to tissues that need it. Once eating resumes, the numbers normalize, and with regular fasting cycles they often end up better than before. My own heart panel at the end of a 10-day fast looked rough and was back to normal a month later. Two practical notes - do not draw a lipid panel during or right after an extended fast if you want a baseline, and if your LDL is already very high, you have thyroid issues, or a genetic factor like elevated Lp(a), talk to your doctor before a long fast.
My heart panel at the end of the fast and one month later: https://fasting.center/fasting-results

During an extended fast, yes - that part is real. The body reads a multi-day fast as an energy shortage and turns down the reproductive axis, so testosterone drops noticeably by the end of a long fast, along with a rise in cortisol. It rebounds during refeeding, usually within a few weeks, and in my own fitness panel the drop at the end of a 10-day fast was followed by a strong bounce back above the pre-fast level. So the pattern is a temporary dip, not a lasting loss. Intermittent fasting is a different story - most studies show little or no effect on testosterone, and any small dip tracks with fat loss, which tends to improve testosterone over time. Three things protect the rebound - refeed properly with enough protein and fat, get back to strength training in the first refeed week, and do not stack extended fasts back to back, since the hormones need eating weeks to recover. If you are on testosterone therapy or being treated for low T, this is a doctor conversation before any long fast.
My testosterone before, during, and after a 10-day fast, and the research: https://fasting.center/fasting-blog/fasting-and-testosterone

Yes, with one distinction that matters - the length of the fast. For intermittent and short fasts (up to about 72 hours), the research is encouraging - better insulin sensitivity, steadier hormones, and in several studies support for reproductive hormone balance and ovulatory function, especially with PCOS. Extended fasts of 3 days or more are different. A long fast is a stress signal, and in some women the body responds by delaying a period, making it lighter or heavier, or temporarily suppressing ovulation. It usually settles within a cycle or two after refeeding, but it is a real effect, not a myth. Practical rules I hear most from women who fast well - start with intermittent fasting and 24-hour fasts before trying anything longer, keep electrolytes up, do not stack long fasts back to back, and if your cycle changes, shorten the next fast rather than push through. Pregnant or breastfeeding - no extended fasting at all, and talk to your doctor before any fasting. Full disclosure - this site is written by a man, so the data here is research and community experience, not my own.
Where fertility and hormone balance sit in the evidence: https://fasting.center/fasting-benefits

It can raise the risk, and this one deserves respect. The gallbladder stores bile and releases it when you eat fat. During a long fast nothing triggers that release, so bile sits and concentrates, and at the same time rapid fat loss pushes more cholesterol into the bile. Concentrated, cholesterol-rich bile is exactly how stones form. The risk is highest for people who already have stones, for rapid weight loss in general (this is well documented after bariatric surgery and very-low-calorie diets, which look a lot like fasting to the gallbladder), and for repeated extended fasts stacked close together. Most healthy fasters never have a problem, but I have seen fasters end up in the ER with a gallbladder attack, so - if you have a history of gallstones or gallbladder pain, talk to your doctor before any fast beyond 24 hours. For everyone else - refeed with a little fat early to get bile moving, space long fasts out, and take sharp pain under the right ribs after eating, especially with nausea or fever, as a reason to see a doctor, not to wait it out.
The full risk list, including the gallbladder: https://fasting.center/fasting-blog/fasting-risks-and-downsides

Not without your doctor. Everything on this site is for healthy adults, and a few groups are outside that line - anyone on diabetes, blood pressure, or blood-thinning medication (doses often need adjusting when you stop eating, and getting that wrong is dangerous), anyone with a heart, kidney, or liver condition, pregnant or breastfeeding women, anyone under 18, anyone underweight, and anyone with a history of an eating disorder. That is not a reason to give up on fasting - many people in these groups fast safely with a doctor who knows their case and adjusts medication and monitoring. It is a reason to have that conversation first, before the first fast, not during it. Never stop or change a prescribed medication on your own to make a fast work. And regardless of health status, any fast beyond 10 days is worth discussing with a doctor.
The full list of risks and who they apply to: https://fasting.center/fasting-blog/fasting-risks-and-downsides