What 25 Hours Without Food or Water Does to Your Gut, and Why the Break-Fast Bagel Hits So Hard

Yom Kippur begins at sundown on Sunday, September 20 and ends at nightfall on Monday the 21st: roughly 25 hours in which observant Jews take in no food and no water. It is the most widely kept fast in Judaism, kept by many people who observe little else, and it comes with a break-fast table that is a cultural institution in its own right — bagels, lox, kugel, coffee, whatever the family does. It also comes, every year, with the same questions typed into the same search bar: why the headache, why the bathroom situation after, why a bagel you eat every other week of the year can feel like a brick on this one night.

Those questions have answers, and unusually good ones, because a 25-hour fast by hundreds of thousands of people on a known date is a natural experiment that Israeli hospitals have been studying for thirty years. This is a physiology explainer, not a religious one: it describes what a dry fast does to a healthy gut, hour by hour, and what the break-fast does after. Whether and how to fast is a question Jewish law itself routes through a rabbi and a doctor — “one who is required to eat on Yom Kippur due to medical reasons, should not attempt to fast against the doctor’s orders” — and the exemptions for illness, pregnancy and nursing are theirs to explain, not ours.

The empty gutNot idle, just on a different schedule

The first thing to know is that a fasting gut does not switch off. Once the pre-fast meal has cleared, the stomach and small intestine shift into a pattern that only exists between meals: the migrating motor complex, described in a 2012 review as “a cyclic, recurring motility pattern that occurs in the stomach and small bowel during fasting; it is interrupted by feeding.” Its most active phase is “a burst of contractions originating from the antrum or duodenum and migrating distally” — a sweep, roughly every hour and a half to two hours, that clears residue and bacteria down the line. The review notes that its absence “has been associated with gastroparesis, intestinal pseudo-obstruction and small intestinal bacterial overgrowth,” which is a way of saying the fasting sweep is one of the gut’s housekeeping routines, and a day of it is not harm. The growling you hear around hour six is, in part, that sweep running through an empty stomach.

Hunger, meanwhile, is a clock more than a gauge. In the study that established it, ghrelin — the stomach’s hunger hormone — “increased nearly twofold immediately before each meal and fell to trough levels within 1 h after eating” in people sampled every 20 minutes for a day. It rises toward the times you usually eat, whether or not you do, and then it recedes. That is why the hardest stretches of a fast tend to be breakfast time and lunchtime, and why the wave passes if you wait it out. The stomach keeps making some acid around the clock on a well-documented daily rhythm, which is the source of the sour, hollow feeling by afternoon; it is not injury, and it ends when food arrives.

The headacheTwo headaches, actually, and neither is dehydration

The headache is the fast’s best-studied symptom, and the research on it is specifically about this fast. In 1993, two Tel Aviv neurologists tracked 370 hospital employees through Yom Kippur: 39 percent of those who fasted developed a headache, against 7 percent of those who did not, and “the number of headache sufferers increased in direct relation to the duration of the fast.” People with any history of headaches were far more likely to get one (66 versus 29 percent). The international headache classification now lists it as its own entity: a “diffuse non-pulsating headache, usually mild to moderate, occurring during and caused by fasting for at least eight hours,” and “relieved after eating.” A 2010 review puts its lifetime prevalence at 4.1 percent of people and notes it usually appears after at least 16 hours and clears within 72 hours of eating.

What it is not, surprisingly, is dehydration. The same neurologists went back and weighed 56 fasters before and after: everyone lost weight, about 1.2 to 1.4 kilograms, and the loss was the same in the half who got a headache and the half who did not. Their conclusion: “dehydration, as reflected by acute weight loss, is an unlikely cause of headache during a single day of fasting.” Nor, in that first study, did caffeine withdrawal appear to explain it. But caffeine withdrawal is a separate headache running on its own clock, and for the two-coffees-a-day majority it lands inside the fast: in the definitive review, withdrawal symptoms begin 12 to 24 hours after the last dose, peak at 20 to 51 hours, produce a headache in about half of people, and appear at intakes as low as 100 milligrams a day. A fast that starts Sunday evening puts hour 24 at Monday evening, right on the peak. Two overlapping headaches, then, one from the fast and one from the coffee — which is why Cornell Health’s standing advice is to “decrease caffeine from coffee, tea, soda, and other sources during the week preceding Yom Kippur.” (Israeli researchers have also run placebo-controlled trials of a long-acting anti-inflammatory taken before the fast, cutting headache from 68 to 36 percent; the drug is not approved in the US, and that is a clinician conversation, not a tip.)

The waterWhat a dry day actually costs

A day without water is the part people worry about most, and the data are more reassuring than the worry. The kilogram or so lost over the fast is mostly water, and the body handles it; the concrete, measurable consequence sits in the kidneys. A 2021 analysis of 11,717 emergency visits for kidney-stone pain at Israel’s largest hospital found that in the three days after the fast, visits averaged 6.66 a day against an annual mean of 4.1 — a real bump, in a population that mostly already had stones, with a mechanism the authors call “unknown.” Against that, a decade of emergency records for people with diabetes found the fast “does not seem to be associated with an increased rate of visits,” a finding that speaks to people who fast with their doctor’s agreement, not against it.

In the gut, the day without fluid shows up afterwards, as constipation. The physiology is straightforward: the colon reclaims water from stool, and when intake drops sharply it reclaims more, so “fluid restriction and thus de- or hypohydration increase constipation.” The month-long Ramadan fast, which is also dry by day, offers a supporting number: in 900 people surveyed, the one gut symptom that changed was constipation, which roughly doubled in odds. A single day is a smaller version of the same effect; the fix is the one Cornell puts first for the break-fast — fluids before food — and the pre-fast advice everyone agrees on: “drink plenty of fluids during the day or two preceding Yom Kippur” and, from one much-shared physician’s guide, “avoid consuming much salt” at the meal before it. Salt is the theme of the next section too.

The break-fastWhy the bagel hits so hard

Then the fast ends and the table is full, and three things happen at once. First, the gut’s fed reflexes come back on with the first swallow: the gastrocolonic response — the colon waking up when the stomach fills — “occurs within minutes after feeding and continues for 30–120 min,” and “fat must be an important component of the ingested meal to induce” it. After 25 hours of the quiet fasting pattern, a plate of lox, cream cheese and kugel is a large, fatty signal into a system that has been running housekeeping sweeps all day, which is the physiological translation of “I ate too fast and now I feel it everywhere.” Second, the meal is salty and the body is a litre or so short: thirst is triggered by just a 2 to 3 percent rise in blood sodium, so a plate of cured fish on a dehydrated body produces a thirst out of proportion to the meal, and the water you then drink in volume sits on top of the food. Third, the sheer size of a first meal after a fast stretches a stomach that has been empty all day; there is evidence that prolonged fasting slows gastric emptying, though that study ran four days, not one, and no one has measured emptying after a 25-hour fast.

None of this is dangerous in a well-nourished person; it is uncomfortable, and it is avoidable. Refeeding syndrome, the genuinely serious electrolyte crash people sometimes worry about, belongs to a different situation entirely: the clinical criteria require little or no intake for five to ten days or more, or existing malnutrition, not a day. What the institutions that answer this question every year actually recommend is nearly identical across sources. Cornell: “Break your fast by first drinking fluids, which will be absorbed quickly on an empty stomach and rehydrate you,” then “progress to regular foods, but try not to eat too rapidly.” A geriatrician writing for My Jewish Learning: “Go slow! Eat some carbs and drink some fluids.” The physician’s guide: begin with “a big glass of fluid, but don’t drink too much at once.” In gut terms that sequence is exactly right: fluid first because the deficit is fluid; something plain and starchy next because it triggers the least gastrocolonic response; the lox and the second bagel twenty minutes later, when the first signal has settled.

The bottom lineA day the gut can do

A healthy adult’s gut is built for a day like this; the fasting sweep, the hunger waves and the sour afternoon are its normal between-meal program running longer than usual. The headache is mostly the fast itself plus the coffee you did not have, and the week-before taper is the one preparation with a mechanism behind it. The water shows up afterwards as a slow day or two in the bathroom, which fluids fix. And the bagel hits hard because it arrives fatty, salty and all at once into a system that has been quiet for 25 hours — so water first, then something plain, then everything else. For anyone with diabetes, a pregnancy, a history of kidney stones or any condition that makes a dry day a medical question rather than a hard one, the tradition itself says the health decision comes first, and it is one to make with a rabbi and a clinician before Sunday. This article is educational, describes the cited research, and is neither medical nor religious guidance.