The Yom Kippur Fast: A Science-Based Guide to Preparing, Fasting, and Recovering

In brief5 points · 1-minute read

• The main driver of the Yom Kippur headache is caffeine withdrawal rather than dehydration, so the effective preparation is a gradual caffeine taper in the week before.
• In Israeli research, headache affected 39% of fasters versus 7% of non-fasters, with headache history and caffeine intake raising the risk.
• A good pre-fast meal is built on fluids, sodium and a normal balanced plate, not on loading extra food or special carbohydrates.
• The tincture is not taken on the fast day itself; it can be taken earlier on the day before, with water, away from the start of the fast.
• Break the fast gradually, fluids first and a small meal, to avoid a sharp rise in blood glucose.

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Yom Kippur — the Day of Atonement, the 10th of Tishrei — is a roughly 25-hour dry fast: no food and no water from before sundown to after nightfall the next day. It is also, remarkably, one of the best-studied ritual fasts in the medical literature, because Israeli neurologists and emergency physicians have been publishing on it since the 1990s. Their numbers are worth knowing before Kol Nidre: 39% of fasters in one Israeli hospital study developed a headache versus 7% of non-fasters; caffeine-withdrawal symptoms peak 20–51 hours after the last cup — squarely inside the fast; and a resting adult in temperate weather loses only about 1% of body mass in a day without food or fluid. This guide walks through the week before, the pre-fast meal, the fast itself, how to break it, and the day after — with a PubMed link on every number, and an honest word about our own tinctures.

When is Yom Kippur this year, and when should preparation start?

Yom Kippur 5787 begins at sundown on Sunday, September 20, 2026, and ends at nightfall on Monday, September 21 — about 25 hours. Physiological preparation starts about a week out: note your daily caffeine dose around September 13–14, begin the five-day caffeine taper around September 15–16, and shift salt and fluids in the final two days.

The date matters because the two mechanisms that make the fast hard both run on clocks you can set in advance. Caffeine withdrawal typically begins 12–24 hours after the last dose and peaks at 20–51 hours — which means a last coffee before the pre-fast meal delivers the peak in the middle of Yom Kippur day. And glycogen, hydration and sodium status are set by what you eat and drink in the 48 hours before Kol Nidre, not by heroics at the last meal. Everything below is organized on that timeline: a week out, two days out, the eve, the day, the break, the morning after.

What happens to your body during 25 hours without food or water?

Less than most people fear. At rest in a temperate environment, a day without food or fluid costs about 1% of body mass in water — a loss described in the sports-science literature as unlikely to meaningfully impair physical or cognitive performance. Hunger comes in waves, alertness dips at predictable hours, and the body defends its fluid volume well.

The 1% figure comes from a review of hydration during Ramadan by Maughan and colleagues, which also notes the two caveats that matter for a September fast in a warm country: losses grow “in hot weather or if exercise is undertaken,” and performance can begin to be affected at as little as 1–2% of body mass. A 2023 bioelectrical-impedance study of 73 healthy fasters adds a reassuring detail: of roughly 1,030 g lost, over 80% was fat mass; total body water and extracellular water did not change significantly, with only intracellular water dipping by about 160 ml. That was repeated daily Ramadan fasting rather than a single 25-hour fast, so read it as direction, not destiny — but the direction is clear: a healthy body protects its water. The practical conclusion writes itself: the fast is manageable at rest, in shade, without exertion. Walking to synagogue is fine; the afternoon is not the time to prove anything physical.

Why does the fast give you a headache — and what actually causes it?

“Yom Kippur headache” is a term from the medical literature: in a study of 370 Israeli hospital workers, 39% of fasters developed a headache versus 7% of non-fasters. It is usually a tension-type headache — bilateral, frontal, non-pulsating, mild to moderate. The leading suspects are caffeine withdrawal and low blood sugar, and the probability rises with the length of the fast.

The founding study, Mosek and Korczyn’s 1995 paper in Neurology — titled simply “Yom Kippur headache” — found the risk was far from uniform: among people with any history of headaches, 66% got one during the fast, versus 29% of those without (both differences vanishingly unlikely to be chance). A later placebo-controlled trial gives an even starker baseline: in its placebo arm, 67.7% of fasters developed a headache over the 25 hours. The 2009 literature review by Torelli and colleagues — whose own search terms included “Yom Kippur” — summarizes the field: fasting headache carries the clinical features of tension-type headache, its probability “increases directly with the duration of fasting,” and hypoglycemia and caffeine withdrawal are the implicated factors. The International Classification of Headache Disorders even has an official category for it: headache attributed to fasting. In other words — if your head pounds on Yom Kippur afternoon, you are a textbook case, and most of the fix happens in the week before, not on the day.

One historical footnote worth knowing: this headache is so predictable that Israeli researchers ran two randomized, double-blind trials preventing it with long-acting anti-inflammatory drugs before the fast. In the 2010 trial, 120 mg of etoricoxib cut headache incidence from 67.7% to 36.4% and roughly halved its severity. The earlier 2006 trial used rofecoxib — a drug withdrawn from the world market in 2004 over cardiovascular risk — which is exactly why we cite these as proof of mechanism, not as a recommendation. Whether any preventive medication is appropriate for you is a question for your doctor, before the fast.

How do you taper off caffeine in the week before?

Gradually, starting about five days out. The tapering protocol documented in the research took moderate-to-heavy coffee drinkers from 300 mg of caffeine a day to zero over five days. Even 100 mg a day — a single cup — is enough to produce withdrawal symptoms on abstinence, so “I barely drink coffee” is not an exemption.

The definitive review of caffeine withdrawal (Juliano & Griffiths, 2004) validated ten symptoms — headache first, then fatigue, drowsiness, low mood, irritability and difficulty concentrating — and mapped their arc: onset 12–24 hours after the last dose, peak intensity at 20–51 hours, duration 2–9 days. In experimental studies, half of abstainers got a headache. Overlay that arc on the fast and the problem is obvious: quit cold at the pre-fast meal and you schedule your withdrawal peak for Yom Kippur afternoon, on top of the fast itself. The mechanism is vascular: in a transcranial-Doppler study, 24 hours of complete abstinence left 10 of 20 volunteers with moderate-to-severe headaches, and blood-flow velocities in the brain’s arteries rose measurably — caffeine constricts cerebral vessels, and withdrawal dilates them. Recovery came within an hour of caffeine — a luxury the fast does not offer.

Two practical findings refine the taper. First, a 2019 trial found that people who knew their dose was being reduced reported stronger withdrawal than people who didn’t — a nocebo effect — so don’t rehearse your suffering; just quietly cut the dose. Second, a 2023 trial in 61 heavy coffee drinkers found that openly drinking decaf — knowing full well it was decaf — still reduced withdrawal symptoms by a significant 9.5 points. The ritual of the cup does real work. So: five days out, start substituting decaf cup by cup, and arrive at the pre-fast meal already at zero, with the worst behind you.

What should you eat and drink in the two days before?

Normal, unheroic meals with three deliberate shifts: fluids spread across both days rather than chugged at the end; salt present at meals, because sodium is what makes the water stay; and carbohydrates at each meal to walk into the fast with full glycogen stores. No crash diets, no new supplements, no experiments.

The sodium point has the best data behind it. In a controlled rehydration trial, doubling the sodium concentration of a drink (from 25 to 50 mmol/L) cut urine output from 1,796 ml to 1,450 ml — roughly a fifth less water lost — and the authors concluded that electrolyte content mattered more for fluid retention than small changes in alcohol content. The context was recovery after exercise, not a pre-fast meal, so treat it as a physiological principle rather than a Yom Kippur trial: water you drink without salt runs through you; water accompanied by sodium is retained. That is the honest scientific case for the traditional advice to eat properly salted food before a fast — and, at the same time, the case against a very salty final meal, which simply makes you thirsty at the worst possible moment. Aim for normally seasoned food across two days, not a salt bomb at the end. Here is the whole preparation, on one clock:

What should the pre-fast meal look like — and what should it not?

A normal, balanced dinner eaten unhurried: carbohydrates, protein, vegetables, normally salted, water alongside — finished before you are uncomfortably full. Skip the very salty, the very spicy, the alcohol and the espresso. And the famous “complex carbs keep you full longer” rule? The direct trial found no such advantage.

That last sentence deserves its evidence, because it contradicts almost every fasting guide ever written. A randomized crossover trial fed fasting men two pre-fast meals identical in macronutrients but different in glycemic index (GI 37 vs about 57), then measured them through the next fasted day and a 60-minute run 12 hours later. The result: “no significant differences between the two meals for ratings in perceived satiety, fullness, appetite and mood states” — and none in metabolic or performance measures either. A separate trial makes the picture stranger still: under matched conditions, the high-GI meal produced about 25% greater fullness (in people with type 1 diabetes after exercise — a caveated population, but the direction is the opposite of the folklore). Our conclusion, stated the way we wish more guides would: eat carbohydrates at the pre-fast meal because glycogen matters, choose whichever kind sits well with you, and don’t expect any particular carb to buy you extra hours of satiety — the evidence for that is not there.

How much water should you drink before the fast — and does “water loading” work?

Spread roughly 8–10 unhurried glasses across the eve of the fast rather than forcing litres at the end — beyond a comfortable full tank, extra water is simply urinated away before the fast begins, and salt-free chugging leaves even faster. And know its limits: pre-fast drinking did not prevent the fast headache in the study that checked.

Two findings put water in its correct, humbler place. First, the loss itself is modest — about 1% of body mass at rest in temperate weather, as above — so you are topping off a small deficit, not provisioning a desert crossing. Second, and this surprises almost everyone: dehydration is probably not why fasting hurts. Mosek and Korczyn followed 56 women through the 25-hour fast and weighed them: those who developed a headache had lost 1.4 ± 0.8 kg, those who didn’t lost 1.2 ± 0.5 kg — statistically indistinguishable. Nearly all lost weight; only about half got the headache. Their conclusion: dehydration “is an unlikely cause of headache during a single day of fasting.” So drink well before the fast — for comfort, for the kidneys, for the margin it buys in heat — but drink with food and salt, across the day, and put your real preventive effort where the evidence points: the caffeine taper.

Should you take supplements or tinctures on the fast day itself?

Our position as a supplement brand: no. Yom Kippur is a full fast from food and drink, and tincture drops are both. We do not recommend taking our extracts during the fast, and we make no claim that any mushroom “helps you fast.” The one category that is genuinely different is prescription medication — and that decision belongs to your doctor and, in your tradition, your rabbi.

We want to be unambiguous here, because this is where a supplement company could quietly sell you something. There is no evidence that mushroom extracts — ours or anyone’s — ease fasting, and we have never looked for a way to claim it. A missed day of reishi or lion’s mane costs nothing meaningful; these are routines measured in weeks, as our page on how long mushroom extracts take to work explains. Prescription medications are the opposite case: some must not be skipped, some can be re-timed, and the medical world maintains detailed frameworks for adjusting treatment around ritual fasts — that is a pre-fast conversation with a physician, never an improvisation on the day. If you take any regular medication, ask about it in the week before, not at Ne’ilah.

So when should you take your extract around the fast?

Earlier in the day before the fast, with a full glass of water — and not close to the start of the fast. The reason is specific: our extracts contain alcohol from the seven-week alcohol phase of the triple extraction, and alcohol’s diuretic effect is measurable precisely when the body is well hydrated — exactly your state during the fluid-loading hours before Kol Nidre.

The physiology is worth spelling out, because it is the opposite of the folk version. In a controlled trial, Hobson and Maughan gave volunteers a litre of beer — 4% alcohol or alcohol-free — in two states. When the volunteers were well hydrated, the alcohol produced significantly more urine (1,279 vs 1,121 ml over four hours); when they were already short of fluid, the difference shrank to non-significance, leading the authors to conclude that “the diuretic action of alcohol is blunted when the body is hypohydrated.” A concentration study by the same group adds nuance: drinks of up to 2% alcohol behaved like water for rehydration, while 4% tended to delay recovery — and a tincture, though its absolute alcohol amount per serving is tiny, is a high-concentration form. So the careful statement is this: the hours before a fast, when you are deliberately hydrated, are exactly when alcohol has a measurable diuretic effect — which is why we say take the drops early in the day, with a full glass of water, and not adjacent to the fast. After the fast, once you have eaten and drunk, the evening routine resumes as usual; our timing and stacking guide covers where each extract sits in a normal week.

Who should talk to a doctor before fasting?

Anyone on regular medication; anyone with diabetes, a history of kidney stones, or a pregnancy; anyone recovering from illness or significantly underweight. For each of these the literature has something concrete to say — and for none of them is the answer an automatic “don’t fast.” The formula we stand by: your doctor for the medicine, your rabbi for the tradition.

The data here are more reassuring than the worry. An Israeli database study covering eleven Yom Kippurs (1999–2009) examined emergency-department visits among people with diabetes in the 48 hours after the fast began — and found no increase. That is not permission; it is context: with planning, most people manage. The planning itself is a real discipline — the international diabetes community publishes practical guidelines for risk assessment and medication adjustment around ritual fasting, which is precisely why the conversation belongs before the fast. The clearest single risk number belongs to kidney stones: an Israeli study of over 18,000 renal-colic emergency visits, controlled for ambient temperature, found a 27% higher rate during the first two weeks of Ramadan fasting — so a stone history earns both a medical conversation and extra pre-fast attention to fluids. In pregnancy, an evidence-based review for obstetricians found little to no clinically significant effect of ritual fasting on birthweight or preterm delivery, with the burden falling mainly on the mother — fatigue and signs of dehydration; a systematic review of 43 studies on offspring outcomes rated only 3 as high quality and called the evidence weak. Translation: not a blanket prohibition, but a genuinely individual decision, made with your obstetrician — and made easier by the fact that the tradition itself exempts those whose health requires it.

What are the hardest hours of the fast — and how do you get through them?

The late afternoon. In a study that tracked fasters through the day, subjective alertness dipped at 9:00 and 16:00 and mood dipped at 16:00 — the hour when the fast, the caffeine withdrawal arc and the circadian low all converge. The strategy: plan the afternoon as the quiet stretch — rest, shade, slow pacing — and know that the wave passes.

The 16:00 trough comes from a small study (ten participants, Ramadan conditions, altered sleep — read it as a sketch, not a law), but it matches what every faster knows: the morning is easier than it has any right to be, and the wall arrives mid-afternoon. Two other findings help you frame the day. The withdrawal literature says caffeine symptoms peak at 20–51 hours — so if your last coffee was at the pre-fast meal, the peak lands squarely in that afternoon window; if you tapered, it doesn’t. And the Ramadan first-day study found headache struck 46% of fasters on placebo in the first days and the effect faded by the second week — fasting is hardest when the body hasn’t practiced, which is an argument for the gradual week-before ramp rather than a cold plunge. A review of cognition under fasting adds one caution worth respecting: some aspects of decision-making measurably dip in the fasted state. Yom Kippur asks for reflection, not sharp decisions — the physiology agrees.

How do you break the fast without crashing?

Fluids first, then something small and unhurried, then — only after a pause — the meal. Continuous glucose monitoring around fast-breaking meals shows exaggerated surges in blood glucose, and sustained overnight hyperglycemia when the evening slides into overfeeding. The traditional light break — a sweet drink, a piece of cake, a pause — is physiologically sound; the buffet assault is not.

The monitoring data come from Ramadan, where the sunset meal (iftar) plays the same role as the post-Yom Kippur meal: the researchers describe “exaggerated surges in blood glucose and sustained overnight hyperglycaemia in cases of nocturnal overfeeding,” worst in people whose glucose control was already strained. Add the GI finding from earlier — a high-glycemic meal produced about 60% greater post-meal glucose exposure than a low-glycemic one — and the shape of a good break emerges: water or diluted juice, a modest carbohydrate to restart the engine, a real pause, then a normal dinner. One fear you can retire: refeeding syndrome, the dangerous electrolyte crash described in the clinical literature, belongs to prolonged malnutrition — its consensus criteria talk about days of depleted feeding in compromised patients, not a healthy person’s single 25-hour fast. If you are healthy, the risk after Yom Kippur is not a syndrome; it is a stomachache from eating a week’s worth of kugel in forty minutes.

What should you do the day after to get back to routine?

Rehydrate across the whole day rather than in one sitting, return to normal meals, reintroduce caffeine deliberately at your new lower dose if the taper taught you something, and resume your regular supplement routine in the evening. The body’s own data say recovery from a single-day fast is fast: most of the lost weight is back with the first day of normal eating and drinking.

The morning after is also a rare opportunity. You have just run a five-day caffeine taper and discovered — most people do — that life at zero or near-zero is survivable. The withdrawal review notes symptoms resolve within 2–9 days; by the morning after Yom Kippur you are through the worst of it, and whatever dose you return to is a choice rather than a habit. Many of our customers use the post-fast evening to restart the routines the holiday paused — the reishi drops after dinner, the regular sleep hour. If you are rebuilding a routine from scratch, our dosage guide and side-effects page are the places to start, and anyone combining extracts with medication should read the interactions page — and ask their doctor, in that order of authority reversed.

Which three myths about the fast does the research reject?

Three beliefs repeated in almost every fasting guide fail against the direct evidence: that the fast headache is caused by dehydration; that complex, low-glycemic carbs at the pre-fast meal keep you full longer; and that alcohol taken before the fast “dehydrates you during it.” Each has a study that tested it — and each study said no.

We keep this section on the page for a reason. A guide that repeats comfortable myths is easy to write and easy to distrust; a guide that shows you where the evidence contradicts the folklore — including folklore that would flatter our own product — is the only kind worth your bookmark.

What does this fast not do — and what don’t we claim?

A 25-hour fast is not a “detox,” not a metabolic reset, and not a health treatment — and it doesn’t need to be; its purpose is spiritual, and the body’s job is simply to carry you through it comfortably. We claim no health benefit for the fast, and no role for our products in it. The science on this page is about comfort, not enhancement.

Scientific humility cuts both ways here. The alarmist version of this page — fasting as danger — fails against the Israeli emergency-room data showing no surge in visits among people with diabetes across eleven Yom Kippurs. The wellness version — fasting as detox and renewal — has no support in anything we reviewed, and “detox” in particular is a marketing word without a measurable referent in a one-day fast. What the honest evidence supports is narrow and useful: prepared correctly, a healthy adult passes 25 hours without food and water with modest fluid loss, a preventable headache, an afternoon dip, and a full recovery by the next day. That is enough. The meaning of the day was never going to come from a supplement company anyway — our job is only to make sure our bottle, which contains alcohol, is timed honestly around yours.

The bottom line

The Yom Kippur fast is a 25-hour dry fast that the medical literature has measured with unusual care, much of it in Israel. The headache that shadows it strikes about 4 in 10 fasters — up to two-thirds in the unprepared — and it is driven by caffeine withdrawal and low blood sugar, not by dehydration; fluid loss at rest is a modest 1% of body mass. The preparation that works is unglamorous: a five-day caffeine taper (decaf helps, even openly), salt and fluids spread over two days, a balanced pre-fast meal with no illusions about magic carbohydrates, shade and stillness through the afternoon dip, and a gentle, staged break. Our tinctures contain alcohol, and we do not recommend taking them on the fast day itself — earlier in the day before, with a full glass of water, and back to the routine the evening after. May the fast be meaningful, and easy — in that order.

Frequently asked questions

Is the Yom Kippur headache caused by dehydration?

Probably not. In the Israeli study that weighed 56 fasters, weight loss was statistically identical in those who developed a headache (1.4 kg) and those who didn’t (1.2 kg), and the authors concluded dehydration is an unlikely cause over a single day. The implicated factors are caffeine withdrawal — whose symptoms peak 20–51 hours after the last dose — and low blood sugar.

How do I prevent the fast headache?

Start a caffeine taper about five days out — the documented protocol went from 300 mg a day to zero over five days, and switching openly to decaf reduced withdrawal symptoms in a controlled trial. Eat regular meals with carbohydrates and normal salt through the final two days, and drink across the day of the eve. If headaches hit you hard every year, talk to your doctor before the fast — preventive approaches exist and were studied in randomized trials.

Can I take Triterra extracts during the fast?

We don’t recommend it — and we’re the ones selling them. Yom Kippur is a full fast from food and drink, and tincture drops are both; our extracts also contain alcohol from the seven-week alcohol phase of the extraction. Take your drops earlier in the day before the fast, with a full glass of water, and resume the routine the evening after. A single missed day changes nothing in a routine measured in weeks.

Does drinking a lot of water right before the fast help?

Only up to a comfortable full tank. A resting adult in temperate weather loses about 1% of body mass in fluid over a day without food or drink — a small deficit. Water gulped without food or salt is largely urinated away before the fast begins; fluids spread across the day, taken with normally salted meals, are retained far better. And pre-fast drinking did not prevent the headache in the study that checked.

Should people with diabetes fast on Yom Kippur?

That is a decision to make with your doctor before the fast — medication timing and monitoring can often be adjusted, and formal medical guidelines exist for exactly this planning. The reassuring context: an Israeli database study across eleven Yom Kippurs found no increase in emergency-room visits among people with diabetes in the 48 hours after the fast began. In your tradition, your rabbi is the other half of the conversation.

What is the best way to break the fast?

Fluids first, then something small, then a pause, then a normal meal. Glucose monitoring around fast-breaking meals shows exaggerated blood-sugar surges, with sustained overnight highs when the evening becomes overfeeding — and high-glycemic meals produced about 60% greater glucose exposure in a matched trial. The traditional sweet drink and light bite before the real meal is physiology-approved.

Is fasting on Yom Kippur a detox?

No. “Detox” has no measurable meaning in a single 25-hour fast; the liver and kidneys do that job every day. The honest physiological summary is narrower: modest fluid loss (about 1% of body mass at rest), a preventable headache, an afternoon dip in alertness, and full recovery within a day. The fast’s value is spiritual — it doesn’t need a health claim, and we don’t make one.

What about fasting while pregnant?

The evidence review for obstetricians found little to no clinically significant effect of ritual fasting on birthweight or preterm delivery, with the burden falling mainly on the mother as fatigue and signs of dehydration; a systematic review rated only 3 of 43 studies as high quality and called the evidence weak overall. That makes it an individual decision, made with your obstetrician — and the tradition itself exempts those whose health requires it.

Scientific sources (peer-reviewed)

• The founding “Yom Kippur headache” study: among 370 hospital workers, 39% of fasters developed headache vs 7% of non-fasters; 66% vs 29% in those with a headache history; typically bilateral, frontal, non-pulsating — Mosek A, Korczyn AD. Neurology, 1995;45(11):1953-1955. View on PubMed
• Dehydration ruled unlikely as the cause: weight loss over the 25-hour fast was 1.4 ± 0.8 kg in women with headache vs 1.2 ± 0.5 kg without — not significant — Mosek A, Korczyn AD. Headache, 1999;39(3):225-227. View on PubMed
• Randomized, double-blind prevention trial: etoricoxib 120 mg before the 25-hour fast cut headache incidence to 36.4% vs 67.7% on placebo, with lower median severity (3.0 vs 5.0) — Drescher MJ, Alpert EA, Zalut T, et al. Headache, 2010;50(8):1328-1334. View on PubMed
• The earlier rofecoxib trial: 18.9% headache vs 65.4% on placebo — cited as historical evidence of mechanism only; the drug was withdrawn worldwide in 2004 — Drescher MJ, Elstein Y. Headache, 2006;46(10):1487-1491. View on PubMed
• The Ramadan analogue: headache in 46% on placebo vs 21% on etoricoxib in the first days, with the difference fading by the second week — the first fast day is the hardest — Drescher MJ, et al. Headache, 2012;52(4):573-581. View on PubMed
• The definitive caffeine-withdrawal review: headache in 50% of abstainers in experimental studies; onset 12–24 h, peak 20–51 h, duration 2–9 days; doses as low as 100 mg/day produce withdrawal — Juliano LM, Griffiths RR. Psychopharmacology (Berl), 2004;176(1):1-29. View on PubMed
• The vascular mechanism: after 24 h of abstinence, 10 of 20 volunteers had moderate-to-severe headache with cerebral blood-flow velocities significantly increased — recovery within 1 h of caffeine — Couturier EG, Laman DM, van Duijn MA, van Duijn H. Cephalalgia, 1997;17(3):188-190. View on PubMed
• The documented taper protocol — 300 mg/day to zero over five days — and the nocebo finding that knowing about dose reductions worsens reported withdrawal — Mills L, Boakes RA, Colagiuri B. Journal of Psychopharmacology, 2019;33(8):994-1002. View on PubMed
• Open-label decaf reduced withdrawal symptoms by 9.5 points (95% CI 4.7–14.3, p = 0.002) in 61 heavy coffee drinkers after 24 h of abstinence — Mills L, Lee JC, Boakes R, Colagiuri B. Journal of Psychopharmacology, 2023;37(2):181-191. View on PubMed
• The fasting-headache literature review: tension-type clinical features, probability rising with fast duration, hypoglycemia and caffeine withdrawal implicated; lifetime prevalence of homeostasis-disorder headaches 22% in a Danish population — Torelli P, Evangelista A, Bini A, et al. Headache, 2009;49(5):744-752. View on PubMed
• The official classification recognizing “headache attributed to fasting” among disorders of homeostasis — Headache Classification Committee of the International Headache Society, ICHD-3. Cephalalgia, 2018;38(1):1-211. View on PubMed
• Fluid loss in a day without food or drink at rest in temperate conditions is typically about 1% of body mass — unlikely to majorly impair performance; heat and exercise increase losses — Maughan RJ, Zerguini Y, Chalabi H, Dvorak J. Journal of Sports Sciences, 2012. View on PubMed
• Body-water compartments in 73 fasters: of ~1,030 g lost, over 80% was fat; total body water and extracellular water unchanged, intracellular water down 160 ml — Frontiers in Nutrition, 2023. View on PubMed
• The daytime map: subjective alertness dipped at 09:00 and 16:00 and mood at 16:00 during fasted days (n = 10, Ramadan conditions) — Roky R, Iraki L, HajKhlifa R, et al. Annals of Nutrition & Metabolism, 2000;44(3):101-107. View on PubMed
• Review of cognition under fasting: certain cognitive aspects, including decision-making, measurably dip in the fasted state — Cherif A, Roelands B, Meeusen R, Chamari K. Sports Medicine, 2016;46(1):35-47. View on PubMed
• The low-GI pre-fast meal trial: no significant differences in satiety, fullness, appetite, mood or metabolic and performance measures 12 hours later vs a mixed meal — International Journal of Food Sciences and Nutrition, 2014. View on PubMed
• Sodium retains fluid: doubling drink sodium (25 to 50 mmol/L) cut urine output from 1,796 to 1,450 ml, with electrolyte content mattering more than small alcohol changes — Desbrow B, Cecchin D, Jones A, et al. International Journal of Sport Nutrition and Exercise Metabolism, 2015;25(3):262-270. View on PubMed
• Under matched conditions, the high-GI meal produced ~60% greater post-meal glucose exposure and ~25% greater fullness than the low-GI meal — Campbell MD, Gonzalez JT, Rumbold PLS, et al. The American Journal of Clinical Nutrition, 2015;101(3):478-486. View on PubMed
• Continuous glucose monitoring around fast-breaking meals: exaggerated glucose surges and sustained overnight hyperglycemia with evening overfeeding — Monnier L, El Azrak A, Lessan N, et al. Diabetes & Metabolism, 2015;41(6):456-462. View on PubMed
• Refeeding-syndrome consensus criteria — defined for prolonged malnutrition, not a healthy person’s single-day fast: graded electrolyte drops within 5 days of renewed feeding — da Silva JSV, et al. Nutrition in Clinical Practice, 2020;35(2):178-195. View on PubMed
• Alcohol’s diuretic action is blunted when the body is hypohydrated and measurable when euhydrated: 1,279 vs 1,121 ml urine over 4 h after a litre of 4% vs alcohol-free beer (p < 0.001) — Hobson RM, Maughan RJ. Alcohol and Alcoholism, 2010;45(4):366-373. View on PubMed
• Concentration decides: drinks up to 2% alcohol rehydrated like water after exercise; 4% tended to delay recovery — a negligible diuretic effect in dilute solution — Shirreffs SM, Maughan RJ. Journal of Applied Physiology, 1997;83(4):1152-1158. View on PubMed
• Formal international guidelines exist for risk stratification and medication adjustment before ritual fasting in diabetes — the reason the doctor conversation belongs before the fast — Hassanein M, et al. Diabetes Research and Clinical Practice, 2017;126:303-316. View on PubMed
• Eleven Yom Kippurs in an Israeli database (1999–2009): no increase in emergency-department visits among people with diabetes in the 48 h after the fast began — Becker M, Karpati T, Valinsky L, Heymann A. Diabetes Research and Clinical Practice, 2013;99(1):e12-e13. View on PubMed
• Kidney-stone history is the clearest risk: renal-colic emergency visits rose 27% (RR 1.27, 95% CI 1.03–1.50) in the first two weeks of Ramadan fasting, controlled for ambient temperature — Sagy I, Zeldetz V, Halperin D, et al. QJM, 2017;110(9):571-576. View on PubMed
• Pregnancy review for obstetricians: little to no clinically significant effect of ritual fasting on birthweight or preterm delivery; the burden is mainly maternal fatigue and signs of dehydration — Shahawy S, Al Kassab L, Rattani A. American Journal of Obstetrics and Gynecology, 2023;228(6):689-695. View on PubMed
• Systematic review of fasting in pregnancy and offspring outcomes: only 3 of 43 included studies rated high quality; the association “is not supported by strong evidence” — Oosterwijk VNL, Molenaar JM, van Bilsen LA, Kiefte-de Jong JC. Nutrients, 2021;13(10). View on PubMed

Read next

• Rosh Hashanah and Renewal: What Nature Knows About New Beginnings
• When to take mushroom extracts — and how to combine them
• How long do medicinal mushrooms take to work?
• Dosage guide
• Side effects of medicinal mushrooms
• Mushroom extracts and medications — the interactions page
• Medicinal Mushrooms by Goal — the hub
• The research hub
• The complete guide to medicinal mushroom extracts
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This page is educational and does not constitute medical, nutritional or religious advice. It describes what published studies measured; it is not a prediction of what any individual will experience during a fast, and it is not a recommendation to fast, not to fast, or to start, stop, re-time or dose any medication or supplement — those decisions belong with your physician and, in your tradition, your rabbi. These statements have not been evaluated by the Food and Drug Administration. This product is not intended to diagnose, treat, cure, or prevent any disease.